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 VV = I x R

Combined:P = I2 RP = V2 / R

AND

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

P = V x IP = V x I

P = 9 V x 0.25 AP = 9 V x 0.25 A

P = 2.25 WP = 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 = V2/R

P = 1202/50

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 tt = 4.5 hrs X 30 days= 135 hrsP = 200 W = 0.2 kW

SolveE = 0.2 kW X 135 h

= 27 kWhCost = 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