2. Current
Convention : Current
depicts flow of
positive (+) charges
Sunday, July 24, 2011
3. Current
Convention : Current
depicts flow of
positive (+) charges
Area
+
Sunday, July 24, 2011
4. Current
Convention : Current
depicts flow of
positive (+) charges
Area
+
Ammeter
(measures current)
Sunday, July 24, 2011
5. Current
Convention : Current
depicts flow of
positive (+) charges
Area
+ +
+
Ammeter
(measures current)
Sunday, July 24, 2011
6. Current
Convention : Current
depicts flow of
positive (+) charges
Area
+ +
+
Ammeter
(measures current)
Sunday, July 24, 2011
7. Current
A measure of how much charge passes through an amount of time
+
+
+ Ammeter
(measures current)
Sunday, July 24, 2011
8. Current
Count how many charges flow through
+ +
+
Sunday, July 24, 2011
9. Current
Count how many charges flow through
Expand surface to a volume
+ +
+
Sunday, July 24, 2011
10. Current
Count how many charges flow through
Expand surface to a volume
+ +
+
Area = A
Sunday, July 24, 2011
11. Current
Count how many charges flow through
Expand surface to a volume
+ +
+
Area = A
length = !x
Sunday, July 24, 2011
12. Current
Count how many charges flow through
Expand surface to a volume
+ + Total volume
V = (A)(!x)
+
Area = A
length = !x
Sunday, July 24, 2011
13. Current
Count how many charges flow through
Expand surface to a volume
+ + Total volume
V = (A)(!x)
+
Area = A
length = !x
Number of charges = (charge density or charge per volume)*(volume)
Number of charges = (n) * (A!x)
Sunday, July 24, 2011
14. Current
Count how many charges flow through
Expand surface to a volume
+ + Total volume
V = (A)(!x)
+
Area = A
length = !x
Number of charges = (charge density or charge per volume)*(volume)
Number of charges = (n) * (A!x)
Total amount of charge = (number of charges)*(charge)
!Q = (n A !x)*(q)
Sunday, July 24, 2011
15. Current
!Q = (n A !x)*(q)
+ + Total volume
V = (A)(!x)
+
Area = A
length = !x
Sunday, July 24, 2011
16. Current
!Q = (n A !x)*(q)
but charges have drift velocity vd = !x/!t
+ + Total volume
V = (A)(!x)
+
Area = A
length = !x = vd !t
Sunday, July 24, 2011
17. Current
!Q = (n A !x)*(q)
but charges have drift velocity vd = !x/!t
+ + Total volume
V = (A)(!x)
+
Area = A
length = !x = vd !t
!Q = (n A vd !t)*(q)
Sunday, July 24, 2011
18. Current
!Q = (n A !x)*(q)
but charges have drift velocity vd = !x/!t
+ + Total volume
V = (A)(!x)
+
Area = A
length = !x = vd !t
!Q = (n A vd !t)*(q)
!Q/!t = (n A vd)*(q)
I = n q vd A
Sunday, July 24, 2011
19. Current
This is the reason why large wires are
needed to support large currents
Sunday, July 24, 2011
20. Current
This is the reason why large wires are
needed to support large currents
Sunday, July 24, 2011
21. Resistance
Current density (J)
current per area
Sunday, July 24, 2011
22. Resistance
Current density (J)
current per area
Direction of current (flow of positive charges)
is same with direction of electric field
Sunday, July 24, 2011
23. Resistance
Current density (J)
current per area
Direction of current (flow of positive charges)
is same with direction of electric field
conductivity
Sunday, July 24, 2011
24. Resistance
Current density (J)
current per area
Direction of current (flow of positive charges)
is same with direction of electric field
conductivity (material property)
resistivity (material property)
Sunday, July 24, 2011
25. Resistance
Current density (J)
current per area
Direction of current (flow of positive charges)
is same with direction of electric field
conductivity
resistivity
Current is proportional to conductivity but
inversely proportional to resistivity!
Sunday, July 24, 2011
26. Resistance
Current is proportional to conductivity but
inversely proportional to resistivity!
Sunday, July 24, 2011
27. Resistance
Current is proportional to conductivity but
inversely proportional to resistivity!
Current is proportional to the electric potential
(specifically potential difference)
Sunday, July 24, 2011
28. Resistance
Current is proportional to conductivity but
inversely proportional to resistivity!
Current is proportional to the electric potential
(specifically potential difference)
Ohm’s Law Potential difference
Resistance
current
Sunday, July 24, 2011
29. Resistance
Current is proportional to conductivity but
inversely proportional to resistivity!
Current is proportional to the electric potential
(specifically potential difference)
Ohm’s Law Potential difference
Resistance
current
a much better form
than ΔV = I R
Sunday, July 24, 2011
30. Resistance
Current is proportional to conductivity but
inversely proportional to resistivity!
Current is proportional to the electric potential
(specifically potential difference)
Ohm’s Law Potential difference
Resistance
current
a much better form Increasing !V increases I
than ΔV = I R Increasing R decreases I
Sunday, July 24, 2011
31. Resistance
Current is proportional to conductivity but
inversely proportional to resistivity!
Current is proportional to the electric potential
(specifically potential difference)
Ohm’s Law Potential difference
Resistance
current
a much better form Increasing !V increases I
than ΔV = I R Increasing R decreases I
!V = I R Increasing R does not increase !V
Current (I) is increased because !V is increased
Sunday, July 24, 2011
33. Resistance
Important points:
same with capacitance, resistance does not
depend on !V and I
Resistance depends on material property
resistivity ", length of wire l and cross
sectional area A
conventional current is flowing positive (+) charges though
in reality electrons flow
direction of the current I is same as direction of electric field
Sunday, July 24, 2011
34. Recent Equations
→
→ →E
J = σE =
ρ
→ →
J = nq v d A
→
→ I
J =
A
∆V
I=
R
ρl
R=
A
Sunday, July 24, 2011
35. Exercise
Rank from lowest to highest amount of current
Derive the equation R = "L/A
from V = IR, J = E/" = I/A, V = EL
Sunday, July 24, 2011
36. Resistance and Temperature
ρl
R=
A
ρ = ρ0 (1 + α∆T )
∆T = T − T0
T0 is usually taken to be 25 °C
T ↑ ρ↑
Sunday, July 24, 2011
37. Power
∆U
P =
∆t
∆(q∆V )
P =
∆t
(∆q)(∆V )
P =
∆t
∆q
P = ∆V
∆t
P = I∆V
Sunday, July 24, 2011
38. Power
P = I∆V
∆V
I=
R
V2
P = P = I 2R
R
Sunday, July 24, 2011
39. Exercises
The electron beam emerging from a certain high-energy electron accelerator
has a circular cross section of radius 1.00 mm. (a) The beam current is 8.00 µA.
Find the current density in the beam, assuming that it is uniform throughout. (b)
The speed of the electrons is so close to the speed of light that their speed can
be taken as c = 3.00 x 108 m/s with negligible error. Find the electron density in
the beam. (c) How long does it take for Avogadroʼs number of electrons to
emerge from the accelerator?
An aluminum wire having a cross-sectional area of 4.00 x 10-6 m2 carries a current
of 5.00 A. Find the drift speed of the electrons in the wire. The density of aluminum
is 2700 kg/m3. Assume that one conduction electron is supplied by each atom.
Molar mass of Al is 27 g/mol.
Four wires A, B, C and D are made of the same material but of different lengths
and radii. Wire A has length L but has radius R. Wire B has length 2L but with
radius ½R. Wire C has length ½L but with radius 2R. Wire D has length ½L but
with radius ½R.
Rank with increasing resistance
A 0.900-V potential difference is maintained across a 1.50-m length of tungsten
wire that has a cross-sectional area of 0.600 mm2. What is the current in the
wire?
resistivity of tungsten is 5.6 x 10-8 Ω-m
Sunday, July 24, 2011
40. Exercises
An electric heater is constructed by applying a potential difference of 120 V to a
Nichrome wire that has a total resistance of 8.00 Ω. Find the current carried by
the wire and the power rating of the heater.
A 500-W heating coil designed to operate from 110 V is made of Nichrome wire
0.500 mm in diameter. (a) Assuming that the resistivity of the Nichrome remains
constant at its 20.0°C value, find the length of wire used. (b) What If? Now
consider the variation of resistivity with temperature. What power will the coil of
part (a) actually deliver when it is heated to 1200°C?
ρ = 1.50 x 10-6 Ω-m
Sunday, July 24, 2011
42. A 500-W heating coil designed to operate from 110 V is made of Nichrome wire
0.500 mm in diameter. (a) Assuming that the resistivity of the Nichrome remains
constant at its 20.0°C value, find the length of wire used. (b) What If? Now
consider the variation of resistivity with temperature. What power will the coil of
part (a) actually deliver when it is heated to 1200°C?
ρ = 1.50 x 10-6 Ω-cm
Sunday, July 24, 2011
43. More exercises
A certain lightbulb has a tungsten filament with a resistance of 19.0 Ω when cold
and 140 Ω when hot. Assume that the resistivity of tungsten varies linearly with
temperature even over the large temperature range involved here, and find the
temperature of the hot filament. Assume the initial temperature is 20.0°C.
4.5 x 10-3 C-1
The cost of electricity varies widely through the United States; $0.120/kWh is
one typical value. At this unit price, calculate the cost of (a) leaving a 40.0-W
porch light on for two weeks while you are on vacation, (b) making a piece of
dark toast in 3.00 min with a 970-W toaster, and (c) drying a load of clothes in
40.0 min in a 5 200-W dryer.
Sunday, July 24, 2011
44. A certain lightbulb has a tungsten filament with a resistance of 19.0 Ω when cold
and 140 Ω when hot. Assume that the resistivity of tungsten varies linearly with
temperature even over the large temperature range involved here, and find the
temperature of the hot filament. Assume the initial temperature is 20.0°C.
4.5 x 10-3 C-1
Sunday, July 24, 2011
45. The cost of electricity varies widely through the United States; $0.120/kWh is
one typical value. At this unit price, calculate the cost of (a) leaving a 40.0-W
porch light on for two weeks while you are on vacation, (b) making a piece of
dark toast in 3.00 min with a 970-W toaster, and (c) drying a load of clothes in
40.0 min in a 5 200-W dryer.
$0.120 $0.120 1kW 1hour $3.33 × 10−8
= =
1kWh 1kWh 1000W 3600secs 1Joule
∆U ∆U 1week 1day 1hour ∆U
(a) P =
∆t
=
2weeks 7days 24hours 3600secs
=
1209600secs
∆U
40.0W =
1209600s
$3.33 × 10−8
∆U = 48384kJ 4.84 × 107 J = $1.61
1Joule
(b) $5.82 × 10− 3
(c) $0.416
Sunday, July 24, 2011
46. Electromotive Force
The electromotive force is denoted as “ε”
A force that moves charges
The emf ε is the maximum possible voltage
that the battery can provide.
ε = ∆V in batteries
Direct current - current that is constant in direction and magnitude
Sunday, July 24, 2011
47. Resistors in Series
∆V
Recall: I=
R
use the equation to
calculate the equivalent
resistance Req
Sunday, July 24, 2011
48. Resistors in Series
Convert
to simple
equivalent
circuit
Sunday, July 24, 2011
49. Resistors in Series
I1 I2
∆V1 ∆V2
Conservation of matter = Current is conserved
I = I1 = I2
Sunday, July 24, 2011
50. Resistors in Series
I1 I2
∆V1 ∆V2
Conservation of matter = Current is conserved
I = I1 = I2
Conservation of energy
∆V = ∆V1 + ∆V2
Sunday, July 24, 2011
51. Resistors in Series
I1 I2
∆V1 ∆V2
Conservation of matter = Current is conserved Ohms Law
I = I1 = I2
∆V
Conservation of energy I=
R
∆V = ∆V1 + ∆V2
Sunday, July 24, 2011
52. Resistors in Series
I1 I2
∆V = I1 R1 + I2 R2
∆V1 ∆V2 ∆V = IR1 + IR2
∆V = I(R1 + R2 )
∆V = IReq
Req = R1 + R2
Conservation of matter = Current is conserved Ohms Law
I = I1 = I2
∆V
Conservation of energy I=
R
∆V = ∆V1 + ∆V2
Sunday, July 24, 2011
53. Resistors in Series
I1 I2
∆V1 ∆V2
Conservation of matter = Current is conserved Ohms Law
I = I1 = I2
∆V
Conservation of energy I=
R
∆V = ∆V1 + ∆V2
Sunday, July 24, 2011
54. Resistors in Parallel
1. Imagine positive charges pass first I1 I2
through R1 and then through%R2.
Compared to the current in R1, the
current in R2 is ∆V1 ∆V2
(a) smaller
(b) larger
(c) the same.
2. With the switch in the circuit of closed (left),
there is no current in R2, because the current
has an alternate zero-resistance path through
the switch. There is current in R1 and this
current is measured with the ammeter (a
device for measuring current) at the right side
of the circuit. If the switch is opened (right),
there is current in R2. What happens to the
reading on the ammeter when the switch is
opened?
(a) the reading goes up
(b) the reading goes down
(c) the reading does not change.
Sunday, July 24, 2011
55. Resistors in Parallel
∆V
Recall: I=
R
use the equation to
calculate the equivalent
resistance Req
Sunday, July 24, 2011
57. Resistors in Parallel
I1
∆V1
I2
∆V2
Conservation of matter = Current is conserved
I = I1 + I2
Sunday, July 24, 2011
58. Resistors in Parallel
I1
∆V1
I2
∆V2
Conservation of matter = Current is conserved
I = I1 + I2
Conservation of energy
∆V = ∆V1 = ∆V2
Sunday, July 24, 2011
59. Resistors in Parallel
I1
∆V1
I2
∆V2
Conservation of matter = Current is conserved Ohms Law
I = I1 + I2
∆V
Conservation of energy I=
R
∆V = ∆V1 = ∆V2
Sunday, July 24, 2011
60. Resistors in Parallel
I1
∆V1 I = I1 + I2
∆V ∆V1 ∆V2
I2 = +
∆V2 R R1 R2
∆V ∆V ∆V
= +
R R1 R2
1 1 1
= +
R R1 R2
Conservation of matter = Current is conserved Ohms Law
I = I1 + I2
∆V
Conservation of energy I=
R
∆V = ∆V1 = ∆V2
Sunday, July 24, 2011
61. Resistors in Parallel
I1
∆V1
I2
∆V2
Conservation of matter = Current is conserved Ohms Law
I = I1 + I2
∆V
Conservation of energy I=
R
∆V = ∆V1 = ∆V2
Sunday, July 24, 2011
62. Recall:
Ohms Law Capacitance
∆V
I= Q = C∆V
R
Series
Parallel
Sunday, July 24, 2011
63. Exercise
Find the current passing through each resistor
Find the voltage drop (potential difference) through each resistor
Sunday, July 24, 2011
64. Kirchhoff’s Rules
Junction Rule
“conservation of matter”
Loop Rule
“conservation of energy”
Σ ∆V = 0
closed loop
Sunday, July 24, 2011
65. Exercise
In solving complicated circuit problems
apply Junction rule first (conservation of current)
You may assign any direction of current as long as it
is reasonable (does not violate common sense!) A
Then apply the loop rule
B
Write down the equations for loop rules concerning
loop A, B, C and the outer loop of the circuit following
C
clockwise direction. (there must be four equations!)
Sunday, July 24, 2011