PHYSICS VIVA QUESTIONS 12th SECTION A.pdf
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physics practical
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VIVA VOCE
(Experiment No. A1) / Self-Assessment
0. 1. What is meant by the term electric current?
Ans. The flow of charge through a conductor is called electric current.
Q. 2. Name the S.I. unit of current.
Ans. Ampere (A).
Q. 3. Name the SI. unit of resistance.
Ans. Ohm which is also written in symbolic form as greek letter S2.
(Experiment No. A1) / Self-Assessment
0. 1. What is meant by the term electric current?
Ans. The flow of charge through a conductor is called electric current.
Q. 2. Name the S.I. unit of current.
Ans. Ampere (A).
Q. 3. Name the SI. unit of resistance.
Ans. Ohm which is also written in symbolic form as greek letter S2.
Q. 4. What is an ampere? Define it.
Ans. It is an S.I. unit of current. It is defined as 1 coulomb of charge flowing throuoh
conductor in 1 second. It is also measured as the rate of flow of charge, i.e., I = qh.
Q. 5. What is an ohm? Define i.
Ans. Ohm is the unit of resistance. When a potential difference of 1 volt is maintained acros
the ends of a conductor and a current of 1 A flows through it, its resistance is said to be 1 ob
Q.6. Every metallic conductor has a high number density of free elecirons moving wi%
high thermal velocities. Why does it not show any current wihen connected acrOSs a sensiti
ammeter?
Ans. The free electrons are in random motion. As a result of it, the number of electrons
Crossing an area of cross-section in one direction is the same as that crossing in the opposite
direction. Therefore, there is no net flow of electrons across a cross-section of the conductor in
any particular direction. Hence no current is shown by the ammeter.
Q. 7. How would you achieve a net flow of free electrons in a particular direction in
the conductor?
Ans. It can be achieved by applying a potential difference (p.d.) across the ends of a
conductor by connecting it to a source of e.m.f.
. 8. VWhy do free electrons start moving in a particular direction when a source of
e.m.f. is connected across the ends of the conductor?
Ans. The application of a potential difference across the ends of the conductor, creates an
electric field (E = V) inside the conductor. The electrons experience the force and start drifting
in a direction opposite to the direction of the field.
0.9. What is drift velocity of electrons?
Ans. On account of application of p.d. across the ends of the conductor. the free electrons
Over and above the random motion, have a directed velocity. The average value of this directed
velocity is called drift velocity.
Q. 10. What are the orders of drift velocity for normally applied potentials (0 to 5 V)
and that of random thermal speed at room temperature say (300 K).
Ans. The drift velocity is of the order of 10 m per second whereas random thermal velocity
of electrons is of the order of 10 metre per second.
Q. 11. What is the conventional direction of the flow of current in a metallic conductor?
In what direction do the electrons move?
Ans. The conventional direction of the flow of current is taken as the direction of flow OJ
positive charge from higher potential to lower potential. Actually, it is the electrons (witl
negative charge) which flow in a metallic conductor. Thus the direction of flow of electroni'
current is from negative to positive i.e., opposite to that of the conventional current.
Q. 12. State Ohm's Law.
Ans. The law states that the current passing through a conductor is directly proportiona
the potential drop across its ends, provided the temperature and other physical conditions rem
unchanged.
Q. 13. Is Ohm's Law true for triodes and diodes?
Ans. No, it is not true because potential difference and current in these do not have im
relationship, i.e., the graph between them is not a straight line. They
are said to be
non-ohmic
conductors; actually in vacuum tubes (V o [).
Q. 14. Define one volt.
Ans. It is an S.I. unit of current. It is defined as 1 coulomb of charge flowing throuoh
conductor in 1 second. It is also measured as the rate of flow of charge, i.e., I = qh.
Q. 5. What is an ohm? Define i.
Ans. Ohm is the unit of resistance. When a potential difference of 1 volt is maintained acros
the ends of a conductor and a current of 1 A flows through it, its resistance is said to be 1 ob
Q.6. Every metallic conductor has a high number density of free elecirons moving wi%
high thermal velocities. Why does it not show any current wihen connected acrOSs a sensiti
ammeter?
Ans. The free electrons are in random motion. As a result of it, the number of electrons
Crossing an area of cross-section in one direction is the same as that crossing in the opposite
direction. Therefore, there is no net flow of electrons across a cross-section of the conductor in
any particular direction. Hence no current is shown by the ammeter.
Q. 7. How would you achieve a net flow of free electrons in a particular direction in
the conductor?
Ans. It can be achieved by applying a potential difference (p.d.) across the ends of a
conductor by connecting it to a source of e.m.f.
. 8. VWhy do free electrons start moving in a particular direction when a source of
e.m.f. is connected across the ends of the conductor?
Ans. The application of a potential difference across the ends of the conductor, creates an
electric field (E = V) inside the conductor. The electrons experience the force and start drifting
in a direction opposite to the direction of the field.
0.9. What is drift velocity of electrons?
Ans. On account of application of p.d. across the ends of the conductor. the free electrons
Over and above the random motion, have a directed velocity. The average value of this directed
velocity is called drift velocity.
Q. 10. What are the orders of drift velocity for normally applied potentials (0 to 5 V)
and that of random thermal speed at room temperature say (300 K).
Ans. The drift velocity is of the order of 10 m per second whereas random thermal velocity
of electrons is of the order of 10 metre per second.
Q. 11. What is the conventional direction of the flow of current in a metallic conductor?
In what direction do the electrons move?
Ans. The conventional direction of the flow of current is taken as the direction of flow OJ
positive charge from higher potential to lower potential. Actually, it is the electrons (witl
negative charge) which flow in a metallic conductor. Thus the direction of flow of electroni'
current is from negative to positive i.e., opposite to that of the conventional current.
Q. 12. State Ohm's Law.
Ans. The law states that the current passing through a conductor is directly proportiona
the potential drop across its ends, provided the temperature and other physical conditions rem
unchanged.
Q. 13. Is Ohm's Law true for triodes and diodes?
Ans. No, it is not true because potential difference and current in these do not have im
relationship, i.e., the graph between them is not a straight line. They
are said to be
non-ohmic
conductors; actually in vacuum tubes (V o [).
Q. 14. Define one volt.
Ans. Volt is the S.I. unit of electric potential. One volt is said to be the potential difference
between two points if one joule of work is done in bringing one coulomb of charge from one
point to the other.
Q. 15. What is an electric cell?
Ans. A cell is a device in which e.m.f. is generated due to chemical action taking place
in it.
Q. l6. What are the essential parts of a cell?
Ans. A cell has two essential parts, viz., (i) two electrodes which are known as its positive
pole and negative pole, and (ii) an electrolyte.
Q. 17. What is the essential difference between a primary and a secondary cel?
Ans. In a primary cell the chemical action taking place inside the cell directly supplies the
electrical energy, i.e., chemical energy directly changes into electrical energy, whereas in a
secondary cell, electrical energy is first stored as chemical potential energy in the cell and
afterwards it is reconverted into electrical energy. Primary cells are not rechargeable whereas
secondary cells can be recharged.
Q. 18. Give examples of Primary and Secondary cells.
Ans. Daniell and Lechlanche cells are primary cells, whereas Lead acid accumulator and
Alkali accumulator (Ni-Fe cell) are the examples of secondary cells.
Q. 19. What is a dry cel?
Ans. It is essentially a Lechlanche cell in which the electrolyte is taken in a paste form.
0. 20. What is a battery?
Ans. Combination of cells joined in series is called a battery. Battery is used for drawing
higher currents. The e.m.f. of the battery is equal to the sum of the e.m.f. of all individual cells.
Q. 21. What is a battery eliminator?
Ans. It is basically a rectifier in which an A.C. voltage of 220 V from mains is converted
into a low d.c. voltage of a desired value such as 1.5 V, 3.0 V, 6 V,9 V, 12 V. It is a good
substitute for a battery or a cell.
0. 22. Is there any advantage of battery eliminator over usual source of e.m.f.?
Ans. Yes, the main advantage is that no charging is required for battery eliminator. (One can
also draw large currents). Battery eliminator is easy to handle and maintain, whereas a cell
requires change of chemicals and electrodes.
Q. 23. What is meant by ampere hour capacity of a cell?
measure of the electrical capacity of a cell, i.e., how much quantity of
electricity a cell is capable of supplying. It is measured by the product amperes x hours. A cell
Ans. It is a
of 16 ampere hour capacity can supply a current of one ampere for 16 hours or a current of
2 amperes for 8 hours.
Q. 24. (a) What does the abbreviation e.m.f. stand for?
(b) As the namne implies, is e.m.f. actually a force?
Ans. (a) It stands for electromotive force.
(b) No, it is a misnomer. It is rather the maximum potential difference across the terminale
of a cell when no current is drawn irom 1l, 1.e., the cell is in the open cireuit
O. 25. What is the usual sourCe of e.nn.f. in the laboratorv>
Ans. A primary or secondary cell and
eliminator.
Or a battery (number of cells in
series) or an
between two points if one joule of work is done in bringing one coulomb of charge from one
point to the other.
Q. 15. What is an electric cell?
Ans. A cell is a device in which e.m.f. is generated due to chemical action taking place
in it.
Q. l6. What are the essential parts of a cell?
Ans. A cell has two essential parts, viz., (i) two electrodes which are known as its positive
pole and negative pole, and (ii) an electrolyte.
Q. 17. What is the essential difference between a primary and a secondary cel?
Ans. In a primary cell the chemical action taking place inside the cell directly supplies the
electrical energy, i.e., chemical energy directly changes into electrical energy, whereas in a
secondary cell, electrical energy is first stored as chemical potential energy in the cell and
afterwards it is reconverted into electrical energy. Primary cells are not rechargeable whereas
secondary cells can be recharged.
Q. 18. Give examples of Primary and Secondary cells.
Ans. Daniell and Lechlanche cells are primary cells, whereas Lead acid accumulator and
Alkali accumulator (Ni-Fe cell) are the examples of secondary cells.
Q. 19. What is a dry cel?
Ans. It is essentially a Lechlanche cell in which the electrolyte is taken in a paste form.
0. 20. What is a battery?
Ans. Combination of cells joined in series is called a battery. Battery is used for drawing
higher currents. The e.m.f. of the battery is equal to the sum of the e.m.f. of all individual cells.
Q. 21. What is a battery eliminator?
Ans. It is basically a rectifier in which an A.C. voltage of 220 V from mains is converted
into a low d.c. voltage of a desired value such as 1.5 V, 3.0 V, 6 V,9 V, 12 V. It is a good
substitute for a battery or a cell.
0. 22. Is there any advantage of battery eliminator over usual source of e.m.f.?
Ans. Yes, the main advantage is that no charging is required for battery eliminator. (One can
also draw large currents). Battery eliminator is easy to handle and maintain, whereas a cell
requires change of chemicals and electrodes.
Q. 23. What is meant by ampere hour capacity of a cell?
measure of the electrical capacity of a cell, i.e., how much quantity of
electricity a cell is capable of supplying. It is measured by the product amperes x hours. A cell
Ans. It is a
of 16 ampere hour capacity can supply a current of one ampere for 16 hours or a current of
2 amperes for 8 hours.
Q. 24. (a) What does the abbreviation e.m.f. stand for?
(b) As the namne implies, is e.m.f. actually a force?
Ans. (a) It stands for electromotive force.
(b) No, it is a misnomer. It is rather the maximum potential difference across the terminale
of a cell when no current is drawn irom 1l, 1.e., the cell is in the open cireuit
O. 25. What is the usual sourCe of e.nn.f. in the laboratorv>
Ans. A primary or secondary cell and
eliminator.
Or a battery (number of cells in
series) or an
Q. 26. Why are the accumulators called storage cells?
Ans. These cells act as storehouses for electricity. Current can be drawn from them wheneve.
desired. After being discharged, they can be recharged. Since they store (or accumulate
electricity, they are known as storage cells or accumulators.
Q. 27. What is neant by internal resistance of a cell?
Ans. The resistance offered by the electrolyte of a cell to the flow of electricity is calledl
internal resistance of the cell. For a normal working cell, its value is about 1 to 2 ohm.
Q. 28. Is storage cell or accumulator, a primary cell or secondary cell?
Ans. It is a secondary cell.
Q. 29. Which one -a primary cell or a storage cell -has smaller internal resistance?
Ans. Storage cells have smaller internal resistance (about 0.1 2 in comparison to about
2 2 for primary cells.)
Q. 30. What do you understand by the ternm, "short-circuiting of a cell"?
Ans. When the two poles of a cell are connected by a wire of negligible resistance, then a
large current is drawn from the cell because I
circuited.
Q. 31. What is meant by e.m.f. of a cell? What are the values of e.m.f. of Daniell and
of Lechlanche cells?
Ans. It is the potential drop across the terminals of a cell when cell is in open circuit. E.m.f.
of Lechlanche cell is 1.5 V and that of Daniell cell is 1.1 V.
Ans.
Q. 32. Name the electrodes and electrolytes in Lechlanche and Daniell ceils.
Cell
Lechlanche
R
Daniell
’c as R ’0 and the cell is said to be short
Positive electrode
Carbon rod
Copper vessel
Negative electrode
Zinc rod
Zinc rod
O. 33. What do you mean by a cell being in an open circuit?
a current is drawn from the cell, then it is said to be in closed circuit.
Ans. When no current is drawn from a cell, it is said to be in open circuit whereas when
0. 34. What is meant by terminal potential drop of a cell?
Electrolyte
0. 35. On what factors does the e.m.f. of a cell depend?
NH,CI
Dil. H,SO,
Ans. It is the drop of potential across the electrodes of a cell when current is being dran
from it, i.e., the cell is in the closed circuit.
0. 36. Which one is greater -C.m.f. or terminal p.d. and why?
Ans. (1) Nature of plates, (2) Nature of electrolyte, (3) Composition of the electrolyte
(4) It is indepdendent of the separation between the plates, and (5) Area of the plates immersed
in the electrolyte.
Ans. The e.m.f. is greater as it is the maximum value of p.d. which can exist across u
terminals of a cell and this happens when no current is drawn from the cell. When the cell sen
out current, a part of the e.m.f. 1s lost in overcoming internal resistance of the cell and the p
at its terminals falls below its maximum value. Hence terminal p.d. is less than the e.m.i. oI
cell.
Ans. These cells act as storehouses for electricity. Current can be drawn from them wheneve.
desired. After being discharged, they can be recharged. Since they store (or accumulate
electricity, they are known as storage cells or accumulators.
Q. 27. What is neant by internal resistance of a cell?
Ans. The resistance offered by the electrolyte of a cell to the flow of electricity is calledl
internal resistance of the cell. For a normal working cell, its value is about 1 to 2 ohm.
Q. 28. Is storage cell or accumulator, a primary cell or secondary cell?
Ans. It is a secondary cell.
Q. 29. Which one -a primary cell or a storage cell -has smaller internal resistance?
Ans. Storage cells have smaller internal resistance (about 0.1 2 in comparison to about
2 2 for primary cells.)
Q. 30. What do you understand by the ternm, "short-circuiting of a cell"?
Ans. When the two poles of a cell are connected by a wire of negligible resistance, then a
large current is drawn from the cell because I
circuited.
Q. 31. What is meant by e.m.f. of a cell? What are the values of e.m.f. of Daniell and
of Lechlanche cells?
Ans. It is the potential drop across the terminals of a cell when cell is in open circuit. E.m.f.
of Lechlanche cell is 1.5 V and that of Daniell cell is 1.1 V.
Ans.
Q. 32. Name the electrodes and electrolytes in Lechlanche and Daniell ceils.
Cell
Lechlanche
R
Daniell
’c as R ’0 and the cell is said to be short
Positive electrode
Carbon rod
Copper vessel
Negative electrode
Zinc rod
Zinc rod
O. 33. What do you mean by a cell being in an open circuit?
a current is drawn from the cell, then it is said to be in closed circuit.
Ans. When no current is drawn from a cell, it is said to be in open circuit whereas when
0. 34. What is meant by terminal potential drop of a cell?
Electrolyte
0. 35. On what factors does the e.m.f. of a cell depend?
NH,CI
Dil. H,SO,
Ans. It is the drop of potential across the electrodes of a cell when current is being dran
from it, i.e., the cell is in the closed circuit.
0. 36. Which one is greater -C.m.f. or terminal p.d. and why?
Ans. (1) Nature of plates, (2) Nature of electrolyte, (3) Composition of the electrolyte
(4) It is indepdendent of the separation between the plates, and (5) Area of the plates immersed
in the electrolyte.
Ans. The e.m.f. is greater as it is the maximum value of p.d. which can exist across u
terminals of a cell and this happens when no current is drawn from the cell. When the cell sen
out current, a part of the e.m.f. 1s lost in overcoming internal resistance of the cell and the p
at its terminals falls below its maximum value. Hence terminal p.d. is less than the e.m.i. oI
cell.
Q. 37. What is a standard cell? Name one such cell?
Ans. A cell whose e.m.f. remains constant with variations of time as well as temperature,
is called a standard cell. Mercury-Cadmiun cell is a standard cell with e.m.f. = 1.0183 volt at
20°C.
Q. 38. What are the defects of ordinary cells?
Ans. There are two defects, viz. :
() Local action, and (ii) Polarisation.
Q. 39. What is local action and how is it remedied?
Ans. This defect arises due to the use of commercial zinc in making the cathode. Such a
material is usually impure and leads to the formation of local cells on the zinc rod. It is remedied
by amalgamating the zinc rod with mercury.
Q. 40. What is polarisation and how is it remedied?
Ans. This defect arises due to the formation of a layer of hydrogen on the copper plate. This
develops a back e.m.f. and as such e.m.f. falls quickly. It is remedied by using a suitable
depolariser which converts hydrogen into water before it reaches the copper plate. Use of MnO,
does this job in a Lechlanche cell.
Q. 41. Name the depolariser for (i) Lechlanche, and (ii) Daniell cells.
Ans. In Lechlanche cell, MnO, is used as depolariser. In Daniell cell CuSO, is used as
depolariser.
Q. 42. Name the cells used for getting (i) large current, (ii) constant current, and
(iii) intermittent current.
Ans. (i) Storage cell or accumulator, (ii) Daniell cell, and (iii) Lechlanche cell.
Q. 43. Why is it not possible for a Lechlanche cell to give a constant current?
Ans. It is because the hydrogen produced does not get depolarised by MnO, at the same rate
at which it is being produced during the chemical reaction.
Q. 44. What is a galvanometer?
Ans. Galyanometer is an instrument which is used to detect the presence of a feeble current
in a circuit.
Q. 45. What is an international ohm?
Ans. It is the resistance of a column of mercury of area of cross-section 1 mm', length
106.300 cm and mass 14.452l gram at 0°C.
Q. 46. What is a shunt?
Ans. A low resistance when connected in parallel to a galvanometer is called a shunt. It is
generally used for converting a galvanometer into an ammeter.
0. 47. What is the law of resistances in series?
Ans When more than one resistors are connected in series, their combined resistance is
equal to the sum of their individual resistances. Symbolically,
R= R, + R, + R, +
0. 48. What is the law of resistances in parallel?
Ane When a number of resistors are connected in parallel, the reciprocal of the resistance
of the combination is equal to the sum of the reciprocals of their individual resistances. Symbolically,
Ans. A cell whose e.m.f. remains constant with variations of time as well as temperature,
is called a standard cell. Mercury-Cadmiun cell is a standard cell with e.m.f. = 1.0183 volt at
20°C.
Q. 38. What are the defects of ordinary cells?
Ans. There are two defects, viz. :
() Local action, and (ii) Polarisation.
Q. 39. What is local action and how is it remedied?
Ans. This defect arises due to the use of commercial zinc in making the cathode. Such a
material is usually impure and leads to the formation of local cells on the zinc rod. It is remedied
by amalgamating the zinc rod with mercury.
Q. 40. What is polarisation and how is it remedied?
Ans. This defect arises due to the formation of a layer of hydrogen on the copper plate. This
develops a back e.m.f. and as such e.m.f. falls quickly. It is remedied by using a suitable
depolariser which converts hydrogen into water before it reaches the copper plate. Use of MnO,
does this job in a Lechlanche cell.
Q. 41. Name the depolariser for (i) Lechlanche, and (ii) Daniell cells.
Ans. In Lechlanche cell, MnO, is used as depolariser. In Daniell cell CuSO, is used as
depolariser.
Q. 42. Name the cells used for getting (i) large current, (ii) constant current, and
(iii) intermittent current.
Ans. (i) Storage cell or accumulator, (ii) Daniell cell, and (iii) Lechlanche cell.
Q. 43. Why is it not possible for a Lechlanche cell to give a constant current?
Ans. It is because the hydrogen produced does not get depolarised by MnO, at the same rate
at which it is being produced during the chemical reaction.
Q. 44. What is a galvanometer?
Ans. Galyanometer is an instrument which is used to detect the presence of a feeble current
in a circuit.
Q. 45. What is an international ohm?
Ans. It is the resistance of a column of mercury of area of cross-section 1 mm', length
106.300 cm and mass 14.452l gram at 0°C.
Q. 46. What is a shunt?
Ans. A low resistance when connected in parallel to a galvanometer is called a shunt. It is
generally used for converting a galvanometer into an ammeter.
0. 47. What is the law of resistances in series?
Ans When more than one resistors are connected in series, their combined resistance is
equal to the sum of their individual resistances. Symbolically,
R= R, + R, + R, +
0. 48. What is the law of resistances in parallel?
Ane When a number of resistors are connected in parallel, the reciprocal of the resistance
of the combination is equal to the sum of the reciprocals of their individual resistances. Symbolically,
in series.
1
Rp
Q. 49. What is an ammeter? Why is it always connccted in series in a circuit?
Ans. It is essentially a shunted moving coil galvanometer. It has a very low resistance and
measures the current through a circuit without modifying its magnitude only when it is connected
1,1,1
R R, R,
Q. 50. What is a voltmeter? Why is it always connected in parallel in a circuit?
Ans, It is a moving coil galvanometer with a high resistance in series. It measures potential
drop across two points without changing its magnitude when it is connected in parallel.
Q. 51. What should be the resistance of an ideal (i) voltmeter, and (ii) ammeter?
Ans. (i) Ideal voltmeter : Infinity, (ii) Ideal ammeter : Zero.
Q 52. How will you convert a moving coil galvanometer into (i) an ammeter,
(i) a voltmeter?
Ans. (i) By connecting a low resistance of suitable value ie., a shunt across the terminals
of the galvanometer. (ii) By connecting a high resistance in series with the galvanometer.
Q. 53. Can you measure e.m.f. of a cell with a voltmeter?
Ans. No, because it requires some curent from the cell for its reading.
Q. 54. Four resistors of 0.1, 1, 10 and 100 S2 resistances are connected in parallel. Give
the approximate value of the combined resistance without making caleulations.
Ans. It is less than the least resistance, ie., less than 0.1 2. Its actual value is 0.09 2.
Q. 55. What are milli-ammeter and milli-voltmeter?
Ans. Milli stands for 103, therefore milli-ammeter is an instrument used for measuring
currents of the order of 10 amperes, whereas milli-voltmeter is an instrument used for measuring
voltages of the order of 10 volts.
Q. 56. What is meant by a current of one micro-ampere and a p.d. of 1 micro-volt?
Ans. One micro-ampere, or 1 u A = 10 A,
One micro-volt, or, 1 u V= 106 V
Q. 57. What is the effect of tenmperature on the resistance of a conductor?
Ans The resistance of a conductor increases with the rise of temperature.
Q. 58. Name some substances whose resistance decreases with the rise of temperature.
Ans. Resistance of carbon and semiconductorS, germaniunm and silicon decreases with rise
of temperature. Variation is more or less exponential.
Q. 59. What is the basic difference between a conductor and an insulator? Name sone
conductors and insulators.
Ans. Presence of free electrons is responsible for the electrical conductivity of a nmaterial.
() Conductors. In conductors, a large number of free electrons are available for electrical
conduction. All metals are good conductors, e.g., copper and silver.
1014 2 m.
() Insulators. The substance in which no free electrons are available to conduct electricity.
such as mica, wOod, ebonite and rubber.
Q. 60. What is the order of magnitude of resistivity of conductors and insulators?
Ans. Resistivity of conductors is of the order of 10-% SQ m and that of insulators is
1
Rp
Q. 49. What is an ammeter? Why is it always connccted in series in a circuit?
Ans. It is essentially a shunted moving coil galvanometer. It has a very low resistance and
measures the current through a circuit without modifying its magnitude only when it is connected
1,1,1
R R, R,
Q. 50. What is a voltmeter? Why is it always connected in parallel in a circuit?
Ans, It is a moving coil galvanometer with a high resistance in series. It measures potential
drop across two points without changing its magnitude when it is connected in parallel.
Q. 51. What should be the resistance of an ideal (i) voltmeter, and (ii) ammeter?
Ans. (i) Ideal voltmeter : Infinity, (ii) Ideal ammeter : Zero.
Q 52. How will you convert a moving coil galvanometer into (i) an ammeter,
(i) a voltmeter?
Ans. (i) By connecting a low resistance of suitable value ie., a shunt across the terminals
of the galvanometer. (ii) By connecting a high resistance in series with the galvanometer.
Q. 53. Can you measure e.m.f. of a cell with a voltmeter?
Ans. No, because it requires some curent from the cell for its reading.
Q. 54. Four resistors of 0.1, 1, 10 and 100 S2 resistances are connected in parallel. Give
the approximate value of the combined resistance without making caleulations.
Ans. It is less than the least resistance, ie., less than 0.1 2. Its actual value is 0.09 2.
Q. 55. What are milli-ammeter and milli-voltmeter?
Ans. Milli stands for 103, therefore milli-ammeter is an instrument used for measuring
currents of the order of 10 amperes, whereas milli-voltmeter is an instrument used for measuring
voltages of the order of 10 volts.
Q. 56. What is meant by a current of one micro-ampere and a p.d. of 1 micro-volt?
Ans. One micro-ampere, or 1 u A = 10 A,
One micro-volt, or, 1 u V= 106 V
Q. 57. What is the effect of tenmperature on the resistance of a conductor?
Ans The resistance of a conductor increases with the rise of temperature.
Q. 58. Name some substances whose resistance decreases with the rise of temperature.
Ans. Resistance of carbon and semiconductorS, germaniunm and silicon decreases with rise
of temperature. Variation is more or less exponential.
Q. 59. What is the basic difference between a conductor and an insulator? Name sone
conductors and insulators.
Ans. Presence of free electrons is responsible for the electrical conductivity of a nmaterial.
() Conductors. In conductors, a large number of free electrons are available for electrical
conduction. All metals are good conductors, e.g., copper and silver.
1014 2 m.
() Insulators. The substance in which no free electrons are available to conduct electricity.
such as mica, wOod, ebonite and rubber.
Q. 60. What is the order of magnitude of resistivity of conductors and insulators?
Ans. Resistivity of conductors is of the order of 10-% SQ m and that of insulators is
Q. 61. What happens to the resistance of a conductor if its length is doubled without
changing its cross-sectional area?
Ans. (0) Resistance of a conductor is directly proportional to its length i.e., R ox l.
(ii) R o where a is crosS-sectional area.
a
Thus, when l is doubled, R is also doubled.
Q. 62. What happens to the resistance of a conductor if its area of cross-section is
reduced to half and its length is doubled?
Aus. Its resistance becomes 4 times the original value.
Q. 63. What is the difference between micro-ohm resistance and mega-ohm resistance?
Ans. 1 micr0-ohm = 10-6 Q
1 mega-ohm = 106 Q
Q. 64. What is meant by specific resistance?
Ans. Specific resistance of a material is the resistance to the flow of current offered by the
conductor of the given material having length one metre and area of cross-section one square
metre. In the relation R =p
Q. 65. What is the S.I. unit of specific resistance?
Ans. Ohm-metre (2 m).
, when l= l m and a = 1 m', then R = p.
0. 66. If the length of a conductor is doubled and area of cross-section reduced to half
as done in Q. 62, what happens to specific resistance?
Ans. It remains the same. Specific resistance is the property of the material of the conductor
and it is independent of the dimensions of the conductor.
Q. 67. What is resistivity?
Ans. It is another name for specific resistance.
Q. 68. What is electrical conductivity of a material?
Ans. It is the reciprocal of the specific resistance.
Q. 69. What is the S.I. unit of electrical conductivity?
Ans. It is, siemen m' or S m-l.
0. 70. Siemen is the S.I. Unit for which physical quantity?
Ans. Conductance. 1 siemen = 1 (ohm) or mho.
Q. 71. What is a resistance box?
Ans. A resistance box consists of a large
number of standard resistances of different values
(1, 2, 2, 5, 10, 20...) ohms fixed in a box. The upper
ends of these resistances are connected to brass studs
arranged in such a way that these resistances can be
joined together in series by removing plugs from the
gaps between the studs as shown in Fig. 3.5.
Fig.
3.5.Connections of resistance coils
inside a
resistance box.
changing its cross-sectional area?
Ans. (0) Resistance of a conductor is directly proportional to its length i.e., R ox l.
(ii) R o where a is crosS-sectional area.
a
Thus, when l is doubled, R is also doubled.
Q. 62. What happens to the resistance of a conductor if its area of cross-section is
reduced to half and its length is doubled?
Aus. Its resistance becomes 4 times the original value.
Q. 63. What is the difference between micro-ohm resistance and mega-ohm resistance?
Ans. 1 micr0-ohm = 10-6 Q
1 mega-ohm = 106 Q
Q. 64. What is meant by specific resistance?
Ans. Specific resistance of a material is the resistance to the flow of current offered by the
conductor of the given material having length one metre and area of cross-section one square
metre. In the relation R =p
Q. 65. What is the S.I. unit of specific resistance?
Ans. Ohm-metre (2 m).
, when l= l m and a = 1 m', then R = p.
0. 66. If the length of a conductor is doubled and area of cross-section reduced to half
as done in Q. 62, what happens to specific resistance?
Ans. It remains the same. Specific resistance is the property of the material of the conductor
and it is independent of the dimensions of the conductor.
Q. 67. What is resistivity?
Ans. It is another name for specific resistance.
Q. 68. What is electrical conductivity of a material?
Ans. It is the reciprocal of the specific resistance.
Q. 69. What is the S.I. unit of electrical conductivity?
Ans. It is, siemen m' or S m-l.
0. 70. Siemen is the S.I. Unit for which physical quantity?
Ans. Conductance. 1 siemen = 1 (ohm) or mho.
Q. 71. What is a resistance box?
Ans. A resistance box consists of a large
number of standard resistances of different values
(1, 2, 2, 5, 10, 20...) ohms fixed in a box. The upper
ends of these resistances are connected to brass studs
arranged in such a way that these resistances can be
joined together in series by removing plugs from the
gaps between the studs as shown in Fig. 3.5.
Fig.
3.5.Connections of resistance coils
inside a
resistance box.
Q. 72. How are the resistances of different magnitude designed and fixed in th.
resistance box?
Ans. The insulated resistance wire of required length is taken. It is doubled over itself anda
then wound over a bobbin of wood or porcelain. The two free ends are then connected to bras
studs.
Q. 73. WVhy is the wire doubled over itsclf before it is wound over a bobbin?
Ans. This is done so as to avoid induced current effects.
Q. 74. How are the values of different resistances in the resistance box controlled and
manipulated?
Ans. Lengths of all resistance coils are more or less the same. The different values are
achieved by controling their thicknesses. For high resistances, wires of high resistivity material
are used.
Q. 75. What is the approximate thickness and length of the wire for infinite resistance
in the resistance box?
Ans. There is no resistance wire below the infinity plug. When the infinity plug is taken out,
the two studs remain unconnected and no current flows across. So there is infinite resistance.
Q. 76.
Which material is suitable for the construction of standard resistances for
resistance boxes etc.?
Ans. The two common materials used for this purpose are
(i) Constantan, and (ii) Manganin.
Q. 77. Why is copper not used?
Ans. Actually the material suitable for the construction of standard resistances should have
() high specific resistance, and
(ii) low temperature coefficient of resistance, i.e., its resistance should not change
appreciably with the rise of temperature. Manganin and constantan satisfy these
two conditions, whereas copper does not.
0. 78. Why is a material named Eureka also sometimes used for this purpose?
Ans. Eureka is nothing but another name of constantan.
Q. 79. What is the composition of manganin and constantan?
Ans. Manganin (Cu 83%, Mn 13% and Ni 4%); Constantan (Cu 60%, Ni 40%).
Q. 80. For making a standard resistance, why should the material possess high speeie
resistance?
Ans. So that even a small length of the wire is enough.
Q. 81. What is a rheostat?
Ans. It is a device to increase or decrease the curent strength in a circuit by introduc1ng
variable resistance in the circuit. A rheostat is effectively a variable resistor.
Q. 82. Windings of the rheostat wire are quite close to each other. Don't they get sl
circuited?
Ans. The wire has a coating of insulating oxide over it. This insulates the windings
each other.
resistance box?
Ans. The insulated resistance wire of required length is taken. It is doubled over itself anda
then wound over a bobbin of wood or porcelain. The two free ends are then connected to bras
studs.
Q. 73. WVhy is the wire doubled over itsclf before it is wound over a bobbin?
Ans. This is done so as to avoid induced current effects.
Q. 74. How are the values of different resistances in the resistance box controlled and
manipulated?
Ans. Lengths of all resistance coils are more or less the same. The different values are
achieved by controling their thicknesses. For high resistances, wires of high resistivity material
are used.
Q. 75. What is the approximate thickness and length of the wire for infinite resistance
in the resistance box?
Ans. There is no resistance wire below the infinity plug. When the infinity plug is taken out,
the two studs remain unconnected and no current flows across. So there is infinite resistance.
Q. 76.
Which material is suitable for the construction of standard resistances for
resistance boxes etc.?
Ans. The two common materials used for this purpose are
(i) Constantan, and (ii) Manganin.
Q. 77. Why is copper not used?
Ans. Actually the material suitable for the construction of standard resistances should have
() high specific resistance, and
(ii) low temperature coefficient of resistance, i.e., its resistance should not change
appreciably with the rise of temperature. Manganin and constantan satisfy these
two conditions, whereas copper does not.
0. 78. Why is a material named Eureka also sometimes used for this purpose?
Ans. Eureka is nothing but another name of constantan.
Q. 79. What is the composition of manganin and constantan?
Ans. Manganin (Cu 83%, Mn 13% and Ni 4%); Constantan (Cu 60%, Ni 40%).
Q. 80. For making a standard resistance, why should the material possess high speeie
resistance?
Ans. So that even a small length of the wire is enough.
Q. 81. What is a rheostat?
Ans. It is a device to increase or decrease the curent strength in a circuit by introduc1ng
variable resistance in the circuit. A rheostat is effectively a variable resistor.
Q. 82. Windings of the rheostat wire are quite close to each other. Don't they get sl
circuited?
Ans. The wire has a coating of insulating oxide over it. This insulates the windings
each other.
0. 83. If the windings are insulated then how does the slider make a contact with the
wire when the rheostat is in use?
Q.
Ans. The insulation is only above the slider. Just below it where the slider is to make the
contact the insulating oxide is missing throughout the entire length of the rheostat.
Q. 84. What material is chosen for the rheostat wire and why?
Ans. It is constantan. Because its temperature coefficient of increase of resistance is low.
Q. 85. What is potential divider arrangement?
Ans. It is an arrangement which provides a variable p.d. In this arrangement, a cell is
connected across the two ends of the rheostat wire. The e.m.f. of the cell is distributed along the
whole length of this wire. The circuit (across which a variable p.d. is required) is connected
between the terminal of the slider and one end of the rheostat wire. Thus by shifting the slider
to various positions, one gets various values of p.d. across the circuit. Thus it provides a means
of applying a desired low value of the p.d. in a circuit.
0. 86. Name the material of the tube over which the constantan wire is wound for
making a rheostat.
Ans. Any non-conducting material is OK. Generally porcelain is used.
wire when the rheostat is in use?
Q.
Ans. The insulation is only above the slider. Just below it where the slider is to make the
contact the insulating oxide is missing throughout the entire length of the rheostat.
Q. 84. What material is chosen for the rheostat wire and why?
Ans. It is constantan. Because its temperature coefficient of increase of resistance is low.
Q. 85. What is potential divider arrangement?
Ans. It is an arrangement which provides a variable p.d. In this arrangement, a cell is
connected across the two ends of the rheostat wire. The e.m.f. of the cell is distributed along the
whole length of this wire. The circuit (across which a variable p.d. is required) is connected
between the terminal of the slider and one end of the rheostat wire. Thus by shifting the slider
to various positions, one gets various values of p.d. across the circuit. Thus it provides a means
of applying a desired low value of the p.d. in a circuit.
0. 86. Name the material of the tube over which the constantan wire is wound for
making a rheostat.
Ans. Any non-conducting material is OK. Generally porcelain is used.
VIVA VOCE / SELF ASSESSMENT
(Experiment Nos. A2 and A3 : Metre Bridge)
Q. 1. What is a metre bridge?
Ans. It is an instrument used for determining the unknown value of the given resistance.
0. 2. What is the alternative term used for metre bridge?
Ans. Slide wire bridge.
Q. 3. Name the principle on which metre bridge is based.
Ans. Wheatstone's bridge.
Q. 4. When is the Wheatstone's bridge said to be most sensitive?
Ans. When resistances in all the four arms P, Q, R and S are of nearly the same order of
magnitude.
Q. 5. When is the bridge balanced?
Ans. The potential of the common end joining the resistors P and is the sanme as that of
the common end joining the resistors R and S.
0. 6. What is the relation between P, 0, R and S when the bridge is balanced?
(Experiment Nos. A2 and A3 : Metre Bridge)
Q. 1. What is a metre bridge?
Ans. It is an instrument used for determining the unknown value of the given resistance.
0. 2. What is the alternative term used for metre bridge?
Ans. Slide wire bridge.
Q. 3. Name the principle on which metre bridge is based.
Ans. Wheatstone's bridge.
Q. 4. When is the Wheatstone's bridge said to be most sensitive?
Ans. When resistances in all the four arms P, Q, R and S are of nearly the same order of
magnitude.
Q. 5. When is the bridge balanced?
Ans. The potential of the common end joining the resistors P and is the sanme as that of
the common end joining the resistors R and S.
0. 6. What is the relation between P, 0, R and S when the bridge is balanced?
Ans.
PR
Q S
Q.7. What will happen in Expt. No. 2 (Circuit Fig. 3.8) if the positions of the cell
galvanometer are interchanged?
Ans. The balance point is not affected on interchanging the positions of the cell and
galvanometer as such arms BD and AC are called conjugate arms.
Q. 8. WVhy should the moving contact of jockey not be pressed too hard or scratchd
along the wire?
Ans. If done so, it may damage the uniformity of the bridge wire.
Q. 9. Why should the current be passed only while taking an observation?
Ans. A continuous flow of current would cause heating and hence an increase in the yalues of resistances.
Q. 10. Why is the metre bridge suitable for resistances of moderate values only?
Ans. The bridge becomes insensitive for too high or too low values and the readings become undependable. When determining low resistances, the end resistances of the metre bridge wire and resistance of connecting wires contribute towards the major part of error.
Q. 11. Why should the bridge wire be of uniform cross-section throughout?
Ans. If it is not so, the resistance per unit length of the wire would vary from position to
position and the relation at balance condition, i.e.,
P R
0. 12. For
determination of resistance of a coil, which of two methods is better -Ohm's Law method or metre bridge method?
Ans. Obviously, the bridge method is better because it is the null point method which is superior to all other methods.
would no longer be valid.
(100 -7)
Q. 13. Why should the battery key be pressed before the
galvanometer key? Ans. This is done to avoid any
electromagnetic induction.
0. 14. Sometimes it is advisable to shunt the
galvanometer while trying for a balance
point. Why?
Ans, During the first trial of the balance, the current that passes through the galvanomete
may be large and may damage it. "Therefore, to protect the
galvanometer, a shunt is used. When
the balance point reaches near the null position, shunt is removed and exact null position S
located.
Ans. Constantan.
0. 15. What is the material of the wire of metre bridge?
Q. 16. Why is
constantan used for the bridge wire?
Ans. The
coefficient increase of
resistance with rise of
temperature a is very
SImall for this material; also its resistivity is high.
The respective values for
constantan are 0.4 x 10 (°C)-! and 49 x 10-8 ohm-M corresponding values for copper are 43 x 10 (°C) and 1.69 x 108 ohm-m.
PR
Q S
Q.7. What will happen in Expt. No. 2 (Circuit Fig. 3.8) if the positions of the cell
galvanometer are interchanged?
Ans. The balance point is not affected on interchanging the positions of the cell and
galvanometer as such arms BD and AC are called conjugate arms.
Q. 8. WVhy should the moving contact of jockey not be pressed too hard or scratchd
along the wire?
Ans. If done so, it may damage the uniformity of the bridge wire.
Q. 9. Why should the current be passed only while taking an observation?
Ans. A continuous flow of current would cause heating and hence an increase in the yalues of resistances.
Q. 10. Why is the metre bridge suitable for resistances of moderate values only?
Ans. The bridge becomes insensitive for too high or too low values and the readings become undependable. When determining low resistances, the end resistances of the metre bridge wire and resistance of connecting wires contribute towards the major part of error.
Q. 11. Why should the bridge wire be of uniform cross-section throughout?
Ans. If it is not so, the resistance per unit length of the wire would vary from position to
position and the relation at balance condition, i.e.,
P R
0. 12. For
determination of resistance of a coil, which of two methods is better -Ohm's Law method or metre bridge method?
Ans. Obviously, the bridge method is better because it is the null point method which is superior to all other methods.
would no longer be valid.
(100 -7)
Q. 13. Why should the battery key be pressed before the
galvanometer key? Ans. This is done to avoid any
electromagnetic induction.
0. 14. Sometimes it is advisable to shunt the
galvanometer while trying for a balance
point. Why?
Ans, During the first trial of the balance, the current that passes through the galvanomete
may be large and may damage it. "Therefore, to protect the
galvanometer, a shunt is used. When
the balance point reaches near the null position, shunt is removed and exact null position S
located.
Ans. Constantan.
0. 15. What is the material of the wire of metre bridge?
Q. 16. Why is
constantan used for the bridge wire?
Ans. The
coefficient increase of
resistance with rise of
temperature a is very
SImall for this material; also its resistivity is high.
The respective values for
constantan are 0.4 x 10 (°C)-! and 49 x 10-8 ohm-M corresponding values for copper are 43 x 10 (°C) and 1.69 x 108 ohm-m.
0. 17. Why is it necessary to obtain the balance point in the middle of the bridge wire?
Explain in detail.
Ans. The sensitivity of the Wheatstone's bridge is maximum when the resistance of all its
four arms are nearly of the same order. For this, the null point or the balance point should be near
the middle of the wire. It can be shown (proof given below) that when the balance point is in the
middle of the wire, a small error in determining its position introduces the least error in the value
of the unknown resistance, i.e., the accuracy of the result is the highest.
Proof. Suppose dl is a small error introduced in the determination of l and the corresponding
error produced in X is dX.
In the balanced condition for the bridge, we have
Or
But.
X=
X =
Or
On differentiating the above, we get
X
(100-)
dX
(L-I)
log X = Log (L -) -log I + log
-dl dl
L-|
0. where L stands for the total length of the wire
+0=
The magnitude of (dX/X) is minimum when (L -) in the denominator is maximum.
l+ (L-) = L, constant
l=L-I
-Ldl
Hence, the product (L -) is maximum when
I(L-I)
0. 19. Define an international ohm.
l= (U2), i.e., the balance point is in the middle of L.
Q. 18. Define resistivity, or specific resistance.
Ans. It is the resistance of a unit cube of a material. R=p.
(: dÍ =0 and dL = 0)
A
A =1; p= R. So it is the resistance across any two opposite faces of a cube of edge of unit length.
1 International ohm = 1.00052 ohm.
-; suchthat forl= land
Ans. International ohm is used as a standard for expressing a unit resistance. It is defined
as the resistance of a column of mercury of uniform cross-section 1 mm², weighing 14.4521 g
and measuring 106.300 cm at 0°C. The international ohm is slightly bigger than true ohm.
0. 20. Can we measure a resistance of the order of 0.l60 2 using a Wheatstone's
bridge? Support your answer with reasoning?
Ans. No, the resistance of the connecting wires and at the junctions of metre bridge and the
other terminals is itself of the order of the resistance to be measured. It would create uncertainty
in the measurement of low resistance.
0. 21. How can you measure the value of such ow resistance of the order of 0.2 ohm?
Ans. (i) The resistance of low value (0.2 ohm) can be determined by using a metre bridee
of wire of resistance 0.5 2 and homogeneous composition and uniform area of cross-section
Explain in detail.
Ans. The sensitivity of the Wheatstone's bridge is maximum when the resistance of all its
four arms are nearly of the same order. For this, the null point or the balance point should be near
the middle of the wire. It can be shown (proof given below) that when the balance point is in the
middle of the wire, a small error in determining its position introduces the least error in the value
of the unknown resistance, i.e., the accuracy of the result is the highest.
Proof. Suppose dl is a small error introduced in the determination of l and the corresponding
error produced in X is dX.
In the balanced condition for the bridge, we have
Or
But.
X=
X =
Or
On differentiating the above, we get
X
(100-)
dX
(L-I)
log X = Log (L -) -log I + log
-dl dl
L-|
0. where L stands for the total length of the wire
+0=
The magnitude of (dX/X) is minimum when (L -) in the denominator is maximum.
l+ (L-) = L, constant
l=L-I
-Ldl
Hence, the product (L -) is maximum when
I(L-I)
0. 19. Define an international ohm.
l= (U2), i.e., the balance point is in the middle of L.
Q. 18. Define resistivity, or specific resistance.
Ans. It is the resistance of a unit cube of a material. R=p.
(: dÍ =0 and dL = 0)
A
A =1; p= R. So it is the resistance across any two opposite faces of a cube of edge of unit length.
1 International ohm = 1.00052 ohm.
-; suchthat forl= land
Ans. International ohm is used as a standard for expressing a unit resistance. It is defined
as the resistance of a column of mercury of uniform cross-section 1 mm², weighing 14.4521 g
and measuring 106.300 cm at 0°C. The international ohm is slightly bigger than true ohm.
0. 20. Can we measure a resistance of the order of 0.l60 2 using a Wheatstone's
bridge? Support your answer with reasoning?
Ans. No, the resistance of the connecting wires and at the junctions of metre bridge and the
other terminals is itself of the order of the resistance to be measured. It would create uncertainty
in the measurement of low resistance.
0. 21. How can you measure the value of such ow resistance of the order of 0.2 ohm?
Ans. (i) The resistance of low value (0.2 ohm) can be determined by using a metre bridee
of wire of resistance 0.5 2 and homogeneous composition and uniform area of cross-section
throughout, making the second arm resistance of magnitude 0.2 ohm using standard
resistance,
The readings are taken by interchanging the arms and calculating the mean.
(ii) Using thick copper wires for connections, a voltmeter of nearly infinite.
resistance and a sensitive ammeter of nearly zero resistance can be used to measure the potential
drop across the given resistance and the current through it. Applying Ohm's law, the value of resistance can
determined.
Q. 22. What is S.I. unit of specific resistance?
Ans. Ohm-m.
Q. 23. State the relation of p with R, I and d.
Ans. p = R. A/l = R
Q. 24. What is the shape of the graph of R vs. I for a wire of uniform cross-section and
of a given homogeneous composition material?
Ans. It would be a straight line through the origin.
Q. 25. The radius of a bridge wire increases uniformly, how wil its resistance vary as
you move along the length?
Ans. Resistance will not increase linearly with length. As area of cross-section increases, it
will cause decrease in resistance. Therefore, increase will not be uniform rather as we move along
the wire, rate of increase of resistance will be less.
0. 26. What is the general formula for combination of n resistances R, R, Ry ..., R, to be connected in series?
41
Ans. Combined resistance is given by R, = 2K= R, + R, + R, +
Ans.
R,
0. 27. What is the general formula for combination of n resistances R,, R,, R9 .., R,
to be connected in parallel? EM
Ans. Current.
i=l R
1,1
R R,
t ... t
i=1
0. 28. If three resistances of different values are connected in series in a circuit, which
would be the same in both --current or the potential difference across their ends?
1
R
0. 29. Resistances of 10 2 and 15 2 are connected across a source of e.m.f. 12 Y
(r= 0). What current would be drawn from the battery?
1
Ans. When in series : R. = 25 S2 and I = 0.48 A, when in parallel : R = 6 2 and 1, = 2.0 A.
P.
Q. 30. Two appliances of 100 W and 200 W are connected across mains of 250 V. Find the equivalent power consumption by them if connected in (a) parallel, and (b) series.
Ans. In parallel, P, = P, + P, = 100 + 200 = 300 W
......... t R.
1
In series, the current through them will decrease, thereby decreasing the power consumpuo
P P >P,=
P+P,
= 66.67 W
resistance,
The readings are taken by interchanging the arms and calculating the mean.
(ii) Using thick copper wires for connections, a voltmeter of nearly infinite.
resistance and a sensitive ammeter of nearly zero resistance can be used to measure the potential
drop across the given resistance and the current through it. Applying Ohm's law, the value of resistance can
determined.
Q. 22. What is S.I. unit of specific resistance?
Ans. Ohm-m.
Q. 23. State the relation of p with R, I and d.
Ans. p = R. A/l = R
Q. 24. What is the shape of the graph of R vs. I for a wire of uniform cross-section and
of a given homogeneous composition material?
Ans. It would be a straight line through the origin.
Q. 25. The radius of a bridge wire increases uniformly, how wil its resistance vary as
you move along the length?
Ans. Resistance will not increase linearly with length. As area of cross-section increases, it
will cause decrease in resistance. Therefore, increase will not be uniform rather as we move along
the wire, rate of increase of resistance will be less.
0. 26. What is the general formula for combination of n resistances R, R, Ry ..., R, to be connected in series?
41
Ans. Combined resistance is given by R, = 2K= R, + R, + R, +
Ans.
R,
0. 27. What is the general formula for combination of n resistances R,, R,, R9 .., R,
to be connected in parallel? EM
Ans. Current.
i=l R
1,1
R R,
t ... t
i=1
0. 28. If three resistances of different values are connected in series in a circuit, which
would be the same in both --current or the potential difference across their ends?
1
R
0. 29. Resistances of 10 2 and 15 2 are connected across a source of e.m.f. 12 Y
(r= 0). What current would be drawn from the battery?
1
Ans. When in series : R. = 25 S2 and I = 0.48 A, when in parallel : R = 6 2 and 1, = 2.0 A.
P.
Q. 30. Two appliances of 100 W and 200 W are connected across mains of 250 V. Find the equivalent power consumption by them if connected in (a) parallel, and (b) series.
Ans. In parallel, P, = P, + P, = 100 + 200 = 300 W
......... t R.
1
In series, the current through them will decrease, thereby decreasing the power consumpuo
P P >P,=
P+P,
= 66.67 W
0. 31. In series connection in Q. No. 30, how is it that P. =
Ans. When connected in series, the curent is the same through both of them and resistances
get added up.
Since,
Therefore, P =
R
is
R,
P=
whereas in parallel, V is the same for both.
1
=
0. 34. What are o and 9?
Ans. 0 =
= P + P,.
R
RÍ-Ro
R, .0
R, R, + R,
y2
v2 y2 P P,
Q. 32. Why are the appliances connected in parallel in domestic and industrial circuits?
Ans. (1) The potential across each appliance would be the same.
(2) Each appliance can be operated independently by using a switch.
(3) If one appliance fuses, the others would continue functioning.
1
0. 33. How does the resistance R of a conductor vary with rise in temperature 0?
Ans. Ra = R, (1 + a ) up to a rise of temperature of about 400°C.
Ans. . is the coefficient of increase of resistance with temperature in S.I. unit (degree)-, whereas is the rise in its temperature.
0. 35. How do you express a in terms of Rg, Ro and 0?
P+P
and its unit is (°C!.
Q. 36. Can you name a material whose resistance decreases with the rise in
temperature?
Ans. Semiconductors (Ge, Si), semiconducting diode etc.
0. 37. How does the resistance of a platinum resistance thermometer vary when it is
heated to a very high temperature?
Ans. R = R, (1 + a + Be), where and B are temperature coefficients of resistance of
platinum.
0. 38. If the same experiment is performed with a metre bridge wire of length 50 cm
in place of 1 metre long, what changes do you expect in the result?
Ans. It will introduce more error in the final result. Permissible eITor in length measurement
Increases, hence percentage error increases.
where Al is the least count of the metre scale which remains the same. When r decreg ges
Ans. When connected in series, the curent is the same through both of them and resistances
get added up.
Since,
Therefore, P =
R
is
R,
P=
whereas in parallel, V is the same for both.
1
=
0. 34. What are o and 9?
Ans. 0 =
= P + P,.
R
RÍ-Ro
R, .0
R, R, + R,
y2
v2 y2 P P,
Q. 32. Why are the appliances connected in parallel in domestic and industrial circuits?
Ans. (1) The potential across each appliance would be the same.
(2) Each appliance can be operated independently by using a switch.
(3) If one appliance fuses, the others would continue functioning.
1
0. 33. How does the resistance R of a conductor vary with rise in temperature 0?
Ans. Ra = R, (1 + a ) up to a rise of temperature of about 400°C.
Ans. . is the coefficient of increase of resistance with temperature in S.I. unit (degree)-, whereas is the rise in its temperature.
0. 35. How do you express a in terms of Rg, Ro and 0?
P+P
and its unit is (°C!.
Q. 36. Can you name a material whose resistance decreases with the rise in
temperature?
Ans. Semiconductors (Ge, Si), semiconducting diode etc.
0. 37. How does the resistance of a platinum resistance thermometer vary when it is
heated to a very high temperature?
Ans. R = R, (1 + a + Be), where and B are temperature coefficients of resistance of
platinum.
0. 38. If the same experiment is performed with a metre bridge wire of length 50 cm
in place of 1 metre long, what changes do you expect in the result?
Ans. It will introduce more error in the final result. Permissible eITor in length measurement
Increases, hence percentage error increases.
where Al is the least count of the metre scale which remains the same. When r decreg ges
0. 1. What is a galvanometer?
VIVA VOCE / SELF ASSESSMENT
(Experiment Nos. A6 and A7:
Ans. lt is an instrument used to detect feeble electric currents.
Conversion of Galvanometer into Ammeter
and Voltmeter)
Q. 2. What type of galvanometer is Weston galvanometer?
Ans. lt is a pivoted type moving coil galvanometer.
Ans.
0. 3. Why is the scale of a galvanometer marked on both sides of zero?
It is so because galvanometer is used to detect null deflection.
Ans.
through it.
Q. 4. What do you understand by resistance of a galvanometer?
It is the resistance offered by the coil of the galvanometer to the flow of current
Q. 5. Which part of the galvanometer offers this resistance?
Q. 7.
Ans. It is the coil of the galvanometer which offers this resistance.
Q. 6. How do you determine this resistance of the galvanometer?
Ans. By half deflection method.
Draw the circuit diagram for this method.
Ans. Refer to and draw the circuit as in Fig. 5.4 on page 95.
Q. 8. Does the value of G always equal S under the half deflection condition?
Ans. No, it is true only when R >> S.
Q. 9. What will happen if R= S?
Ans. In that case G# S but it will be given by the relation
VIVA VOCE / SELF ASSESSMENT
(Experiment Nos. A6 and A7:
Ans. lt is an instrument used to detect feeble electric currents.
Conversion of Galvanometer into Ammeter
and Voltmeter)
Q. 2. What type of galvanometer is Weston galvanometer?
Ans. lt is a pivoted type moving coil galvanometer.
Ans.
0. 3. Why is the scale of a galvanometer marked on both sides of zero?
It is so because galvanometer is used to detect null deflection.
Ans.
through it.
Q. 4. What do you understand by resistance of a galvanometer?
It is the resistance offered by the coil of the galvanometer to the flow of current
Q. 5. Which part of the galvanometer offers this resistance?
Q. 7.
Ans. It is the coil of the galvanometer which offers this resistance.
Q. 6. How do you determine this resistance of the galvanometer?
Ans. By half deflection method.
Draw the circuit diagram for this method.
Ans. Refer to and draw the circuit as in Fig. 5.4 on page 95.
Q. 8. Does the value of G always equal S under the half deflection condition?
Ans. No, it is true only when R >> S.
Q. 9. What will happen if R= S?
Ans. In that case G# S but it will be given by the relation
G=
RS
G = S.
R S
R
S. S
RS
R
Q. 10. For the determination of G of a galvanometer by balf deilection method. w
should we use high value of R, i.e., R >> G ?
Ans. It prevents the galvanometer from damage. Also we can use the relation
Q. 12. What is an ammeter?
Q. 11. Is there any other method for determining G ?
t....which equals S when approaches zero.
Ans. G can also be determined by Kelvin's method.
S
Ans. An ammeter is also a galvanometer with a suitable value of shunt, i.e., low resistanos
connected in parallel to it. It gives the value of the current to be measured directly.
Q. 13. Do you mean that there is no difference between a galvanometer and an ammeter
except for the scale graduations?
Ans. Yes, there is no fundamental difference in its construction. But the resistance of a
ammeter is very low as compared to that of a galvanometer hecause a shunt of low value is
attached in parallel to it.
Q. 14. Why is it necessary for an ammeter to have a low resistance?
Ans. An ammeter is always connected in series with a circuit in which current is to be
measured. Iff it has a high resistance, 1t wi it will alter the value of actual current to be measured.
Q. 15. According to you an ideal ammeter shouid have zero resistance. Isn't it?
Ans. Yes, ideally speaking, a current-measuring device must have a zero resistance but this
cannot be realised in practice, so we try to have it as low as possible.
Q. 16. How do you achieve it practically?
S=
Ans. This is achieved practically by connecting a very small resistance in parallel with the
galvanometer. This small resistance is known as shunt.
Q. 17. VWhy does the resistance of the galvanometer become very low when a shunt is
connected acrOSs its terminals?
Ans. From the law of resistances in parallel, we know that the combined resistance is smaller
than even the smallest individual resistance. Thus the resistance of the shunted galvanometer
becomes less than even the shunt resistance.
lç.G
0. 18. Do you connect a shunt of any arbitrary resistance value to convert a gve
galvanometer into an ammeter?
Ans No, the value of shunt res..ws depends upon the desired range of the ammeter
its value for range I, and full deflection current l, would be
Q. 19. What do you understand by the range of an ammeter?
Ans It is the maximum value of the current which can be safely measured by it.
RS
G = S.
R S
R
S. S
RS
R
Q. 10. For the determination of G of a galvanometer by balf deilection method. w
should we use high value of R, i.e., R >> G ?
Ans. It prevents the galvanometer from damage. Also we can use the relation
Q. 12. What is an ammeter?
Q. 11. Is there any other method for determining G ?
t....which equals S when approaches zero.
Ans. G can also be determined by Kelvin's method.
S
Ans. An ammeter is also a galvanometer with a suitable value of shunt, i.e., low resistanos
connected in parallel to it. It gives the value of the current to be measured directly.
Q. 13. Do you mean that there is no difference between a galvanometer and an ammeter
except for the scale graduations?
Ans. Yes, there is no fundamental difference in its construction. But the resistance of a
ammeter is very low as compared to that of a galvanometer hecause a shunt of low value is
attached in parallel to it.
Q. 14. Why is it necessary for an ammeter to have a low resistance?
Ans. An ammeter is always connected in series with a circuit in which current is to be
measured. Iff it has a high resistance, 1t wi it will alter the value of actual current to be measured.
Q. 15. According to you an ideal ammeter shouid have zero resistance. Isn't it?
Ans. Yes, ideally speaking, a current-measuring device must have a zero resistance but this
cannot be realised in practice, so we try to have it as low as possible.
Q. 16. How do you achieve it practically?
S=
Ans. This is achieved practically by connecting a very small resistance in parallel with the
galvanometer. This small resistance is known as shunt.
Q. 17. VWhy does the resistance of the galvanometer become very low when a shunt is
connected acrOSs its terminals?
Ans. From the law of resistances in parallel, we know that the combined resistance is smaller
than even the smallest individual resistance. Thus the resistance of the shunted galvanometer
becomes less than even the shunt resistance.
lç.G
0. 18. Do you connect a shunt of any arbitrary resistance value to convert a gve
galvanometer into an ammeter?
Ans No, the value of shunt res..ws depends upon the desired range of the ammeter
its value for range I, and full deflection current l, would be
Q. 19. What do you understand by the range of an ammeter?
Ans It is the maximum value of the current which can be safely measured by it.
0. 20. If a given galvanometer is to be converted firstly into an ammeter of rage
1 ampere and then to range of 100 mA, in which case will the value of shunt resistance be
lower?
Ans. The value of the shunt resistance will be lower when 1 ampere current is to be
measured.
Q. 21. Explain, why is it so?
Ans. The value of shunt resistance S to be applied to a galvanometer of resistance G, to
change it into an ammeter of range I, is given by
S=
where I, is the current through the coil of the galvanometer which gives the full scale deflection.
Now from the formula it is clear that higher the value of I, lower will be the value of S.
Q. 22. How do you determine I?
-G= G
Ans. , is given by the product of the figure of merit and the total number of divisions on
either side of the zero of the galvanometer. (le= kN
Q. 23. What do you mean by the figure of merit of a galvanometer?
Ans. Figure of merit of a given galvanometer is the amount of current required to produce
a deflection of one division on the galvanometer scale. Sensitivity is reciprocal of figure of merit.
Q. 24. VWhat is the order of figure of merit of pivoted type moving coil galvano-meters
which you use in the laboratory?
Ans. It is generally of the order of 10 ampere, i.e., (10 uA per division).
Q. 25. Once you know the vaiue of the shunt resistance to be connected, in what form
will you connect it?
Ans. A shunt, in the form of a wire of suitable length and material, is connected across the
galvanometer in parallel with it.
Q. 26. How do you compute the suitabie length of the shunt wire?
Ans. Resistance S of a given wire is given as
S=p
where is its length, a is the area of cross-section and p is its specific resistance. Since a, the
area of cross-section of the wire is (Tr),
Sr
Radius r of the wire can be determined by using a screw gauge, p of the material of the wire
can be obtained from the standard tables. Hence we can get the length l of the wire corresponding
to the required shunt resistance.
Q. 27. Why should a slightly greater length of the wire taken than the calculated
length 1?
Ans, Yes.
Ans. It is done because a small part of the wire near its ends is required for the terminal
Connections, so that the exact calculated length of the wire is left out between the two terminals.
Q. 28. Can you check the accuracy of the converted ammcter?
Q. 29. When the current in the circuit is I, we know that only I; passes through the
galvanonmeter. How does your converted ammeter measure I?
1 ampere and then to range of 100 mA, in which case will the value of shunt resistance be
lower?
Ans. The value of the shunt resistance will be lower when 1 ampere current is to be
measured.
Q. 21. Explain, why is it so?
Ans. The value of shunt resistance S to be applied to a galvanometer of resistance G, to
change it into an ammeter of range I, is given by
S=
where I, is the current through the coil of the galvanometer which gives the full scale deflection.
Now from the formula it is clear that higher the value of I, lower will be the value of S.
Q. 22. How do you determine I?
-G= G
Ans. , is given by the product of the figure of merit and the total number of divisions on
either side of the zero of the galvanometer. (le= kN
Q. 23. What do you mean by the figure of merit of a galvanometer?
Ans. Figure of merit of a given galvanometer is the amount of current required to produce
a deflection of one division on the galvanometer scale. Sensitivity is reciprocal of figure of merit.
Q. 24. VWhat is the order of figure of merit of pivoted type moving coil galvano-meters
which you use in the laboratory?
Ans. It is generally of the order of 10 ampere, i.e., (10 uA per division).
Q. 25. Once you know the vaiue of the shunt resistance to be connected, in what form
will you connect it?
Ans. A shunt, in the form of a wire of suitable length and material, is connected across the
galvanometer in parallel with it.
Q. 26. How do you compute the suitabie length of the shunt wire?
Ans. Resistance S of a given wire is given as
S=p
where is its length, a is the area of cross-section and p is its specific resistance. Since a, the
area of cross-section of the wire is (Tr),
Sr
Radius r of the wire can be determined by using a screw gauge, p of the material of the wire
can be obtained from the standard tables. Hence we can get the length l of the wire corresponding
to the required shunt resistance.
Q. 27. Why should a slightly greater length of the wire taken than the calculated
length 1?
Ans, Yes.
Ans. It is done because a small part of the wire near its ends is required for the terminal
Connections, so that the exact calculated length of the wire is left out between the two terminals.
Q. 28. Can you check the accuracy of the converted ammcter?
Q. 29. When the current in the circuit is I, we know that only I; passes through the
galvanonmeter. How does your converted ammeter measure I?
Ans. From the formula,
S(U-1) =l;.G
I. = 1.S/(S + G)
It is clear that I,, I, as G and S are constants for a given converted ammeter. Thus the scala
of converted ammeter directly measures I after proper calibration.
Conversion of Galvanometer into Voltmeter
Q. 30. What is a voltmeter?
Ans. It is an instrument which is used to measure the value of the potential difference
between two points directly.
Q. 31. What is the difference between a galvanometer and a voltmeter?
Ans.
difference.
Galvanometer detects feeble currents whereas a voltmeter measures potential
Q. 32. Is there any difference in the construction of a galvanometer and voltmeter?
Ans. Yes, a voltmeter is a galvanometer with a very high resistance connected in series
with it.
Q. 33. Why is it necessary for a voltmeter to have a high resistance?
Ans. Since the voltmeter is to be connected across two points in parallel if it has a low
resistance it will decrease the actual potential difference to be measured.
0. 34. An ideal voltmeter should have infnite resistance, isn't it?
Ans. Yes. A voltage measuring device nmust have infinite resistance, but practically this is not
possible.
Q. 35. Why is it not possible practically?
Ans. For any p.d. to be measured, the voltmeter has to deflect its pointer, for which it needs
energy, i.e., it must draw some current.
Q. 36. How do you do this in actual practice?
Ans. This is done by connecting a suitable high resistance in series with the galvanometer.
Q. 37. Do you connect a high resistance of any arbitrary value in series with a grven
galvanometer for this purpose?
Ans. No, the value of the series resistance depends upon the sensitivity and desired rang°
of the converted voltmeter.
Q. 38. VWhat do you understand by the range of a voltmeter?
Ans. It is the maximum value of the p.d. which can be measured by it.
0. 39. If a galvanometer is first converted into a voltmeter of range 10 volis and theu
into one of range of 1 volt, in which case is the series resistance of higher value?
Ans. In the case of the 10 volts range voltmeter because, R = (VJL) G.
0. 40. How do you deternmine the resistance to be connected iu series vith galvanomete
to change it into a volimeter of a required range?
Ans. It is determined by using the following formula
R= (VI) -G
However, there are other kinds of voltmeter like vacuum tube voltmeter (VTVM) with resistance a0ou
10 mega ohm and quadrant electrometer to measure p.d. without consuming any current. Tne
discussion lies beyond the scope of this book.
S(U-1) =l;.G
I. = 1.S/(S + G)
It is clear that I,, I, as G and S are constants for a given converted ammeter. Thus the scala
of converted ammeter directly measures I after proper calibration.
Conversion of Galvanometer into Voltmeter
Q. 30. What is a voltmeter?
Ans. It is an instrument which is used to measure the value of the potential difference
between two points directly.
Q. 31. What is the difference between a galvanometer and a voltmeter?
Ans.
difference.
Galvanometer detects feeble currents whereas a voltmeter measures potential
Q. 32. Is there any difference in the construction of a galvanometer and voltmeter?
Ans. Yes, a voltmeter is a galvanometer with a very high resistance connected in series
with it.
Q. 33. Why is it necessary for a voltmeter to have a high resistance?
Ans. Since the voltmeter is to be connected across two points in parallel if it has a low
resistance it will decrease the actual potential difference to be measured.
0. 34. An ideal voltmeter should have infnite resistance, isn't it?
Ans. Yes. A voltage measuring device nmust have infinite resistance, but practically this is not
possible.
Q. 35. Why is it not possible practically?
Ans. For any p.d. to be measured, the voltmeter has to deflect its pointer, for which it needs
energy, i.e., it must draw some current.
Q. 36. How do you do this in actual practice?
Ans. This is done by connecting a suitable high resistance in series with the galvanometer.
Q. 37. Do you connect a high resistance of any arbitrary value in series with a grven
galvanometer for this purpose?
Ans. No, the value of the series resistance depends upon the sensitivity and desired rang°
of the converted voltmeter.
Q. 38. VWhat do you understand by the range of a voltmeter?
Ans. It is the maximum value of the p.d. which can be measured by it.
0. 39. If a galvanometer is first converted into a voltmeter of range 10 volis and theu
into one of range of 1 volt, in which case is the series resistance of higher value?
Ans. In the case of the 10 volts range voltmeter because, R = (VJL) G.
0. 40. How do you deternmine the resistance to be connected iu series vith galvanomete
to change it into a volimeter of a required range?
Ans. It is determined by using the following formula
R= (VI) -G
However, there are other kinds of voltmeter like vacuum tube voltmeter (VTVM) with resistance a0ou
10 mega ohm and quadrant electrometer to measure p.d. without consuming any current. Tne
discussion lies beyond the scope of this book.
where the symbols have their usual meanings.
0. 41. How do you check the aceuracy of this converted voltmeter?
Ans. Refer to the circuit given in Fig. 5.10. A standard voltmeter V of the same range is
connected in parallel with this converted voltmeter with a rheostat ete. as shown. For a particular
position of rheostat slider, the readings in the standard and the converted voltmeters are noted.
The difference in these two readings gives the error in the converted voltmeter.
0. 42. A resistance of 10000 ohms is connected in series with a galvanometer of
resistance 50 ohms. Is the converted galvanonmefer an ammeter or voltmeter?
Ans, A voltmeter.
Q. 43. In Q. No. 42, if the range of the converted voltmeter is 10 V and it has
50 divisions. what is the full scale deflection current and figure of merit of the
galvanometer?
Ans. I, (for full deflection) is nearly
k, figure of merit
Q. 45.
manipulated?
lG_1000 LA
No
50
Vo
R
10V
10 000 S2
Q. 44. How are the resistances of different magnitudes designed and fixed in the
resistance box?
Ans. The insulated resistance wire of desired value is taken and it is doubled over itself.
Then it is wound over a bobbin of wood or porcelain. The two free ends are connected to the
two brass studs, separated by a plug key.
= 20 LA div.1
How are the values of different resistances in the box controlled and
0. 49. What is a milli-volimeter?
Ans. We know that R os la. The lengths of different resistance wires taken are almost the
same. Their values are controlled by their thickness. Thinner the wire, higher is its resistance.
= = 1 mA
Q. 46. What is appropriate thickness and length of the wire for infinite resistance in
the box?
Ans. There is no resistance wire marked infinity. When infinity plug is taken out, there is
only air gap left between the two brass studs and thus it introduces an infinite resistance in it.
0. 47. Why is the wire doubled over itself ?
Ans. It is done to avoid induced current effects.
Q. 50. What is a micro-ammeter?
0. 48, What is the material of the wires to be used in the resistanee boxes and why?
Ans. Constantan or manganin, because they have low temperature coefficient of resistance
and high specific resistance.
Ans. It is a voltmeter which measures up to
1
1000
Ans. t is an ammeter which can measure currents of the order of 1/10° of an ampere, i.e.,
0. 41. How do you check the aceuracy of this converted voltmeter?
Ans. Refer to the circuit given in Fig. 5.10. A standard voltmeter V of the same range is
connected in parallel with this converted voltmeter with a rheostat ete. as shown. For a particular
position of rheostat slider, the readings in the standard and the converted voltmeters are noted.
The difference in these two readings gives the error in the converted voltmeter.
0. 42. A resistance of 10000 ohms is connected in series with a galvanometer of
resistance 50 ohms. Is the converted galvanonmefer an ammeter or voltmeter?
Ans, A voltmeter.
Q. 43. In Q. No. 42, if the range of the converted voltmeter is 10 V and it has
50 divisions. what is the full scale deflection current and figure of merit of the
galvanometer?
Ans. I, (for full deflection) is nearly
k, figure of merit
Q. 45.
manipulated?
lG_1000 LA
No
50
Vo
R
10V
10 000 S2
Q. 44. How are the resistances of different magnitudes designed and fixed in the
resistance box?
Ans. The insulated resistance wire of desired value is taken and it is doubled over itself.
Then it is wound over a bobbin of wood or porcelain. The two free ends are connected to the
two brass studs, separated by a plug key.
= 20 LA div.1
How are the values of different resistances in the box controlled and
0. 49. What is a milli-volimeter?
Ans. We know that R os la. The lengths of different resistance wires taken are almost the
same. Their values are controlled by their thickness. Thinner the wire, higher is its resistance.
= = 1 mA
Q. 46. What is appropriate thickness and length of the wire for infinite resistance in
the box?
Ans. There is no resistance wire marked infinity. When infinity plug is taken out, there is
only air gap left between the two brass studs and thus it introduces an infinite resistance in it.
0. 47. Why is the wire doubled over itself ?
Ans. It is done to avoid induced current effects.
Q. 50. What is a micro-ammeter?
0. 48, What is the material of the wires to be used in the resistanee boxes and why?
Ans. Constantan or manganin, because they have low temperature coefficient of resistance
and high specific resistance.
Ans. It is a voltmeter which measures up to
1
1000
Ans. t is an ammeter which can measure currents of the order of 1/10° of an ampere, i.e.,
Q. 51. What is the minimum
current measurabie by a moving coil
galvanometer
(D'Arsonval type)?
Ans. 0.1 LA.
Q 52. The required value of shunt for converting a galvanometer into an ammeter of
range 3A, is O.05 ohm, Find the shunt resistance required if the required range of the
ammeter is 30 mA.
Ans. Shunt value S is given as S =
If 1, << I, , then
lG_-xG.
of the new shunt S' required should be,
S lo
Therefore, if I, range is to be reduced by a factor of
S' = 100 × S = 100 × 0.05 = 5 ohms.
3A
30× 10
Q. 53. Can you use a d.c. ammeter for the measurement of alternating current? If not,
why?
Ans. No. A d.c. ammeter functions on magnetic effect of current and the force experienced
by a conductor placed in a magnetic field. When a.c. is passed through the coil of d.c. ammeter,
its direction of deflection would reverse at the same frequency as the frequency of the alternating
current (say 50 Hz). The inertia of the coil would not respond to frequent changes in the direction
of deflection, and hence would show zero reading when alternating current is passed through a
d.c. ammeter.
-, ie.. 100, then the resistance
A'
Q. 54. What is the obvious difference between an a.c. voitmeter and a d.c. voltmeter
which you can tel! just by seeing them?
Ans. 1. Scale of d.c. voltmeter has divisions uniformly spaced whereas that of a.c. one is non
uniform, i.e., as you go to higher values, the divisions become closer.
2. There is a marking ~ On a.c. voltmeter and on d.c. voltmeter.
3. In a d.c. voltmeter, there is a coil pivoted between the magnetic pole pieces whereas in
a.c. there is a heating coil where heat produced varies as the square of current.
Q. 55. How can you increase the range of a galvanometer?
Ans. We can increase the range of a galvanometer by connecting a resistance in series
with it.
0. 56. How can you measure alternating current?
Ans. Same as d.c. ammeter, i.e., 1.5 A.
. Ans. Heating effect of current, Q= '. R.t. is independent of the direction of flow of current,
because P would always be positive. Therefore, Hot Wire Ammeter is used for this purpose.
0. 57. n a circuit, a d.c. ammeter reads 1.5 A. If the meter is replaced by an a.c. meter,
what would it read?
0. 58. In Q. No. 57, what would be the peak value of alternating current?
Ans. Z, = 2.gs or V2 l Therefore, the peak value I, = 1.5 2 A = 2.115 A.
Q. 59. An ideal voltmeter should have infinite resistance. Do you know of arny su
voltmeter in practice?
Ans. Yes, the electronic voltmeter VTVM (Vacuum Tube Volt Meter) has
nearly
infinite
resistance and is used fOr accurate measurements of p.d.
current measurabie by a moving coil
galvanometer
(D'Arsonval type)?
Ans. 0.1 LA.
Q 52. The required value of shunt for converting a galvanometer into an ammeter of
range 3A, is O.05 ohm, Find the shunt resistance required if the required range of the
ammeter is 30 mA.
Ans. Shunt value S is given as S =
If 1, << I, , then
lG_-xG.
of the new shunt S' required should be,
S lo
Therefore, if I, range is to be reduced by a factor of
S' = 100 × S = 100 × 0.05 = 5 ohms.
3A
30× 10
Q. 53. Can you use a d.c. ammeter for the measurement of alternating current? If not,
why?
Ans. No. A d.c. ammeter functions on magnetic effect of current and the force experienced
by a conductor placed in a magnetic field. When a.c. is passed through the coil of d.c. ammeter,
its direction of deflection would reverse at the same frequency as the frequency of the alternating
current (say 50 Hz). The inertia of the coil would not respond to frequent changes in the direction
of deflection, and hence would show zero reading when alternating current is passed through a
d.c. ammeter.
-, ie.. 100, then the resistance
A'
Q. 54. What is the obvious difference between an a.c. voitmeter and a d.c. voltmeter
which you can tel! just by seeing them?
Ans. 1. Scale of d.c. voltmeter has divisions uniformly spaced whereas that of a.c. one is non
uniform, i.e., as you go to higher values, the divisions become closer.
2. There is a marking ~ On a.c. voltmeter and on d.c. voltmeter.
3. In a d.c. voltmeter, there is a coil pivoted between the magnetic pole pieces whereas in
a.c. there is a heating coil where heat produced varies as the square of current.
Q. 55. How can you increase the range of a galvanometer?
Ans. We can increase the range of a galvanometer by connecting a resistance in series
with it.
0. 56. How can you measure alternating current?
Ans. Same as d.c. ammeter, i.e., 1.5 A.
. Ans. Heating effect of current, Q= '. R.t. is independent of the direction of flow of current,
because P would always be positive. Therefore, Hot Wire Ammeter is used for this purpose.
0. 57. n a circuit, a d.c. ammeter reads 1.5 A. If the meter is replaced by an a.c. meter,
what would it read?
0. 58. In Q. No. 57, what would be the peak value of alternating current?
Ans. Z, = 2.gs or V2 l Therefore, the peak value I, = 1.5 2 A = 2.115 A.
Q. 59. An ideal voltmeter should have infinite resistance. Do you know of arny su
voltmeter in practice?
Ans. Yes, the electronic voltmeter VTVM (Vacuum Tube Volt Meter) has
nearly
infinite
resistance and is used fOr accurate measurements of p.d.
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