Electric Power
Answer Me!!!

How is the brightness of a light bulb
measured? What do you think is the
significance of this?
Electric Power
Power is the rate at which work is done or
energy is expended.
 Electrical Power is the rate at which
electrical energy is expended.

Electrical Power


Electric power is the product of electric current
and potential difference
P = V·I
Power is still measured in Watts:
1volt  1ampere
1
Joule
Coulomb
1volt  1ampere
1
Joule
second
1
Coulomb
second
 1 watt
Electrical Power and Ohm’s Law
P  VI  I R 
2
V
2
R
Electrical Energy

Since work is define by:
W  Pt
2
W  VIt  I Rt 
2
V t
R

Electrical energy is measured in Joules.
Energy is a scalar quantity.
Practice Problem 1

A microwave oven operating at 120 V is
used to heat a hot dog. If the oven draws
12.5 A of current for 45 seconds, what is
the power dissipated by the oven?
Practice Problem 2

If the potential drop across an operating
300 W floodlight is 120 V, what is the
current through the floodlight?
Practice Problem 3

A potential drop of 50 V is measured
across a 250 ohm resistor. What is the
power developed in the resistor?
Practice Problem 4

As the potential difference across a given
resistor is increased, the power expended
in moving the charge through the resistor
will _____________.
Practice Problem 5

A toaster dissipates 1500 W of power in 90
seconds. What is the amount of electric
energy used by the toaster?
Practice Problem 6

An electric iron operating at 120 V draws
10 A of current. How much heat energy is
delivered by the iron in 30 seconds?
Practice Problem 7

An electric dryer consumes 6.0 x 106 J of
energy when operating at 220 V for 30
minutes. During operation, the dryer
draws how much current?
Practice Problem 8

An operating electric heater draws a
current of 10 A and has a resistance of 12
ohms. How much energy does the heater
use in 60 seconds?