Power (OCR GCSE Physics A (Gateway))

Power

Definition of Power

• 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

• Time is an important consideration when it comes to power

• Two cars transfer the same amount of energy, or do the same amount of work to accelerate over a distance

• If one car has more power, it will transfer that energy, or do that work, in a shorter amount of time

Two cars accelerate to the same final speed, but the one with the most power will reach that speed sooner

• 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 Rating Table

Calculating Power

• 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, measured in joules (J)

  • W = 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

• 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)