Power (Cambridge O Level Physics)

• 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

• 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

• Since power is defined as

The rate of doing work

• And work is

Work done = energy transferred

• Then, power can be expressed in equation form as

$P = \frac{W}{t}$ or $P=\frac{∆E}{t}$

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

• Note that these two equations may be written slightly differently, but are representing the same thing - a transfer of energy over time

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

Formula Triangle for Power

Work, power, time formula triangle