Usefulness of Electrical Devices

power = energy transferred/time taken

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  • Created by: Harri
  • Created on: 10-06-11 11:43


  • energy is measured in joules (J)
  • power is the reat of energy transfer. power = energy transfered/time
  • power is measured in watts (W)
  • one watt is an energy transfer of one joule per second (J/s)
  • example
  • a kettle transfers 6000 J of energy in 3 secs
  •        6000/3= 2000W or 2kW
  • work is a name for energy transfer, therefore the formula for power can also be written as power= work/time
  • current = rate of flow of electrons
  • volatage = joules per coulomb of energy
  • electrical power = voltage x current (P=IV)
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The Cost of Electricity

energy transfered (kWh) = power (kW) x time (h)

kilowatt-hours are also known as units

example a 2000W electric fire is used for 3 hours. what is the cost at 8p per unit?

energy transferred = 2kW x 3 hours = 6kWh (6 units)

6 x 8 = 48p

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