electric power is the rate at which the energy is absorbed or delivered.

2. • Energy is the ability or the capacity to do work.
• It is a quantity that can take on many forms,
among them heat, light, sound, motion of
objects with mass.

3. Power is the rate at which the energy is
absorbed or delivered.
• Mathematically, power is defined as:
Current, typically in Ampere
[A]
Power,
typically in
Watts [W]
P
VI
Voltage, typically in Volts [V]

4. Example 1
How much power is expended in the resistor if
a voltage of 220 V across a resistor causes a
current of 10 A to flow through the resistor?
Given:
I = 10 A
V = 220 V
Solution:
P VI
P
P
( 220V )(
10 A)
2200
W

5. Example 2
A 700 watt hair dryer is connected to 220 V
circuit. How much current does it draw?
Given:
P = 700 W
V = 220 V
P
Solution:
I
V
I
I
700W
220V
3.18 A

6. Another expression for electric power may be
written in terms of resistance by:
Current, typically in Ampere (A)
P
Resistance, typically in Ohms
( )
P
2
I R
Voltage, typically in Volts (V)
2
V
R
Resistance , typically in Ohms
( )

7. Example 3
What is the resistance of a 25.0 W
incandescent bulb connected at 220 V?
Given:
P = 25.0 W
V = 220 V
V
P
Solution:
R
2
R
R
R
V2
P
( 220V ) 2
25.0W
1,940

8. Example 4
Calculate the resistance of a 1000 Watt
microwave designed for 27 Volts.
Given:
P = 1000 W
V = 27 V
Solution:
V2
R
R
R
P
( 27V ) 2
1000 W
0.729

9. Activity
1. A 60 W light bulb is connected to a 120 V
plug. What is the current in the light bulb?
2. What is the power rating of a light bulb when
0.8 A flow through it when connected to a
120 V outlet?
3. An alarm clock uses 5.0 W of electric power. If
the clock is plugged into a 120 V outlet, what
electric current is in the clock’s circuit?

10. 4. A nightlight uses 4.00 W of power when
plugged into an outlet. Assume that the only
resistance in the circuit is provided by the light
bulb’s filament. The current in the circuit is
3.40 × 10^−2 A. What is the voltage across the
filament?
5. A generator at a power plant produces
electricity with a voltage of 2.5 × 10^4 V and a
current of 20.0 A. How much power does the
generator produce?
6. Calculate the resistance of an electric blanket
that draws 9 A to transform 50 W of energy.

11. Quiz
A computer with a resistance of 57.5 Ω has
a power input of 230.0 W. Calculate the
voltage across and current in the computer,
using the formulas relating power to
resistance.