Worksheet Overview

QUESTION 1 of 10

When an electrical appliance is on and working, it uses electricity from the mains supply. Electrical energy is transferred from the mains to the appliance. The rate at which energy is transferred is measured in kilowatt hours (kWh). It can be calculated by multiplying the appliance power with the time it is being used.

energy transferred = power × time

Note that since the unit for energy transferred to an appliance is kWh, when using the equation, you must make sure power is in kW and the time is converted into hours.

Electricity meters (see diagram below) are used at home to measure the energy transferred to all electrical appliances used at that particular house or building. The total cost is calculated using this formula:

total cost = number of units × cost per unit

In the formula one unit is 1 kWh.

It is essential that energy is used efficiently by everyone nowadays, as excessive use causes more pollution to the environment and costs more. All appliances waste some energy when on, so only a proportion of the energy is utilised for the actual intended use. For example, a light bulb uses electricity when lit, but some of the output energy is heat, which is not actually needed from a light bulb. Heat is wasted into the environment. The more input energy (electricity) is utilised by an appliance for the intended use, the more efficient an appliance is.

Sankeydiagrams show the input and output energy of appliances; you will see the Sankey diagram for a light bulb below. The electrical energy transferred to the light bulb is 100 J. 75 J are actually converted to light energy, which is the intended use of a light bulb. 25 J are wasted as heat.

What type of energy is used by most appliances at home?

heat

kinetic

electrical

What is the unit of the energy transferred to electrical appliances by mains electricity?

J

W

kWh

Calculate the energy transferred to a 2 kW appliance over 3 hours.

1 kWh

5 kWh

6 kWh

Calculate the energy transferred to a 500 W appliance over 3 hours.

1500 kWh

1.5 kWh

4.5 kWh

Calculate the energy transferred to a 1 kW appliance over 30 minutes.

0.5 kWh

30 kWh

31 kWh

What does an electricity meter measure?

watt

joules

kilowatt-hours

Calculate the cost of electricity in pence for a particular household, if one unit costs 10 pence and 10 units have been used.

1 penny

20 pence

100 pence

Calculate the cost of electricity in pounds for a particular household, if one unit costs 10 pence and 10 units have been used.

1 pound

20 pounds

100 pounds

Calculate the cost of electricity in pounds for a particular household, if one unit costs 20 pence and 10 units have been used.

1 pound

2 pounds

200 pounds

What is essential to do in order to save fuels and pay less for electricity?

Use less efficient appliances.

Use more efficient appliances.

Use old and outdated appliances.

ANSWERS

Question 1

What type of energy is used by most appliances at home?

CORRECT ANSWER

electrical

EDDIE SAYS

Electrical energy is used by most appliances at home.

ANSWERS

Question 2

What is the unit of the energy transferred to electrical appliances by mains electricity?

CORRECT ANSWER

kWh

EDDIE SAYS

The unit of electricity transferred to appliances is kilowatt-hour (kWh).

ANSWERS

Question 3

Calculate the energy transferred to a 2 kW appliance over 3 hours.

CORRECT ANSWER

6 kWh

EDDIE SAYS

energy transferred = 2 kW x 3 h = 6 kWh

ANSWERS

Question 4

Calculate the energy transferred to a 500 W appliance over 3 hours.

CORRECT ANSWER

1.5 kWh

EDDIE SAYS

energy transferred = 0.5 kW x 3 h = 1.5 kWh. If the power is given in W, you must divide by 1000 to get the kW, so your calculation is right.

ANSWERS

Question 5

Calculate the energy transferred to a 1 kW appliance over 30 minutes.

CORRECT ANSWER

0.5 kWh

EDDIE SAYS

energy transferred = 1 kW x 0.5 h = 0.5 kWh. If the time is given in minutes, you must divide by 60 to get the time in hours, so your calculation is right.

ANSWERS

Question 6

What does an electricity meter measure?

CORRECT ANSWER

kilowatt-hours

EDDIE SAYS

Electricity meters measure the units of electricity, which are kilowatt-hours.

ANSWERS

Question 7

Calculate the cost of electricity in pence for a particular household, if one unit costs 10 pence and 10 units have been used.

CORRECT ANSWER

100 pence

EDDIE SAYS

cost = number of units x cost per unit = 10 x 10 = 100 pence

ANSWERS

Question 8

Calculate the cost of electricity in pounds for a particular household, if one unit costs 10 pence and 10 units have been used.

CORRECT ANSWER

1 pound

EDDIE SAYS

cost = number of units x cost per unit = 10 x 10 = 100 pence = £1.00. Alternatively, one can convert pence into pounds before the calculation. Let's try that at the next question.

ANSWERS

Question 9

Calculate the cost of electricity in pounds for a particular household, if one unit costs 20 pence and 10 units have been used.

CORRECT ANSWER

2 pounds

EDDIE SAYS

cost = number of units x cost per unit = 10 x 0.2 = £2.00. There are 100 pence to a pound, so 20 pence would be 20/100 = £0.20 (0.2).

ANSWERS

Question 10

What is essential to do in order to save fuels and pay less for electricity?

CORRECT ANSWER

Use more efficient appliances.

EDDIE SAYS

The use of more efficient appliances ensures that we do not waste fuels, we reduce pollution and we save money in electricity.

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