1. Power, Energy and Electrical Safety

2. Power = Current x Voltage
Electric Power
The measure of the rate at which electric energy does work or provides energy
Power = Current x Voltage
Unit = Watts (W)
Large quantities measured in kilowatts (kW)

3. Electric Power Power (P) = Watts Current (I) = Amps
Voltage (V) = Volts

4. P V I
Practice Problem 1
A toaster oven is plugged into an outlet that provides a voltage difference of 120 V. What power does the oven use if the current is 10 A?
1200 W

5. P V I
Practice Problem 2
A VCR that is not playing still uses 10.0 W of power. What is the current if the VCR is plugged into a 120-V electrical outlet?
1200 W

6. P V I
Practice Problem 3
A flashlight bulb uses 2.4 W of power when the current in the bulb is 0.8 A. What is the voltage difference?
1200 W

7. P V I
Practice Problem 4
A circuit breaker is tripped when the current in the circuit is greater than 15 A. If the voltage difference is 120 V, what is the power being used when the circuit breaker is tripped?
1200 W

8. Electrical Energy
Electric companies charge a certain amount for electricity
The bill is based on how much energy is used in the household
Electric meter records this amount of energy

9. Electrical Energy Energy (E) = kilowatt hours (kWh)
Power (P) = kilowatts (kW)
Time (t) = seconds

10. E t P
Practice Problem 1
A microwave uses 1,200 W for 0.25 hr. How much electrical energy is used by the microwave?
1200 W

11. E t P
Practice Problem 2
A refrigerator operates on average for 10.0 h a day. If the power rating of the refrigerator is 700 W, how much electrical energy does the refrigerator use in one day?
1200 W

12. E t P
Practice Problem 3
A TV with a power rating of 200 W uses 0.8 kWh in one day. For how many hours was the TV on during this day?
1200 W

13. E t P
Practice Problem 4
Suppose electricity costs $0.08 per kilowatt-hour. How much would it cost to run a 100-watt light bulb for 5 hours?
1200 W

14. E t P
Practice Problem 5
Calculate the monthly cost of using a 700-W refrigerator that runs for 10 h a day if the cost per kWh is $0.09.
1200 W

15. Electrical Safety
Never handle electrical appliances if your hands are wet or you are standing in water.
Never run wires under carpets.
Never overload a circuit by attaching too many appliances to it.

16. Electrical Safety
Always repair worn or frayed wires to avoid short circuits.
Never stick your finger in an electric socket or a utensil in an appliance that is plugged in.
Never touch wires that have come loose from power poles or buildings