Electric Power and EMF
Electrical Power
Electromotive Force (emf)
Real Batteries
Homework
Electric Power
The rate at which a system loses electric potential
energy as a charge Q moves through a potential dif ference
is
Since power is defined as the rate at which work is
done or energy is converted from one form to another, the electric power is defined as
The unit of power is the watt (W): 1 W = 1 V A = 1
J/s
For a resistor or some other device with resistance
R, we can use Ohm’s law to express the rate of electric energy dissipation due to the resistance as
Example 1
An electron machine delivers 200 A at 250 kV for 1.10
s. (a) What is the power delivered during this time
interval? (b) How much energy is delivered in this time
interval?
Example 1 Solution
An electron machine delivers 200 A at 250 kV for 1.10
s. (a) What is the power delivered during this time
interval?
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(b) How much energy is delivered in this time interval?
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#$ *! !+ ,.-
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Electromotive Force (emf)
Any device that maintains a potential difference in
a circuit is called a source of emf.
Can be thought of as a "charge pump".
The emf 0 of a source is defined as the work done
per unit charge, and thus the SI unit of emf is the
volt.
Figure 21.12
Real Batteries
Real batteries have internal resistance 1
The terminal potential of the battery in the circuit
shown below is
If = 0, then
32 4 65
=0
0
4
1
Real Batteries (cont’d)
From the figure below, we see that the emf provided
by the battery is equal to the voltage drops across
the resistances
0 7 8 1
and the current is
0
8 1
Real Batteries (cont’d)
The rate at which energy is delivered by the battery
is equal to the rate at which energy is converted to
heat in the resistors
0 ! 8 1
Example 2
A battery having an emf of 7.00 V produces a current
of 0.500 A when connected to an external resistance.
If 150 J of energy are produced in 1.00 minute in the
external resistor, determine the values of the internal
and external resistances.
Example 2 Solution
A battery having an emf of 7.00 V produces a current
of 0.500 A when connected to an external resistance.
If 150 J of energy are produced in 1.00 minute in the
external resistor, determine the values of the internal
and external resistances.
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=7 1 0
4
7
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0 4 7 8 1 )
=7 @!7%'>
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7A
Homework Set 18 - Due Friday May14
Read Sections 21.6
Answer Questions 21.10
Do problems 21.15, 21.16, 12.21, 21.23 & 21.24