Power (Edexcel GCSE Physics: Combined Science)

• Machines, such as car engines, transfer energy from one energy store to another constantly over a period of time

• The rate of this energy transfer, or the rate of work done, is called power

• Two electric motors:
  • lift the same weight
  • by the same height
  • but one motor lifts it faster than the other

• The motor that lifts the weight faster has more power

Two motors with different powers

• Power ratings are given to appliances to show the amount of energy transferred per unit time
• Common power ratings are shown in the table below:

Power Ratings Table

• Power is defined as

energy transferred per unit time

• And 

work done = energy transferred

• Therefore, power is also

work done per unit time

• Power can be calculated using the following equation:

• Where:
  • P = power, measured in watts (W)
  • E = energy transferred or work done, measured in joules (J)
  • t = time, measured in seconds (s)

• This equation can be rearranged with the help of a formula triangle:

Work, power, time formula triangle

• The watt is the unit of power
• Since power is energy transferred per second, the watt can also be defined as 1 joule per second

1 W = 1 J / s 

• 1 kilowatt (1 kW) is equal to 1000 watts, or 1000 joules of energy transferred per second (1 kJ / s)