P1. 3.3 Using electrical energy

Year 9

Week commencing 23rd April 2012

Objectives

• I must be able to calculate energy transferred using the equation

• I should be able to carry out the equation with a change in units

• I should also be able to re-arrange the equation to find out power or time

Do different appliances use

different amounts of electricity?

© Boardworks Ltd 2003

How much energy?

How much does electricity cost?

How much does it cost every time I

watch TV?

It is important to learn this

because: • When you leave home, you will need to know how

your electricity bill is worked out.

• When you buy new electrical appliances, you will know about power ratings.

• We need to make our fossil fuels last as long as possible. By knowing about how much electricity we use, we can achieve this.

Last lesson you learnt

• About how to calculate power.....

• This lesson will teach you about how to

calculate the energy given to an appliance

and how to calculate the cost of that

How do we measure energy?

• Energy is measured in Joules (J)

• We need to know how much energy something uses

over time = a rate

• We can use Joules per second (J/s)

• 1 Joule / sec = 1 Watt (W)

A 100W light bulb uses 100J of energy every second

Power Rating

• Different electrical devices have different power

ratings

• They use different amounts of electrical energy over

a given time.

15W 100W

VS

Write down the equation

• Energy transferred = power X time

• E = p X t

• E is energy transferred in kilowatt-

hours, kWh

• p is power in kilowatts, kW

• t is time in hours, h

• How could this be re-arranged?

• A 0.5kW appliance is left on for 2

hours. How much energy is

transferred?

= 0.5 X 2

= 1kWh

energy transferred = power rating x time

• A 100W light bulb on for 1 hour will use

100 Watt hours (Wh) of electricity

• This is usually measured in Kilowatt Hours

(kWh)

0.1 kWh

energy transferred = power rating x time

On an electricity bill

1 unit of electricity = 1 kilowatt hour

Cost of Electricity • At the moment, one unit (1 kWh) of electricity costs 12p.

• To run a normal 100W light bulb for 5 hours costs:

Energy used = power X time

100W x 5hours = 500Wh = 0.5kWh

Therefore, cost = energy used X cost per unit

= 0.5kWh x 12p = 6p

• To run a 15W eco bulb for 5 hours costs:

15W x 5hours = 75Wh = 0.075kWh

0.075kWh x 12p = 0.9p

What about for a year?

• To run a normal 100W light bulb for 5 hours a day all year costs:

100W x 5hours = 500Wh = 0.5kWh

0.5kWh x 12p = 6p

6p x 365 days = 2190p = £21.90

• To run a 15W eco bulb for 5 hours a day all year costs:

15W x 5hours = 75Wh = 0.075kWh

0.075kWh x 12p = 0.9p

0.9p x 365 days = 328.5p = £3.29

Meter reading on

Tuesday

Meter reading on

Wednesday

Electricity used in one day:

35 534.9 – 35 497.6 = 37.3 kWh or 37.3 units

If each unit costs 13 pence

The cost of that day’s electricity is 37.3 x 13 p = 484.9 p

OR £ 4. 85

OR £ 4.849

Now carry out your worksheets on the sheet

Plenary

Ultra-Elec has a standing charge of 20p per

day and a cost of 10 pence per kWh.

Top-Power has no standing charge but a

cost of 12p per kWh.

If the Jones family use 300kWh in a period

of 90 days, which company is cheaper for

them to use?