Power and Efficiency

Card 1: What is power?


Power is the rate at which work is done, which means how fast a work is done.


Example 1

An electric motor takes 20 s to lift a box of mass 20kg to a height of 1.5 m. Find the amount of work done by the machine and hence find the power of the electric motor.


Work done,
W = mgh = (20)(10)(1.5) = 300J


  P = \frac{W}
{t} \hfill \\
  P = \frac{{300}}
{{20}} = 15W \hfill \\ 


Card 2: Efficiency


The efficiency of a device is defined as the percentage of the energy input that is transformed into useful energy.

Card 3: Efficiency Example


In the example above, the input power is 100J/s, the desire output power (useful energy) is only 75J/s, the remaining power is lost as undisire output. Therefore, the efficiency of this machine is

75/100 x 100% = 75%

Card 4: Ways of increasing the efficiency - Air conditioner

Air conditioner

  • Switch off the air conditioner when not in use.
  • Buy the air conditioner with suitable capacity according to the room size.
  • Close all the doors and windows of the room to avoid the cool air in the room from flowing out.
Card 5: Ways of increasing the efficiency - Regrigerator


  • Always remember to close the door of refrigerator.
  • Open the refrigerator only when necessarily.
  • Always keep the cooling coil clean.
  • Defrost the refrigerator regularly.
  • Choose the refrigerator with capacity suitable for the family size.
  • Refrigerator of large capacity is more efficient compare with refirgerator of small capacity.
Card 6: Ways of increasing the efficiency - lamp or light bulb

Lamp or Light Bulb

  • Use fluorecent bulb rather than incandescent bulb. Fluorescent bulbs are much more efficient than incandescent bulbs.
  • Use a lamp with reflector so that more light is directed towards thr desirable place.
Card 7: Ways of increasing the efficiency - Washing Machine

Washing Machine

  • Use front-loading washing machine rather than top-loading wahing machine because it uses less water and electricity.
  • Use washing machine only when you have sufficient clothes to be washed. Try to avoid washing small amount of clothes.
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