### 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.