ZIMSEC O Level Combined Science Notes: Electric Power equations and calculations

  • Power calculations that you are going to be presented with in the exam are varied
  • They all depend on your knowing:
  • The equation to calculate whatever variable that you are required to calculate
  • Your ability to substitute and change the subject of the formula to suit your needs
  • Below are some of the calculations that you might encounter during your examinations

Question 1

What is the power of an electric light operating on a 240V and using a current of 0.5A?

Solution: P=VxI


120 watts

Question 2

What current is drawn by an electric heater rated 2 KW operating on a 200V supply?


  • I=\dfrac{P}{V}
  • I=\dfrac{2000}{200}
  • 10 \quad Amps
  • Note 2kW is equal to 2000 watts

Question 3

A 2kW, 250V electric heater is used for 5 hours.

a) Calculate how much current is drawn

b) Calculate the heat energy produced


  • a) Use the equation above:
  • I=\dfrac{2000}{250}
  • 8 Amps
  • b) Heat Produced is found using the formula:
  • Heat=volts x amps x seconds
  • 250 x 8 x 18000(5 hours in seconds)
  • 36000000 Joules
  • 36000kJ

Question 4

An electric iron is rated at 600 watts when used on a 200 Volts mains power supply:

a) Calculate how much heat it produces in 30 minutes

b) What is the resistance of the electric iron?

Solution: Heat=V*A*t(in seconds)

600 x 200 x 1800
216000000 Joules
216000 kJ

b) First find the current being drawn by the iron:

  • I=\dfrac{P}{V}
  • \dfrac{600}{200}
  • 2 Amps

Resistance is found using the formula:

  • R=\dfrac{V}{I}
  • \dfrac{200}{2}
  • 100\Omega

Question 5 

A light bulb draws 0.5 Amps from a 250V supply.

a) What is the power of the light bulb?

b) How much heat energy does it produce in 2 hours?

c) What is the resistance of the light bulb?


a) Power= Volts x Amps

120 Watts

b) Energy produced= 120 * 7200
864000 Joules
864 kJ

c) Resistance is:

  • R=\dfrac{V}{I}
  • \dfrac{240}{0.5}
  • 480\Omega

Question 6

An electric washing machine draws a current of 10 A. If the resistance of the motor is 10 Ohms what is the power of the motor?

Solution: P=I^{2}R

  • 10^{2}*10
  • 1kW

To access more topics go to the Combined Science Notes page.