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B         MainElectricityElectric Circuits � Lesson III.3.3   ### Power - Lesson III.3.3

 Key Terms: Power | Power can be defined as the rate at which energy is used. The unit for power is the watt. Other relationships for power in an electric circuit are:

• • •   The power rating on electrical appliances gives a comparative indication of the cost of operating those appliances (not taking efficiencies into account).

Energy consumed by an appliance depends on

1. its power rating
2. the length of time it is operating

E = Pt

The units usually used to measure electrical energy consumption are:

• the megajoule (MJ) ~ 1MJ = 1 x 106 J
• the kilowatt.hour (kW.h) ~ 1 kW.h = 3.6 x 106 J

The cost of using electricity can be calculated if the amount of energy used is known.

E = Pt
E = VIt

For example:

A toaster uses 9 A when connected on a 110 V circuit. How much does it cost to operate it 0.5 hours per day for 30 days if electricity costs 7 cents per kW.h?

Soultion: Try the following questions:

 Note: When entering answers use two decimal places (with answers that have a decimal, round the 2nd digit). Enter in the correct units [Watts - W, Ohms - Ohms, Kilograms - Kg]. If a voltage source of 10 V is connected across a 9? resistor, the power supplied to the resistor is   A kettle, connected to a 110 V supply, draws a 14 A current. If the electricity costs 6 cents per kW.h, the cost of running the motor for 15 hours is is equivalent to   1N.s divided by 1m/s is equivalent to         