 Power and Energy. Power Power means work over a time period Units = Watts Electric power is measured in Watts.

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Power and Energy

Power Power means work over a time period Units = Watts Electric power is measured in Watts

Power Power = Current x Voltage Watts = Amperes x Volts P = I x V 1 kilowatt = 1000 watts

P = I x V V = I x R Combined: P = I 2 R P = V 2 / R AND

Problem 1 A CD player has 0.25 A of current running through it and runs on a 9 V battery. How much power does it use?

P = V x I P = 9 V x 0.25 A P = 2.25 W

Problem 2 How many watts of power does a TV use if its total resistance is 50  ?

V = 120 V (house current) R = 50  P = V 2 /R P = 120 2 /50 P = 14400/50 P = 288 W

The Cost of Electrical Energy

Power and Energy Electrical power costs \$ You are charged by your rate of power usage By Kilowatt Hours (kWh)

Kilowatt Hours Energy = Kilowatt Hour 1 Kilowatt Hour equals consumption of 1 Kilowatt of power in 1 hour.

Kilowatt Hours E = P x t kWh = Kilowatts x time Cost = Rate x kWh Rate varies: 2.25 ¢ to 8.9 ¢

Problem How much does it cost to run a 200 W refrigerator in a month if it runs 4.5 hours per day? (rate = 8.2 ¢/ kWh)

Solution E = P x t t = 4.5 hrs X 30 days = 135 hrs P = 200 W = 0.2 kW

Solve E = 0.2 kW X 135 h = 27 kWh Cost = 8.2 ¢/kWh = 8.2 X 27 = 221.4 ¢ = \$2.21

Problem: Assume the cost of electricity is 13 cents per kilowatt -hour. Calculate the energy used and its cost for the use of the following items: a. 4 Watt night light for 8 hours b. 1300 Watt microwave for 10 minutes c. 60 Watt light bulb 6 hours a day for one year d. 300 Watt refrigerator for 3 hours a day for one month

Assume the cost of electricity is 17 cents per kilowatt -hour. Calculate the power, energy used and its cost for the use of the following items connected to a 120 Volt electric outlet: a. 0.5 Amp computer monitor for 12 hours b. 10 Amp hair dryer for 5 minutes c. 3 Amp electric drill for 30 seconds d. 25 Amp electric clothes dryer connected to 240 Volts for 30 minutes

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