ELECTRICITY: CHAPTER 7

Section 3 B

Protecting the Building

Two ways to control current in wiring Fuse

Contains a small wire that melts if too much current heats it

Burned out fuse breaks circuit so no more current flows

Circuit Breaker Contains piece of metal that bends if

overheated Switch opens circuit so no more current flows

Electric Power

Power: the rate at which energy is converted from one form to another

Electric power: the rate electricity is converted to another form of energy

Power measured in watts (W)

Electric Power

Power = current x potential difference Watts = amps x volts Power often expressed in kilowatts (kW) P = I V

Electric Energy

Energy used is power multiplied by time Energy = power x time Electric energy use measured in

kilowatt-hours (kWh) E = P t kWh = (kW)(h) The cost of electricity is the energy

times the cost per kWh

Math Skills Activity

You use your fan for 3 hours each day. It has a power rating of 50 W. How much energy does it use in one day? Express your answer in kilowatt-hours.

What we know: P = 50 W t = 3 h

What equation: E = P t

Math Skills Activity Continued Plug in the numbers!

E = (50 W) (3 h) Convert W to kW: Use Mr. Boals’ line! E = (0.050 kW)(3h)

Do the math: E = 0.15 kWh

Check your answer/units: Energy units are kWh!

Practice Problem

A 100-W light bulb has a power rating of 100 W. How much energy in kWh is used when you leave it on for 5 h?

Find the power rating for a hair dryer. How much energy is used if you run it for 12 minutes?