Power (CIE IGCSE Physics: Co-ordinated Sciences (Double Award))

• Power is:

Work done per unit time

• Since work done is equal to energy transferred, power is also:

Energy transferred per unit time

• 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 power

Power ratings

• 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

• 1 kW = 1000 W (1 kilowatt = 1000 watts)
• 1 MW = 1 000 000 W (1 megawatt = 1 million watts)

The power equations

• There are two equivalent forms of the power equation

• Power can be expressed in terms of work done

• Or power can be expressed in terms of energy transferred

• Where:
  • = power measured in watts (W)
  •  = work done, measured in newton metres (N m)
  •  = time measured in seconds (s)
  •  = energy transferred, measured in joules (J)

• Note that these two equations may be written slightly differently, but they represent the same thing
  • A transfer of energy (work done) over time

Power equation triangle

To use a formula triangle, simply cover up the quantity you wish calculate and the structure of the equation is revealed

• Using an equation triangle is covered in more depth in the revision note Speed & velocity