1.8 Units of electricity

Natural sciences

Grade 8

Energy: electricity, heat and light

Module 9

Units of electricity

Activity:

To calculate electricity costs [lo 1.3, lo 2.4]

• You may know that the unit in which electricity is sold is known as kilowatt per hour – kW/hour

1 kWh is the amount of electric energy that an appliance with a power rating of 1 kW uses in one hour.

We use the following formula to calculate the kWh of any appliance:

KWh = power x time

Say you want to know how much energy your hair drier (1 500 W = 1,5 kW) uses in 20 minutes (20 minutes = $\frac{1}{3}$ of an hour):

KWh = power rating x time

= 1 500 W x 0,333

= 1,5 kW x 0,333

= 0,5 kWh

You use the following formula to calculate the cost of the use of an electrical appliance:

Cost = electrical energy in kWh x cost of 1 kWh

Supposing that 1 kWh costs 50c, the above hair drier will cost:

Cost = 0,5 kWh x 50c

= 25c

= R0,25

Assignment 1:

Examine the pictures and calculate the cost of using each appliance. Electricity costs R2, 50 per kW.

Light bulb in use: 100 w for 5 hours

TV in use: 80W for 5 hours

Iron in use: 1500 W for 2 hours

Assessment of Calculations

Were you able to do the calculations correctly?

[LO 1.3; LO 2.4]

Assessment

LO 1: Scientific investigations:

The learner will be able to act confidently on curiosity about natural phenomena, and to investigate relationships and solve problems in scientific, technological and environmental contexts.

This is evident when the learner:

• evaluates data and communicates findings.

LO 2: Constructing Science Knowledge:

The learner will know and be able to interpret and apply scientific, technological and environmental knowledge.

This is evident when the learner:

2.4 applies knowledge.

Memorandum

Assignment 1

UNITS of ELECTRICITY

The amount of electricity that is used by an appliance depends on the type of appliance and the length of time for which it is used.

Appliances used for generating heat, like stoves (ovens), kettles, irons and hot water cylinders general use much more electricity and are more costly. Very powerful halogen light bulbs (spotlights) also consume much electricity, while appliances fitted with microchips or small motors, like electric toothbrushes and razors, will use less.

Each appliance bears an indication of its value - the watt value . This is the appliance's power rating . It is usually indicated on the outer cover of the appliance, like in the case of a light bulb. A 100-Watt light bulb will burn more brightly than a 60-Watt light bulb.

Watt actually indicates the amount of electricity per second that the appliance will use. As the watt unit is relatively small, we usually speak in terms of kilowatt (kW)

• 1 kW = 1000 W

A unit of electricity is read in terms of the amount of electricity that a 1 kW appliance will use in one hour or a 100 W appliance will use in 10 hours. This is the unit in which electricity is sold to consumers.

Your 100-W light bulb will therefore burn for 10 hours on 1 kW of electricity. Your father will be able to operate his electric drill (500W) for two hours only.

We know that electricity is sold units of kilowatt per hour – kW/hour

• 1 kWh is the amount of electrical energy that an appliance with a rating of 1 kW uses in one hour.

To determine the Wh rating of an appliance, we use the following formula:

• kWh = power rating x time

If you, for instance, want to know how much energy your hairdryer (1500W) uses in 20 minutes, you work it out like this:

• kWh = power rating x time
• 1 500w /1000w x 0.3
• = 0.45 kWh

The cost of using an electrical appliance can be determined by means of the following formula:

• Cost = electrical energy in kWh x cost of 1 kWh

Supposing that 1 kWh costs 50c, the use of your hairdryer (above) will cost:

• Cost = 0.45 kWh x 50c
• = R 2,25