Topic 3 Why are electrical devices so useful?

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. 

Question 1  What are the advantages and disadvantages of using a microwave oven compared with using an ordinary oven in a cooker?  ANSWER

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

A radio:

Electrical Energy ® Sound Energy

Question 2  Electrical energy is converted into other energy.  Complete this table.  One has been done as an example:

ANSWER

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.

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

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

Question 4  Complete the following table of powers of different appliances.   One has been done as an example:

ANSWER

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:

Worked Example

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

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.

Electrical energy typically costs about 8 pence per kilowatt-hour.

Worked Example

Question 6:  How much does it cost to watch 4.5 hours of TV which has a power of 300 watts?  Electricity costs 8 p per unit.  ANSWER

How do we get electricity to our homes?

Electrical energy cannot be stored as electricity.  In a battery it is converted to chemical energy.  We are constantly using electricity, so it has to be constantly generated.

Electricity is generated in large quantities in power stations.  We will look at power stations in Topic 4.

Electricity is passed from the power station to our homes through a complex network of wires called the National Grid.  When you see pylons, these are part of the National Grid.  The Grid is constructed in such a way that if one line of wires (a transmission line) fails, other lines can take over so that the electricity supply to our homes does not fail.

Generally the system is very reliable, although overhead power lines are vulnerable to:

• lightning strikes;

• high winds;

• heavy snowfall.

Underground cables are less vulnerable, but are much more expensive to put in.

Mains electricity comes in to our homes at a voltage of 230 Volts (V).  However it is not generated at 230 V because the current needed would be huge.  Big currents need heavy cables that get hot.  In the power station the electricity is generated at 25 000 V (at a current of 100 000 A).  Outside the power station there are huge step-up transformers that take the voltage from 25 000 V to 275 000 V.  The voltage can be as high as 415 000 V in the super grid.

As the voltage goes up ten times, the current comes down ten times.

The picture shows a transformer:

Obviously 275 000 V is far too high a voltage to use, so step-down transformers are used to reduce the voltage as follows:

The drawing shows a very simplified diagram of the National Grid.

The reason for voltage being so high is that the current is lower.  Therefore the wires can be thinner, and don't get so hot.  Therefore less energy is wasted.  Even so there is a fair amount of energy lost in heating up the wires which heats up the countryside.

Answer Question 7

Now try the TOPIC QUIZ

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