Module 4, Lecture 4: Electric Current and Resistance
Current – any motion of ___________ from one region to another.
____ - standard symbol used for current.
I
Current
Current density
Current units: _________
dQ
dt
J
Current density units: ______
I
A
DC - ______________ current – always in the same direction
AC - ______________ current – continuously changes directions
EF 152 2016 Lecture 4-4
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Electric Current Direction
By convention, current is
defined as flowing from + to –
out

in

Electrons actually flow in the
______ direction
We will use the conventional current flow
(which was established well before the discovery of
electrons).
EF 152 2016 Lecture 4-4
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Resistance
Resistivity at Room
Temperature
For many materials, the current is
proportional to the potential (V)
Material
Conductors
V  IR
Silver
Copper
Aluminum
Steel
A material in which V =
is
said to follow “_______ Law”
L
: ___________
A
L: _____________
1.47 x 10-8
1.72 x 10-8
2.75 x 10-8
20 x 10-8
Semiconductors
Graphite
Silicon
V

Ohm
A
: _____________
R
(·m)
3.5 x 10-5
2300
Insulators
108 – 1011
1010 – 1014
> 1013
Wood
Glass
Teflon
A: _____________
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Resistivity and temperature
Resistivity increases with
increasing temperature
Conductors
Resistivity decreases with
increasing temperature
Semiconductors
R  R0 1  T 
:
temperature coefficient of resistance
A copper wire increases
temperature by 20°C. What
is the increase in resistance?
EF 152 2016 Lecture 4-4
At temperatures below
the resistivity is zero
Superconductor
Material
α (1/°C)
Conductors
Silver
Copper
Aluminum
Steel
3.8
3.9
3.9
5.0
x
x
x
x
10-3
10-3
10-3
10-3
Semiconductors
Germanium
Silicon
-50 x 10-3
-70 x 10-3
4
Examples:
An 18 gauge lamp cord copper wire has a diameter of
1.02 mm and a cross sectional area of 8.17x10-7m2.
Determine the resistance of a 15 ft (4.57 m) length of wire.
1.72
10
Ω ∙ m
A typical circuit breaker will be tripped if more than 20 amps
pass through it. For a 120 V circuit, determine the minimum
resistance to keep the circuit breaker from tripping.
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Energy and power in circuits
P  Vab I
Power delivered to a resistor
Units of Watts (W) 1 W =
AC vs. DC
AC power is easier to _____
3000 W generator; 2Ω wire
Volt
Amp
Loss
100
1000
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Electromotive Force
 A ___________ circuit is required for a steady current flow.
 Electromotive force (emf, ) causes current flow from
lower to higher potential.
 emf is not really a force – it is an energy/charge
quantity like potential.
 Sources of emf: ________________________
Determine the current flow in the circuit.
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Example:
Determine the power dissipated
in the 4 Ohm resistor.
Two light bulbs operate at 120 V, but one has a power rating of
25 W while the other has a power rating of 100 W. Which one
has the greater resistance?
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