Electric_Current_and_Circuits Powerpoint

Electric current and electric circuits are intrinsically linked.

1. Electric Current and CircuitsElectric Current and Circuits

2. Circuit Follow-Up QuestionsCircuit Follow-Up Questions
Does the same electron that leaves the battery have to Does the same electron that leaves the battery have to
make it all the way around back to the battery to have make it all the way around back to the battery to have
current?current?
Which light bulb lights up first?Which light bulb lights up first?
How fast do electrons go?How fast do electrons go?
Where did the electrons come from that made both lights Where did the electrons come from that made both lights
bulbs light up?bulbs light up?
What does the battery do?What does the battery do?
How is a hair dryer like a battery?How is a hair dryer like a battery?
How is a Genecon like a battery? How is it different?How is a Genecon like a battery? How is it different?
How is a capacitor like a battery? How is it different?How is a capacitor like a battery? How is it different?

3. Electric CurrentElectric Current
CurrentCurrent means “flow” means “flow”
Electric current Electric current meansmeans “ “flow of charge”.flow of charge”.
Electric current Electric current does does nonot refer to the speed of the t refer to the speed of the
charged particles…. It charged particles…. It doesdoes refer to the quantity of refer to the quantity of
charge that passes a single point in a given charge that passes a single point in a given
amount of time.amount of time.

4. Electric CurrentElectric Current
The SI unit of current is the The SI unit of current is the ampere ampere (A).(A).
1 ampere is equal to one Coulomb per 1 ampere is equal to one Coulomb per
second.second.
The symbol used to represent current is I, The symbol used to represent current is I,
and we can writeand we can write
I = q/t (reference tables)I = q/t (reference tables)

5. Sample ProblemSample Problem
What is the electric current in a conductor if What is the electric current in a conductor if
240 Coulombs of charge pass through it in 240 Coulombs of charge pass through it in
one minute?one minute?
SolutionSolution
I = q/tI = q/t
I = (240 C) / (60. seconds) = 4.0 C/s = 4.0 AI = (240 C) / (60. seconds) = 4.0 C/s = 4.0 A

6. Current and Potential DifferenceCurrent and Potential Difference
What must be necessary to initiate a flow of What must be necessary to initiate a flow of
charge?charge?
A A potential difference.potential difference.
An electric circuitAn electric circuit
–A closed path along which charged particles can move.A closed path along which charged particles can move.

7. Potential DifferencePotential Difference
We call the flow of positive charge a We call the flow of positive charge a conventional current.conventional current.
–Its direction is always opposite to the flow of electrons.Its direction is always opposite to the flow of electrons.
It is often considered more “natural” to choose the electron It is often considered more “natural” to choose the electron
flow as the direction of currentflow as the direction of current
–This is the definition used in your review book.This is the definition used in your review book.
+ -
V
Flow of Electrons
Flow of positive charge

8. Potential DifferencePotential Difference
What supplies the potential difference?What supplies the potential difference?
–The supply of potential difference must come The supply of potential difference must come
from an external energy source, such as…from an external energy source, such as…
A voltaic or galvanic cell (a common dry cell). A voltaic or galvanic cell (a common dry cell).
Converts chemical energy to electrical energy.Converts chemical energy to electrical energy.
A battery (several cells connected together).A battery (several cells connected together).
A photovoltaic cell, or solar cell. Converts light A photovoltaic cell, or solar cell. Converts light
energy into electrical energy.energy into electrical energy.
A generator.A generator.

9. Potential DifferencePotential Difference
How do we measure potential difference?How do we measure potential difference?
–With a device called a With a device called a voltmetervoltmeter..
Symbols for a cell, a battery, and a Symbols for a cell, a battery, and a
voltmeter…voltmeter…

10. Electric CircuitElectric Circuit
A closed path along which charged particles A closed path along which charged particles
move.move.
A A switchswitch
–A device for making, breaking, or changing the A device for making, breaking, or changing the
connections in an electric circuit.connections in an electric circuit.
–Symbol…Symbol…
Switch

11. Build a Simple CircuitBuild a Simple Circuit
With a partner, construct the simple circuit With a partner, construct the simple circuit
whose schematic diagram is on the board.whose schematic diagram is on the board.

12. Electric CircuitElectric Circuit
We have already discussed conductors and We have already discussed conductors and
insulatorsinsulators
Conductors – the ability of a material to conduct Conductors – the ability of a material to conduct
electricity depends on the number of “free” electricity depends on the number of “free”
charges (per unit volume) and their mobility.charges (per unit volume) and their mobility.
–ConductivityConductivity – a property of a material that depends on – a property of a material that depends on
the availability of charges that are free to move under the availability of charges that are free to move under
the influence of an electric field. the influence of an electric field.
–Good conductors – metals

13. Electric CircuitElectric Circuit
Resistance – we call the electrical version of Resistance – we call the electrical version of
friction friction resistance.resistance.
–As electrons travel through a conductor, they As electrons travel through a conductor, they
lose energy because of collisions with the lose energy because of collisions with the
atoms of the conductor. The energy lost due to atoms of the conductor. The energy lost due to
these collisions is converted almost entirely to these collisions is converted almost entirely to
heat.heat.

14. Electric CircuitElectric Circuit
The resistance of a material depends on….The resistance of a material depends on….
–The nature of the materialThe nature of the material
–The geometry of the conductorThe geometry of the conductor
–TemperatureTemperature

15. Electric CircuitElectric Circuit
Resistivity (Resistivity (ρ - “rho”)ρ - “rho”)
–A characteristic of a material that depends on its A characteristic of a material that depends on its
electronic structure and temperature (i.e. the electronic structure and temperature (i.e. the
“nature of the material”)“nature of the material”)
The resistance of a wire is directly proportional to its The resistance of a wire is directly proportional to its
resistivityresistivity.
See “Table of Resistivities at 20
0
C” in your reference
tables.

16. Electric CircuitElectric Circuit
Combining the factors that affect the Combining the factors that affect the
resistance of a conductor gives the following resistance of a conductor gives the following
equationequation
R = R = ρL / A ρL / A (reference tables)(reference tables)
where…
R is the resistance in ohms (Ω)
ρ is the resistivity in ohm • meters (ρ is the resistivity in ohm • meters (Ω • m)• m)
L is the length in metersL is the length in meters
A is the cross-sectional area in square metersA is the cross-sectional area in square meters

17. Sample ProblemSample Problem
Determine the resistance of a 2.0 meter length of Determine the resistance of a 2.0 meter length of
silver wire having a diameter of 2.0 millimeters. silver wire having a diameter of 2.0 millimeters.
Assume a temperature of 20Assume a temperature of 20
00
C.C.
SolutionSolution
ρ = ρ = 1.59 x 10
-8
Ω•m (from reference tables)
A
circle
= πr
2
= π(d/2)
2
(remember to convert to meters)
L = 2.0 meters
R = ρL / A = ρL / π(d/2)
2
R = (1.59 x 10
-8
Ω•m) (2.0 m) / π (0.002 m / 2)
2
R = 1.0 x 10
-2
Ω

18. Ohm’s LawOhm’s Law
German scientist Georg Simon Ohm found German scientist Georg Simon Ohm found
that the ratio of the potential difference to that the ratio of the potential difference to
the current is always a constant for a given the current is always a constant for a given
conductor…conductor…
R = V/I R = V/I (reference tables)(reference tables)
Most metallic conductors obey ohm’s law at Most metallic conductors obey ohm’s law at
constant temperature over a limited range of constant temperature over a limited range of
voltagesvoltages

19. Ohm’s LawOhm’s Law
Many important devices do not obey Ohm’s Law…Many important devices do not obey Ohm’s Law…
–Lightbulbs (tungsten)Lightbulbs (tungsten)
–transistorstransistors
–diodesdiodes
–pocket calculatorspocket calculators
–transistor radiostransistor radios
Resistors are devices designed to have specific Resistors are devices designed to have specific
resistance, and are made of long, thin wires, resistance, and are made of long, thin wires,
graphite, or semiconductors.graphite, or semiconductors.

20. Electric PowerElectric Power
Suppose we wish to calculate the Suppose we wish to calculate the raterate at at
which energy is supplied in a circuit.which energy is supplied in a circuit.
If we multiply the potential difference across If we multiply the potential difference across
the circuit by the current in the circuit and the circuit by the current in the circuit and
examine the units, we find that…examine the units, we find that…
V * I = (Joules/Coulomb) * (Coulombs/Second)V * I = (Joules/Coulomb) * (Coulombs/Second)
= Joules/Second = Power= Joules/Second = Power

21. Electric PowerElectric Power
Therefore, the power supplied to a circuit by Therefore, the power supplied to a circuit by
the source is given by the relationship…the source is given by the relationship…
P = VI (reference tables)P = VI (reference tables)
Since Ohm’s Law says V=IR, we can Since Ohm’s Law says V=IR, we can
substitute this for the voltage and get… substitute this for the voltage and get…
twinkle, twinkle little star….twinkle, twinkle little star….
P = IP = I
22
RR

22. Let’s PracticeLet’s Practice
Calculate the rate at which energy is Calculate the rate at which energy is
supplied by a 120-volt source to a circuit if supplied by a 120-volt source to a circuit if
the circuit is 5.5 amperes.the circuit is 5.5 amperes.
SolutionSolution
P = VIP = VI
P = (120 V) (5.5 amps)P = (120 V) (5.5 amps)
P = (120 J/C) (5.5 C/s)P = (120 J/C) (5.5 C/s)
P = 660 J/s = 660 WP = 660 J/s = 660 W

23. Electric EnergyElectric Energy
Energy = Power x timeEnergy = Power x time
We already know that P = VI…We already know that P = VI…
So it follows that Energy = VIt, or using the So it follows that Energy = VIt, or using the
other equations for powerother equations for power
Energy = Work = VIt = IEnergy = Work = VIt = I
22
Rt = VRt = V
22
t / Rt / R
(reference tables)(reference tables)

24. Sample ProblemSample Problem
How much power is produced by a 50. volt How much power is produced by a 50. volt
source that generates a current of 5.0 source that generates a current of 5.0
amperes? How much energy does it amperes? How much energy does it
generate over a two minute period?generate over a two minute period?
Solution:Solution:
 P = VI = (50. V) (5.0 A) = 250 WattsP = VI = (50. V) (5.0 A) = 250 Watts
W = Pt = (250 W)(2 minutes)( 60 secs / minute)W = Pt = (250 W)(2 minutes)( 60 secs / minute)
= 30,000 J= 30,000 J

25. Mini-ActivityMini-Activity
Intro To Series CircuitsIntro To Series Circuits

26. Series CircuitsSeries Circuits
Definition of a Series CircuitDefinition of a Series Circuit
–A A series circuitseries circuit is a circuit in which all parts are
connected end to end to provide a single path
for the current.
–If the current path is interrupted (or broken), the
entire circuit ceases to operate.

27. Series CircuitsSeries Circuits
Conventions and RemindersConventions and Reminders
–Get good contact when building circuitsGet good contact when building circuits
–Red is positive, black is negativeRed is positive, black is negative

28. Series CircuitsSeries Circuits
VoltmeterVoltmeter
–Very high resistance deviceVery high resistance device
–Measures potential difference across two pointsMeasures potential difference across two points
–Always connected in parallelAlways connected in parallel
–Symbol (in reference tables)Symbol (in reference tables)
AmmeterAmmeter
–Very low resistance deviceVery low resistance device
–Measures current passing through any point in a circuitMeasures current passing through any point in a circuit
–Always connected in seriesAlways connected in series
–Symbol (in reference tables)Symbol (in reference tables)

29. Series CircuitsSeries Circuits
Mini-Activity (Intro to Series Circuits)Mini-Activity (Intro to Series Circuits)

30. Current In A Series CircuitCurrent In A Series Circuit
Since a series circuit contains only one path, the current Since a series circuit contains only one path, the current
throughout the circuit is constant.throughout the circuit is constant.
An ammeter placed at any position in the circuit would An ammeter placed at any position in the circuit would
record the same value.record the same value.
So, for a series circuit…So, for a series circuit…
I = II = I
11 = I = I
22 = I = I
33 = …. = ….
(Reference Tables)(Reference Tables)
Where IWhere I
11 is the current through R is the current through R
11, etc.., etc..

31. Potential Difference In A Series Potential Difference In A Series
CircuitCircuit
The potential difference across two points The potential difference across two points
depends on the work the source must do in depends on the work the source must do in
order to move the charge between the two order to move the charge between the two
points.points.
In the series circuit we built, the resistance In the series circuit we built, the resistance
across the resistors will determine how across the resistors will determine how
much work needs to be done.much work needs to be done.

32. Potential Difference In A Series Potential Difference In A Series
CircuitCircuit
What we find is that the potential difference What we find is that the potential difference
across the entire circuit (Vacross the entire circuit (V
tt), supplied by the ), supplied by the
power source, is equal to the sum of the power source, is equal to the sum of the
potential differences (Vpotential differences (V
11, V, V
22, …) across all , …) across all
the resistances.the resistances.
VV
tt = V = V
11 + V + V
22 +V +V
33 + … + …
(Reference Tables)(Reference Tables)

33. Potential Difference In A Series Potential Difference In A Series
CircuitCircuit
The total voltage of a series circuit, or the The total voltage of a series circuit, or the
voltage drop across a resistor, can be voltage drop across a resistor, can be
calculated as follows…calculated as follows…
VV
nn = I = I
nn R R
nn
Where “n” can be “total” or the number of any Where “n” can be “total” or the number of any
one of the resistors.one of the resistors.

34. Potential Difference In A Series Potential Difference In A Series
CircuitCircuit
If we combine the previous three equations, If we combine the previous three equations,
we see the following…we see the following…
VV
tt = V = V
11 + V + V
22 + … + V + … + V
nn
II
tt R R
tt = I = I
11RR
11 + I + I
22RR
22 + … + I + … + I
nnRR
nn
= I (R= I (R
11 + R + R
22 + … + R + … + R
nn))
So…So…
RR
tt = R = R
eqeq = R = R
11 + R + R
22 + … + R + … + R
nn (reference tables)(reference tables)

35. Series CircuitSeries Circuit
To the right is an To the right is an
animation of a series animation of a series
circuit where electrical circuit where electrical
energy is shown as energy is shown as
gravitational potential gravitational potential
energy (GPE). The energy (GPE). The
greater the change in greater the change in
height, the more height, the more
energy is used or the energy is used or the
more work is done.more work is done.

36. Series CircuitSeries Circuit
In this animation you should notice the In this animation you should notice the
following things:following things:
The battery or The battery or sourcesource is represented by an is represented by an
escalator which raises charges to a higher escalator which raises charges to a higher
level of energy. level of energy.
As the charges move through the As the charges move through the resistorsresistors
(represented by the paddle wheels) they do (represented by the paddle wheels) they do
work on the resistor and as a result, they lose work on the resistor and as a result, they lose
electrical energy. electrical energy.
The charges do more work (give up more The charges do more work (give up more
electrical energy) as they pass through the electrical energy) as they pass through the
larger resistor. larger resistor.
By the time each charge makes it back to the By the time each charge makes it back to the
battery, it has lost all the energy given to it by battery, it has lost all the energy given to it by
the battery. the battery.
The total of the potential drops ( - The total of the potential drops ( - potential potential
differencedifference) across the resistors is the same ) across the resistors is the same
as the potential rise ( + as the potential rise ( + potential differencepotential difference) )
across the battery. This demonstrates that a across the battery. This demonstrates that a
charge can only do as much work as was charge can only do as much work as was
done on it by the battery. done on it by the battery.
The charges are positive so this is a The charges are positive so this is a
representation of representation of Conventional CurrentConventional Current (the (the
apparent flow of positive charges)apparent flow of positive charges)
The charges are only flowing in one direction The charges are only flowing in one direction
so this would be considered so this would be considered direct currentdirect current
( ( D.C.D.C. ). ).

37. Parallel CircuitsParallel Circuits
In contrast with a series circuit, a In contrast with a series circuit, a parallel parallel
circuitcircuit has more than one current path. has more than one current path.
If a segment of a parallel circuit is If a segment of a parallel circuit is
interrupted, the result will not necessarily be interrupted, the result will not necessarily be
that the entire circuit doesn’t work.that the entire circuit doesn’t work.

38. Parallel CircuitsParallel Circuits
We can analyze parallel circuits using the following We can analyze parallel circuits using the following
relationships which are valid for any parallel circuit…relationships which are valid for any parallel circuit…
(in reference tables)(in reference tables)
The potential drops of each branch equals the potential rise of the source.The potential drops of each branch equals the potential rise of the source.
VV
tt = V = V
11 = V = V
22 = V = V
33 = ….. = V = ….. = V
nn

39. Parallel CircuitsParallel Circuits
In a parallel circuit, the current separates into In a parallel circuit, the current separates into
more than one path. The point where this occurs more than one path. The point where this occurs
is known as a is known as a junctionjunction..
Due to the Law of Conservation of Charge, the Due to the Law of Conservation of Charge, the
sum of the currents entering the junction must sum of the currents entering the junction must
equal the sum of the currents leaving the junction.equal the sum of the currents leaving the junction.
This statement is known as This statement is known as Kirchoff’s 2Kirchoff’s 2
ndnd
Rule Rule, or , or
the junction rulethe junction rule..

40. The Junction RuleThe Junction Rule
In this example you will In this example you will
notice that 8 Amps of notice that 8 Amps of
current enter the current enter the
junction and 3 and 5 junction and 3 and 5
Amps leave the Amps leave the
junction. This makes a junction. This makes a
total of 8 Amps total of 8 Amps
entering and 8 Amps entering and 8 Amps
leaving.leaving.

41. The Junction RuleThe Junction Rule
In this example you will In this example you will
notice 8 Amps and 1 notice 8 Amps and 1
Amp entering the Amp entering the
junction and 9 Amps junction and 9 Amps
leaving.
  This makes a
leaving.
  This makes a
total of 9 Amps total of 9 Amps
entering and 9 Amps entering and 9 Amps
leaving.leaving.

42. The Junction RuleThe Junction Rule
In this example you will In this example you will
notice 8 Amps and 1 notice 8 Amps and 1
Amp entering the Amp entering the
junction while 7 Amps junction while 7 Amps
and 2 Amps leave.
 
and 2 Amps leave.
 
This makes a total of 9 This makes a total of 9
Amps entering and 9 Amps entering and 9
Amps leaving.Amps leaving.

43. Parallel CircuitsParallel Circuits
Ohm's LawOhm's Law may be used may be used
in a parallel circuit as long in a parallel circuit as long
as you remember that you as you remember that you
can use the formula with can use the formula with
eithereither partial values or partial values or
with total values but you with total values but you
can not mix parts and can not mix parts and
totals.totals.

44. Parallel CircuitsParallel Circuits
Lastly, if we combine the above three equations, Lastly, if we combine the above three equations,
we can develop a means of finding the resistance we can develop a means of finding the resistance
of the parallel circuit as a whole…of the parallel circuit as a whole…
1/R1/R
tt = 1/R = 1/R
11 + 1/R + 1/R
22 + 1/R + 1/R
33 + ….. + …..
(reference tables)(reference tables)
Where RWhere R
tt is the is the equivalent resistance equivalent resistance of the parallel of the parallel
circuit.circuit.

45. Parallel CircuitsParallel Circuits
A A parallel circuitparallel circuit
has more than one has more than one
resistor resistor (anything (anything
that uses electricity that uses electricity
to do work)to do work) and gets and gets
its name from having its name from having
multiple (multiple (parallelparallel) )
paths to move paths to move
along .along .

46. Parallel CircuitsParallel Circuits
More current flows through the smaller More current flows through the smaller
resistance. (More charges take the easiest resistance. (More charges take the easiest
path.).path.).
The battery or The battery or sourcesource is represented by an is represented by an
escalator which raises charges to a higher escalator which raises charges to a higher
level of energy.level of energy.
As the charges move through the As the charges move through the resistorsresistors
(represented by the paddle wheels) they do (represented by the paddle wheels) they do
work on the resistor and as a result, they work on the resistor and as a result, they
lose electrical energy.lose electrical energy.
By the time each charge makes it back to By the time each charge makes it back to
the battery, it has lost all the electrical the battery, it has lost all the electrical
energy given to it by the battery.energy given to it by the battery.
The total of the potential drops ( - The total of the potential drops ( - potential potential
differencedifference) of each "branch" or path is the ) of each "branch" or path is the
same as the potential rise ( + same as the potential rise ( + potential potential
differencedifference) across the battery. This ) across the battery. This
demonstrates that a charge can only do as demonstrates that a charge can only do as
much work as was done on it by the battery.much work as was done on it by the battery.
The charges are positive so this is a The charges are positive so this is a
representation of representation of conventional currentconventional current (the (the
apparent flow of positive charges).apparent flow of positive charges).
The charges are only flowing in one The charges are only flowing in one
direction so this would be considered direction so this would be considered direct direct
currentcurrent ( ( D.C.D.C. ). ).