Electric Power
Chapter 17
Lesson 4
p. 618
Source of Current: Potential Difference
• If no potential difference:
- Free electrons in a conductor move randomly
when all points in the conductor are the same
potential.
• If potential difference is applied:
- Free electrons in a conductor move from a
higher electric potential to a position of lower
electric potential.
* Potential difference maintains current in a
circuit.
Batteries and generators supply energy
to charge carriers.
• Battery – maintains a potential difference
across its terminals by converting chemical
energy to electric potential energy.
• Generator – converts mechanical energy into
electrical energy.
Current can be direct or alternating.
• There are two types of current:
1) Direct current (DC) – charges move in only
one direction with negative charges moving
from a lower to higher electric potential.
- direction of current does not change
2) Alternating current (AC) – the terminals of
the source of potential difference are
constantly changing sign.
- direction of current continually changes
Energy transfer
Energy transfer
• A charge leaves the battery at A with a
certain amount of electrical potential
energy. The charge loses this energy
while moving from B to C, and then
regains the energy as it moves through
the battery from D to A.
Electric Power is the rate of conversion
of electrical energy.
Electric Power (P) is the rate at which charge
carriers convert electrical potential energy to
nonelectrical forms of energy.
Electric Power = Current x Potential Difference
Formulas:
(1)
(2)
(3)
P  I  V
PI R
2
P
( V )
R
2
Practice E Problems
1. A 1050 W electric toaster operates on a
household circuit of 120 V. What is the
resistance of the wire that makes up the
heating element of the toaster? (#1, p. 621)
2. A calculator is rated at 0.10 W and has an
internal resistance of 22 Ω. What battery
potential difference is required for this
device? (#3, p. 621)
Practice E Problems
3. An electric heater is operated by applying a
potential difference of 50.0 V across a wire of
total resistance 8.00 Ω. Find the current in the
wire and the power rating of the heater.
(#4, p.621)
4. The operating potential difference of a light
bulb is 120 V. The power rating of the bulb is
75 W. Find the current in the bulb and the
bulb’s resistance. (#56,Chapter Review,p. 628)
Electric Power
• The SI unit of power is the watt, W.
• Electric companies measure energy consumed
in kilowatt-hour. One kilowatt-hour (kW  h)
is the energy delivered in 1 h at the constant
rate of kW.
• Electrial energy is transferred at high potential
differences to minimize energy loss.
Homework
1. A computer is connected across a 110 V
power supply. The computer dissipates 130 W
of power in the form of electromagnetic
radiation and heat. Calculate the resistance of
the computer. (#55, Chapter Review, p. 628)
2. A small electronic device is rated at 0.25 W
when connected to 120 V. What is the
resistance of this device?
(#2, Practice E, p. 621)