ppt current electricity -1developed.pptx

It is about current electricity

1. CURRENT ELECTRICITY Prepared by Sheena Punnoose PGT Physics, JNV A lleppey

2. CURRENT ELECTRICITY -1 Electric current Ohm’s law Resistance and Resistivity Current density Drift of electrons and resistivity Mobility Limitations of ohm’s law Colour code of carbon resistor Temperature resistance of resistivity Electrical energy and power Combination of resistors

3. Electric Current It is defined as the rate of flow of charges across a given area of cross section. I = For unsteady currents, electric current is defined as the rate of flow of net charge Δ Q flowing across a cross section of a conductor in time Δ t, in the limit of Δ t tending to zero. I(t) = = The S.I . unit of current is ampere(A)  

4. Electric current in conductors Electric charge in motion, due to the force experienced by an applied electric field, constitutes electric current. In solid conductors the current is carried by negatively charged electrons. In electrolytic solution the current is carried by the motion of positive and negative ions. When no electric field is applied ,the electrons are in random motion and the average velocity of charges is zero . Therefore there will be no net current.

5. Ohm’s law The electric current, I, flowing through a conductor is directly proportional to the potential difference ,V across the two ends of the conductor, provided the physical conditions like temperature, entropy etc. remain a constant. I α V V = RI where the constant of proportionality R is called the resistance of the conductor. The S.I unit of resistance is ohm( Ω ).

6. Ohm’s law

7. One ohm is the resistance of a conductor which develops a potential difference of 1 volt when a current of 1A is passing through it. The resistance of a conductor depends on: ( i ) The length of the conductor R α l (ii) The area of cross-section of the conductor R α 1/A (iii) The temperature of the conductor At a constant temperature, R α R = ρ , where ρ is called the resistivity or the specific resistance of the conductor. Its unit is ohm metre( Ω m).  

8. Resistance and Resistivity Resistance is the property of a substance by virtue of which it opposes the flow of current or charge. Its unit is Ω . Factors affecting resistance are length of the conductor, area of the cross-section of the conductor, temperature of the conductor and nature of material of the conductor Resistivity or specific resistance is the property of the material of the conducting substance and is defined as the resistance of a conductor of unit length and unit area of cross-section. Its unit is Ω m. Factors affecting resistivity are temperature, number density of free electrons, type of conductor and relaxation time.

9. Current density : Current per unit area( taken normal to the current)is called current density .It is denoted by . Its S.I unit is A/ .   According to ohm’s law, V =IR =I ρ =j ρ l If E is the magnitude of uniform electric field in the conductor of length I, then V = El ⇒ El = j ρ l E = j ρ = = = σ where σ is conductivity .Its S.I unit is mho/m or siemen /metre  

10. Drift of electrons and resistivity Drift velocity is defined as the velocity with which the free electrons are moving or drifting towards the positive terminal under the effect of the applied (external) electric field. Consider a conductor of length l, area of cross section A across which an electric field E is present due to potential V being applied. The acceleration a, produced in the free electron due to the field a = -eE/m The electrons undergo collisions. The average velocity of the electrons in the presence of an electric field is called drift velocity, .  

11. The average time interval between two successive collisions of free electrons is called relaxation time , τ . = a e If n is the number density of free electrons and nA Δ t is the number of electrons crossing area A in time Δ t, then Current flowing, I= = = neA I= τ Also, I = A Therefore, = . τ In vector form, = . τ , As j= E= , = . τ Therefore, ρ =  

12. Mobility ( μ ) Conductivity is due to mobile charge carriers. Moblity , μ is defined as the magnitude of the drift velocity per unit electric field μ = = τ The S.I unit of mobility is /Vs, where V is volt and s is second.  

13. Limitations of Ohm’s law Ohm’s law is valid over a large class of materials , but in some materials and devices it does not hold true . ( i ) V ceases to be proportional to I. (ii) The relation between V and I depends on the sign of V. ,e.g. in a diode . (iii) The relation between V and I is not unique. e.g . in Ga As.

14. Commercial Resistors : Commercial resistors are of two major types. Wire bound resistors: They are made by winding the wires of an alloy e.g. manganin , nichrome and constantan. The resistances are in the range of a fraction of an ohm to a few hundred ohms. (ii) Carbon resistors: They are made from carbon. They are compact, inexpensive and small in size and their values are given using a colour code.

15. Colour code for carbon resistors : The resistors have a set of co-axial coloured rings. The first two bands indicate the first two significant figures of resistance in ohms.The third band indicates the decimal multiplier. The last band indicates the tolerance 27x 5%   Table for resistor colour code colour Number Multiplier Tolerance(%) Black 1   Brown 1 10 1   Red 2 10 2   Orange 3 10 3   Yellow 4 10 4   Green 5 10 5   Blue 6 10 6   Violet 7 10 7   Gray 8 10 8   White 9 10 9   Gold   10 -1 5 Silver   10 -2 10 No colour     20 27x 5 % Ω  

16. Electrical Resistivity and Its temperature dependence Depending on the electrical resistivity, materials are classified into: ( i ) Conductors: Resistivity in the range of Ω m to Ω m . The resistivity over a limited range of temperatures is given by , where is the resistivity at temperatureT , is the resistivity at temperature and is the temperature coefficient of resistivity. S.I unit of α is . For metals, is positive. ( ii) Semi conductors: The resistivity is between that of conductors and insulators. Resistivity decreases with increase in temperature. (iii) Insulators: Resistivity is times that of metals. The resistivity decreases with increase in temperature .  

17. Electrical Energy and Power Electrical power is the rate at which electric energy is converted into heat energy. P= VI = R= . Electric power is transmitted from power stations via transmission cables. The energy loss through transmission is called transmission loss . It can be minimised by transmitting electric power at low current and high voltage through a transmission line. Power dissipated, = = , where P is the device power.  

18. Combination of Resistors ( i ) Series combination of resistors If three resistors of resistances , combined in series, Effective resistance, R= The potential drop across the combination, V= + .  

19. (ii) Parallel combination If three resistors of resistances combined in parallel, Effective resistance + The current through the combination = +  

20. Comparison of EMF and Potential difference EMF EMF is the maximum potential difference between the two electrodes of the cell when no current is drawn from the cell,i.e , when the circuit is open. It is independent of the resistance of the circuit. The term emf is used only for the source of the emf. It is greater than the potential difference between any two points in a circuit. Potential Difference P.D is the difference of potentials between any two points in a closed circuit. It is proportional to the resistance between the given points. It is measured between any two points of the circuit. However, P.D is greater than emf when the cell is being charged.