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Electrical power

When is done on an object, energy is transferred. The at which this energy is transferred is called . So the more powerful a device is, the more energy it will transfer each second. This is important when considering how much an electricity bill might cost.

Calculating electrical power

Power can be calculated using:

\(power = \frac{work~done}{time}\)

This is when:

  • power is measured in watts (W)
  • work done is measured in joules (J)
  • time is measured in seconds (s)

The power output of a device increases if:

  • more energy is transferred in a given time
  • the same amount of energy is transferred in a shorter time

The energy transferred by an electrical device can be investigated using a joulemeter. The power output of a device can be calculated if the length of time the device is switched on is also measured.

Example

An electric lamp transfers 1,200 J in 2 minutes. Calculate its power.

2 minutes = 2 脳 60 = 120 s

power = 1,200 梅 120

= 10 W

Calculating energy transferred

Joules and seconds

The energy transferred can be calculated using:

energy transferred = power 脳 time

This is when:

  • energy transferred is measured in joules (J)
  • power is measured in watts (W)
  • time is measured in seconds (s)

Example

A 250 W computer is used for 10 minutes. Calculate the energy transferred.

10 minutes = 10 脳 60 = 600 s

energy transferred = 250 脳 600

= 150,000 J (150 kJ)

Kilowatt-hours and hours

The electricity bills from an energy supplier show the energy used in kWh, rather than in J or KJ.

The energy transferred is still calculated using:

energy transferred = power 脳 time

But:

  • energy transferred is measured in kilowatt-hours (kWh)
  • power is measured in kilowatts (kW)
  • time is measured in hours (h)

Example

A 10 kW shower is used for 12 minutes. Calculate the energy transferred in kWh.

12 minutes = 12 梅 60 = 0.2 h

energy transferred = power 脳 time

= 10 脳 0.2

2 kWh

Learn more on energy transfers in everyday appliances in this podcast