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II. Electric current
1. Definition
Q
I
t
Units: [ I ] = 1A = 1 C/s
Conventional
current
Electron
flow
Example: 1020 electrons passed through the electric conductor during 4
seconds. Find the electric current through this conductor.
q (1.6 10 19 C )(10 20 )
I 
 4A
t
4s
Example: The electric current of 0.5 A is flowing through the electric conductor.
a) What electric charge is passing through the conductor during each second?
b) What electric charge will pass through the conductor during 1 minute?
a)
q  It  (0.5 A)(1 s)  0.5 C
b)
q  It  (0.5 A)(60 s)  30 C
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2. Ohm’s Law
Nonohmic device
I
I
V
V
3. Resistance
Definition:
V
I
R
V
R
I
Units: [ R ] = 1Ω = 1 V/A
Ohm’s Law:
R  const
V  IR
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4. Resistivity
Resistivity:
I
A
L
L
R
A
A
R
L
Example: What is the resistance of 1 m of nichrome wire of 2 mm diameter?
L
1m
6
3
R    10   m

3

10

2

2
A
 10 m


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5a. Resistors in series
Two resistors R1 and R2 are in series when they are connected one after the other
I
R1
V1
R2
V
I  I1  I 2
V2
Req
V
V  IReq
V  V1  V2
IReq  IR1  IR2
I
Req  R1  R2
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5b. Resistors in parallel
Two resistors R1 and R2 are in parallel when they are connected to the same
potential difference
I1
R1
I
I
I
I2
R2
V  V1  V2
V
I  V / Req
I  I1  I 2
V / Req  V / R1  V / R2
Req
1
1
1
 
Req R1 R2
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Example: A 2 A current flows into a series combination of a 3 Ω and
a 4 Ω resistors. What is the voltage drop across the 4 Ω resistor?
I  2A
R1  3
V2  IR2  2 A  4  8V
R2  4
V2  ?
Example: A 3 A current flows into a parallel combination of 4 Ω, 6 Ω, and
12 Ω resistors. What current flows through the 12 Ω resistor?
I  3A
1
1
1
1
1
1
1
1
 





 Reff  2
Reff R1 R2 R3 4 6 12 2
R1  4
1)
R2  6
2) V  IReff  3 A  2   6V
R3  12
I3  ?
3) I 3 
V
6V

 0.5 A
R3 12
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Symbols for circuit elements
Conductor (wire) with negligible resistance
Resistors
- +
Battery
Switch
Ground
V
A
Voltmeter (should be connected in parallel)
Ammeter (should be connected in series)
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6a. Ammeter (used to measure current)
A
• Ammeter should be connected in series
• Ammeter has very small (ideally zero) internal resistance,
so it does not affect the current to be measured
6b. Voltmeter (used to measure voltage or potential difference)
A
V
B
• Voltmeter should be connected in parallel
• Voltmeter has very large (ideally infinite) internal resistance, so
that very little current is diverted through it.
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6c. Voltmeter and Ammeter
•
•
•
•
Voltmeter should be connected in parallel
Ammeter should be connected in series
Voltmeter has very large (ideally infinite) internal resistance
Ammeter has very small (ideally zero) internal resistance
Example: What connection is not correct?
V
A
C
A
A
V
A
B
V
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