Electrical appliances

Appliances, power and energy

All electrical appliances transfer energy from one store to another, for example a chemical energy store in the fuel in power stations. This is transferred into a kinetic energy store of a fan or an internal energy store in a cooker.

The amount of energy transferred depends on the power (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.

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Time should be converted from minutes into seconds - this is done by multiplying the number of minutes by 60.

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)
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When working with mains electricity and appliances, the potential difference is 230 V.

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

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