Electrical power

Electrical energy itself is not a useful form of energy for us, but it supplies energy to our appliances which transform it into other forms of energy which are useful.

An electric fire, a cooker, a toaster and an electric kettle all convert electrical energy into heat energy.

The power of an appliance is the rate at which energy is transferred.

  • \[Power = \frac{{energy}}{{time}}\]
  • \[P = \frac{E}{t}\]
  • The symbol for power is \(P\), it is measured in watts \((W)\)
  • The symbol for energy is \(E\), it is measured in joules \((J)\)
  • The symbol for time is \(t\), it is measured in seconds \((s)\)

Appliances in the home have their power ratings stated on their base plates:

Power rating plate on electrical apparatus
Power rating plate on electrical apparatus

Typical power ratings include:

AppliancePower rating \[(W)\]
Kettle2000
Microwave oven (cooking power of \[800\])1000
Toaster900
Hair dryer1200
Fridge135

Example

Question

Calculate the power rating of a hair dryer that is used for 2 minutes and uses 100 kilojoules of energy.

To calculate the power rating you need to take the following steps:

\[power = energy \div time\]

\[= 100000 \div (2 \times 60)\]

\[= 830 watts\,(830 W)\]

\[Energy = 830 W\]