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

240×0.5

120 watts

Question 2

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

Solution=P=VxI

  • 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

Solution: 

  • 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?

Solution: 

a) Power= Volts x Amps

0.5×240
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.


Quick NetOne, Telecel, Africom, And Econet Airtime Recharge

If anything goes wrong, chat with us using the chat feature at the bottom right of this screen