Power (Cambridge (CIE) O Level Physics): Revision Note

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

Vehicles with Different Powers

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 is the rate of transfer of energy, or rate of work done. The higher power motor will do the same work in a shorter time.

• 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

• 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