Electrical appliances
Appliances, power and energy
All electrical appliances transfer energy from one energy storeThe different ways in which energy can be stored, including chemical, kinetic, gravitational potential, elastic potential and thermal stores. to another, for example a chemical energyThe energy stored in a fuel source, such as petrol or diesel. Measured in Joules (J). Eg the car filled up on chemical energy when it went to the garage. store in the fuel in power stations. This is transferred into a kinetic energyEnergy which an object possesses by being in motion. store of a fan or an internal energy store in a cooker.
The amount of energy transferred depends on the powerThe energy transferred each second, measured in watts (W). Power = work done ÷ time taken. (the energy transferred each second) and the amount of time the appliance is switched on for. The power of an appliance can be calculated using the equation:
\( power = \frac{energy \ transferred}{time}\)
\( P = \frac{E}{t}\)
This is when:
- energy (E) is measured in joules (J)
- power (P) is measured in watts (W)
- time (t) is measured in seconds (s)
One watt is the power when one joule of energy is transferred in one second.
Learn more on energy transfers in everyday appliances in this podcast
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Example
A hair dryer has a power rating of 1,500 W. How much energy is transferred by the hair dryer in 15 minutes?
\( E = P \times t\)
\( E = 1,500 \times (15 \times 60)\)
\( E = 1,500 \times (900)\)
\(E = 1,350,000~J~or~ 1.35~ megajoules~ (MJ)\)
Power can also be calculated using the equation:
power = potential difference × current
\( P = V \times I\)
This is when:
- power (P) is measured in watts (W)
- potential difference (V) is measured in volts (V)
- current (I) is measured in amps (A)
This means that the energy transferred by an electrical appliance can also be calculated from a combination of the equations above:
\( E = V \times I \times t\)
This is when:
- potential difference (V) is measured in volts (V)
- current (I) is measured in amps (A)
- energy (E) is measured in joules (J)
- time (t) is measured in seconds (s)
Example 1
What is the power rating of an electric heater that draws a current of 12.5 amps (A) when plugged into the mains?
\( P = V \times I\)
\( P = 230 \times 12.5\)
\( P = 2,875 \ W\)
The power rating of the heater is 2,875 W.
Example 2
If the electric heater is used for 15 minutes, how much energy is transferred in that time?
\( E = V \times I \times t\)
\( E = 230 \times 12.5 \times (15 \times 60)\)
\( E = 2,875 \times (900)\)
\( E = 2,587,500 \ J \ or \ 2.59 \ MJ\)
