Core Physics Topic 6 - Transferring Electrical Energy

Why is Electricity so Useful?

We take electricity for granted in our homes.  It is very easy to plug in an appliance or switch on a light.  Electricity is clean, and, if used properly, safe.  It can do all sorts of jobs that other energies can do, and more.  It is possible to heat and light with gas.  It could be possible to use a gas-powered hoover, but a TV or computer can only work with electricity.

Doing clever things with electricity is Electronics.

Look at this kitchen:

You can see a range of appliances that all use electricity, each one doing a different job, all easy to use.

We can compare the use of different appliances for a particular job.

 What are the advantages and disadvantages of using a microwave oven compared with using an ordinary oven in a cooker?

Energy Transformations in Everyday Appliances

To do a useful job for us, electrical energy has to be converted into other forms of energy.  For example:

A drill:

Electrical Energy ® Movement (Kinetic) Energy

Electrical Energy ® Sound Energy

Electrical energy is converted into other energy.  Complete this table.  One has been done as an example:

 Appliance Useful Energy Other Energy Drill Kinetic Noise/heat Iron Hoover TV Set

The power of an appliance

Electrical energy is measured in joules (J).

The amount of energy an electric appliance uses depends on:

• how much power it uses;

• how long its on for.

The Power is the rate at which an appliance transforms electrical energy into other forms of energy.  Power is measured in Watts (W).  1 watt is 1 joule every second:

1 W = 1 J/s

On every appliance there is a label that shows the power of the appliance.

This motor operates at a voltage of 230 V and at mains frequency of 50 Hz (50 cycles per second).  Its power is 250 watts.  That means it turns 250 joules of electrical energy into movement energy every second.

The formula that gives you the right answer is shown in the box below:

 energy transferred (kWh) = power (kW) × time (h)

In Physics Code:

E = Pt

 Question 3 How many joules will the motor convert in 10 s?

Often appliances have their power marked in kilowatts (kW):

1 kW = 1000 W

1 W = 1/1000 kW

So our 250 W motor will have a power of 250 ÷ 1000 = 0.25 kW

Complete the following table of powers of different appliances.   One has been done as an example:

 Appliance Power in watts Power in kilowatts Drill 350 0.35 TV set 0.20 Kettle 2200 Shower 7.5

How much does it cost to use an appliance?

When we work out the cost of using an appliance, we pay for the energy that has been used.  We could pay for the number of joules used, but the joule is only a small unit.  So we need a bigger unit.  This is called the kilowatt-hour (kWh) or unit.

1 kilowatt hour is the amount of energy used by a 1 kW appliance running for 1 hour.

A common bear trap is to write "kW/h" (kilowatts per hour).  Don't!

To work out the amount of energy used by an appliance:

1. Work out the power in kilowatts (kW);

2. Work out the time used in hours (h);

3. Multiply the two numbers together.

The formula that gives you the right answer is shown in the box below:

 energy transferred (kWh) = power (kW) × time (h)

In Physics Code:

E = Pt

In triangle form:

 Worked Example Lorraine uses a 7500 W shower for 20 minutes.  How many kilowatt hours has she used? Answer 1.  Work out the power in kilowatts:       Power = 7500 ÷ 1000 = 7.5 kW   2. Convert the minutes to hours:     20 min = 20 ÷ 60 = 1/3 hour   3.  Multiply the two numbers together:      Electrical energy = 7.5 × 1/3 = 2.5 kWh

 Question 5 How many units (kilowatt-hours) are used by a 2.5 kW oven running for 1.5 h?

A common bear trap is to fail to convert watts to kilowatts and minutes to hours.

To work out the cost we simply multiply the number of kilowatt-hours by the cost per unit.

 total cost = number of kilowatt-hours × cost per kilowatt-hour

Electrical energy typically costs about 11 pence per kilowatt-hour.

 Worked Example How much does Lorraine's shower cost? Answer Lorraine has used 2.5 kWh Cost = 2.5 kWh × 11 p = 27.5 pence (28 p)

 Question 6 How much does it cost to watch 4.5 hours of TV which has a power of 300 watts?  Electricity costs 11 p per unit.

 Summary Appliances convert electrical energy into useful forms of energy Power is measured in watts or kilowatts; Electrical energy is purchased in kilowatt hours (units); energy transferred (kWh) = power (kW) × time (h); total cost = number of kilowatt-hours × cost per kilowatt-hour;