Power is a measure of how quickly energy is transferred. Efficiency is a measure of how much useful energy or power is transferred. By considering where energy is ‘wasted’ we can improve efficiency.

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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:

Typical power ratings include:

Appliance | Power rating \[(W)\] |
---|---|

Kettle | 2000 |

Microwave oven (cooking power of \[800\]) | 1000 |

Toaster | 900 |

Hair dryer | 1200 |

Fridge | 135 |

- 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:

\[power = energy \div time\]

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

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

\[Energy = 830 W\]