© boardworks ltd 2003 ks4 electricity – mains electricity

© Boardworks Ltd 2003

KS4 Electricity – Mains Electricity


Mains Electricity

What is mains electricity?

The electricity we get from plug sockets.

NEVER STICK ANYTHING IN A PLUG SOCKET!


Electrical cable

Electrical cable connects the plug to a device.

What does the cable look like inside?

How is it designed for maximum safety?


Electrical cable cross- section

The cable contains three wires.

What material is the wire? Copper

Each wire is encased in plastic.

Why is plastic used?

Plastic is an electrical insulator.

The wires are then encased in

another insulating layer of plastic.


Three copper wires

The three copper wires have different functions.

The colour of the plastic casing tells you what the wire is.

Colour Name

Green and yellow

Blue

Brown

Earth

Neutral

Live


The Live wire

The Live wire carries a current that alternates between negative and positive values.

What is this type of current called?

Alternating Current (A.C.)

The Live wire is connected to the Live pin.

Which is the live pin?


The Neutral wire

The Neutral wire stays at a voltage close to zero with respect to Earth.

What colour is the Neutral wire?

Blue

The Neutral wire is connected to the Neutral pin.

Which is the neutral pin?


The Earth wire

If an appliance has a metal case it needs to be earthed. The Earth wire connects the appliance case to the Earth pin. If there is a fault that connects the live wire to the appliance case a very large current flows to earth and overloads the fuse.

What colour is the Earth wire?

Green and yellow

Which is the Earth pin?


Materials

Material

Brass

Copper

Plastic

Used for

Plug case

Pins

Wires

Useful Property

Good conductor

Good insulator

Good conductor


Electrical surges

If there is a surge of electrical current in the mains supply this could damage any appliances or devices connected to the mains supply.

What could cause such an electrical surge?

A lightning strike!

Name two ways of protecting devices from electrical surges:

1._______________

2._______________

Fuses

Circuit breakers


AC/DC

What is AC?

AC is alternating current, a current which is constantly changing in direction.

What is DC?

DC is direct current, current that is always flowing in one direction.

Source of AC?

Mains electricity (50Hz)

Sources of DC?

Cells or batteries


AC/DC signal simulation

Use the animation below to recreate different a.c. or d.c. signals.


Fuses I

What is a fuse?

A fuse is a device that is used to protect devices from a current which is too high.

How does a fuse work?

As current flows along a conductor there is friction between the electrons and the atoms of the conductor. The more current, the more friction. The friction causes heat, and the more current the hotter the fuse gets.Fuses contain pieces of conductor that melt if the current is too high. When the fuse melts, it breaks the circuit.


Fuses II

What is the circuit symbol for a fuse?

Fuses are labelled with the maximum current they are designed to allow.

When choosing the fuse for a device you should always choose one with the ampage above the operating current of the device.

Example:

So if a kettle operates with an electrical current of 4.3A what fuse should you use? You can choose from fuses of 3A, 5A, 8A and 13A.

use a 5A fuse


For the following devices choose fuses from 3A, 5A and 13A.

Operating Current

Fuse

2.1A

11.2A

4.7A

6.0A

4.5A

8.2A

3.0A

13.0A

5.0A

13.0A

5.0A13.0A

Fuses II


Power calculations and fuses

If you have a lamp and it has a potential difference of 200V across it and it operates at a power of 60W:

A) What current is running through the lamp?

B) What fuse should be fitted to the plug of the lamp?

A) P = IV

I = P/V

I = 60W / 200V

I = 0.3A

B) A 3A fuse should be fitted to the plug.


If you have a cooker and it has a potential difference of 230V across it and it operates at a power of 9.2kW:

A) What current is running through the cooker?

B) Why should a 13A fuse not be fitted to the plug?

A) P = IV

I = P/V

I = 9 200W / 230V

I = 40A

B) The fuse would blow each time the cooker was turned on.

Power calculations and fuses


Can you label the plug?

Live terminal

Neutral terminal

Earth terminal

Cable grip

Fuse


The Circuit breaker

switch

electromagnet

Too much currentcauses the electromagnet to produce a magnetic field strong enough to open the switch.

(The circuit break is said to ‘trip’).

This switches off the current.

What happens if the current is too large?


kWhMeter

Ring MainPower Points

5A

30A

live neutralearth

lights

Domestic wiring system


Buying electricity

0689567406895674

In your home will be an electricity meter.

It records how much electricity you use in units.

From the number of units you use your electricity bill is calculated.

06895702

How many units have been used here? ______26


Cost of electricity

The cost of electricity is the number of units multiplied by the cost per unit:

Cost = Number of units x cost per unit

Example:

How much would 10 units of electricity cost at a price of 9p per unit?

Cost = 10 x 9p

= 90 p


What is one unit of electricity?

Energy = Power x time

Units: kWh

1 unit of electricity = 1 kWh

Example:

How many units of electricity is 17.6 kWh?

17.6 units


Buying electricity – question 1:

A kettle uses 45.2 kWhr of energy.

If electricity costs 10p per unit, how much does it cost to use the kettle?

Number of units:

Number of units of electricity is 45.2 units

Cost of electricity:

Cost = 45.2 x 10 p

= 452p or £4.52


Buying electricity - question 2:

An iron that operates at a power of 3 KW for 4 hours uses electricity that costs 8p per unit.

How much does it cost for the electricity that it uses?

Number of units:

kWhr = 3kW x 4hr

= 12 kWhr

Number of units of electricity is 12 units

Cost of electricity:

Cost = 12 x 8 p

= 96p


What colour is the live wire?

A. Green

B. Yellow

C. Green and Yellow

D. Brown


Why are the wires in an electrical cable made from copper?

A. It is an electrical insulator

B. It is flexible

C. It is an electrical conductor

D. It is cheap


If a kettle operates at an electrical current of 6.7A, what fuse should be fitted to the kettle?

A. 3A

B. 5A

C. 1A

D. 13A


If an electric fan operates at an electrical current of 3.5A, what fuse should be fitted to the fan?

A. 3A

B. 5A

C. 8A

D. 13A


Which of the following is a safety feature of

a 3-pin plug?

A. Brass pins

B. Copper wires

C. Cable-grip

D. Made in U.K.

© boardworks ltd 2003 ks4 electricity – mains electricity

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