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
- its power rating
- 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
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?
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