2/4/2010
1
Chapter 18
s
Direct Current Circuits
So far
s
Current,
resistance,
resistivity, Ohm’s
Law
s
Power
s
Superconductivity
R
V
R
I
P
A
l
R
IR
V
/
2
2
=
=
=
=
ρ
Today
s
Circuits
s
Electromotive force
s
Internal resistance
s
Resistors in series or in parallel
s
Other (more complex) circuits
Sources of emf
s
The source that maintains the current in
a closed circuit is called a source of
emf
s
Any devices that increase the potential
energy of charges circulating in circuits are
sources of emf
s
Examples include batteries and generators
s
SI units are Volts
s
The emf is the work done per unit charge
emf and Internal
Resistance
s
A real battery has
some internal
resistance
s
Therefore, the
terminal voltage
is not equal to the
emf
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More About Internal
Resistance
s
The schematic shows
the internal
resistance, r
s
The terminal voltage
is ∆V = V
b
V
a
s
∆V = ε – Ir
s
For the entire circuit,
ε = IR + Ir
Internal Resistance and
emf, cont
s
ε is equal to the terminal voltage
when the current is zero
s
Also called the
opencircuit voltage
s
R is called the
load resistance
s
The current depends on both the
resistance external to the battery
and the internal resistance
Internal Resistance and
emf, final
s
When R >> r, r can be ignored
s
Generally assumed in problems
s
Power relationship
s
I
ε
= I
2
R + I
2
r
s
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 Spring '08
 Avery
 Physics, Conductivity, Current, Resistance, Superconductivity, Force, Power, Resistor, Electrical resistance, Series and parallel circuits, equivalent resistance

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