CK-12 Physical Science Concepts - For Middle School

5.73. Electric Power and Electrical Energy Use http://www.ck12.org

Electrical Energy Use

Did you ever wonder how much electrical energy it takes to use an appliance such as a hair dryer? Electrical energy
use depends on the power of the appliance and how long it is used. It can be calculated with this equation:

Electrical Energy = Power×Time

If Miranda uses her 1.8-kilowatt hair dryer for 0.2 hours, how much electrical energy does she use?

Electrical Energy = 1.8 kilowatts×0.2 hours = 0.36 kilowatt-hours

Electrical energy use is typically expressed in kilowatt-hours, as in this example. How much energy is this? One
kilowatt-hour equals 3.6 million joules of energy.

Q:Suppose Miranda were to use a 1.2-kilowatt hair dryer for 0.2 hours. How much electrical energy would she use
then?

A:She would use:

Electrical Energy = 1.2 kilowatts×0.2 hour = 0.24 kilowatt-hours

You can learn much more about electrical energy use—including your own—at the URL below. Use the tables and
other information, along with the handy electrical energy calculator, to estimate how much electrical energy you use.

http://michaelbluejay.com/electricity/howmuch.html

Summary

• Electric power is the rate at which a device changes electric current to another form of energy. The SI unit of
  power is the watt. Electric power can be calculated as current times voltage.


• Electrical energy use equals the power of the appliance multiplied by the amount of time the appliance is used.

Vocabulary

• electric power: Rate at which a device changes electric current to another form of energy.

Explore More

At the following URL, review how to calculate electric power and then solve problems 1-7.

http://webs.rps205.com/curriculum/science/files/A3874CAB2CB74F2B9EEEF914DB22E2EA.pdf

Review

1. Define electric power. What is the SI unit for power?


2. A microwave is the only electric device in a 120-volt circuit. If the circuit has 7.5 amps of current, what is the
   electric power of the microwave?


3. Calculate the electrical energy use of the microwave in question 2 if it used for 0.05 hours.