1. ELECTRICITY: CHAPTER 7 Section 3 B

2. 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

3. 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)

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

5. 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

6. 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

7. 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!

8. 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?