Electrical Energy and Power
Quick revise

Energy and Power

Electrical appliances at home transfer energy from the mains supply to heat and light our homes as well as to operate our appliances such as TV, Microwave and Computers etc.

The energy used is constant so a TV will use double the amount of energy in two hours as it will in one hour.  The power of an electrical appliance tells us how much electrical energy it transfers in a second.

Power, P is measured in watts (W) where:
1 W = 1 J/s (joule/second).

Appliances used for heating have a much higher rating than those used to produce light or sound.

The amount of energy transferred from the mains appliance depends on the power rating of the appliance and the time for which it is switched on. Energy transferred from electricity is worked out by:
Energy = power × time
E = P × t

Energy, E is measured in:

  • joules (J) when the power is in watts and the time, t, is in seconds.
  • kilowatt hours (kWh) when the power is in kilowatts and the time, t, is in hours.

Example: A 800 W toaster is switched on
for one minute. The energy used is:
E = 800 W × 60 s
E = 48000 J
Example: A 1.8 kW kettle used for 5 minutes:
E = 1.8 ×   5
= 0.15 kWh

This video explains how a modern light bulb works.

Paying for Electricity

The units on an electricity bill, and measured by an electricity meter, are kilowatt hours. The cost of each unit of electricity varies. The electricity bill is calculated by working out the number of units used and multiplying by the cost of a unit.

Cost of electrical energy used =
power in kW × time in hours × cost of one unit

Cost = number of kW h used × cost of one unit

Example: The 1 KW microwave is used for half an hour and the cost of a unit is 22p:
Cost = 1 kW × 0.5 hours × 22 p/kW h
Cost = 11p

Power, Current and Voltage

The mains voltage in the UK is 230 V. Electrical power depends on the current and the voltage:

Power = current × voltage
P = I × V

Power is measured in watts (W), current, I, in amps (A) and voltage, V, in volts (V).

A torch with a 3.0 V battery has a current of 0.4 A.

Its power is: P = 3.0 × 0.4 = 1.2 W

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