### 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.