Mains electricity

Question 1

What is the mains voltage supply in the U.K.?

Answer 1

230V a.c. at a frequency of 50 Hz.

Question 2

What are the three wires in a cable and what are their colours? Where are they in a plug? What other things are there to label in a plug?

Answer 2

Live (brown), neutral (blue) and earth (green and yellow)

The earth wire is at the top middle, the neutral wire on the bottom left and the live wire middle right.

The cable grip is at the bottom. The fuse is next to the live wire on its right.

Question 3

What is the purpose of the live wire?

Answer 3

The live wire is connected to the National Grid. Its potential varies from about +340V to -340V.

Question 4

What is the purpose of the neutral wire?

Answer 4

It is simply a return wire. It has potential 0.

Question 5

What is the purpose of the earth wire?

Answer 5

The earth wire is a low resistance connection straight to the earth so that should there be a fault in which the live wire touches the casing the current will take the path of least resistance (the earth wire) and a fuse will blow. Because the live wire and neutral wires should form a complete circuit no current should normally flow In the earth wire.

Question 6

Why must the live wire be insulated from the other two wires in electric plugs?

Answer 6

If the live wire comes Into contact with the neutral or earth wires a short circuit will result that will cause a large current to flow for a short time before safety devices are activated.

Question 7

Name 6 safety hazards

Answer 7

Frayed cables, damaged plugs, water around sockets, long cables, pushing metal objects into sockets, too many appliances plugged into one electricity supply

Question 8

What is the use of fuses and circuit breakers?

Answer 8

Fuses and circuit beakers are designed to cut the circuit when the current exceeds a certain value.

Question 9

What do fuses consist of?

Answer 9

Traditional fuses consist of a thin wire contained in a glass or ceramic tube: when the current gets too large the wire melts or breaks. This type of fuse can be replaced when it ‘blows’.

Question 10

What are the most common fuses used?

Answer 10

3A for lighting and 13A for everything else.

Question 11

What are the dangers of an exposed metal part of an electrical appliance?

Answer 11

A person touching it would recurve an electric shock as a current flowed through them to earth - if no safety measures were in place.

Question 12

Why are appliances earthed?

Answer 12

If a fault develops a large current will then flow to earth and that will blow a fuse or activate a circuit breaker, thus disconnecting the circuit. However the earth wire is no use if it’s not connected to a fuse, the person will still receive a shock if it’s not.

Question 13

What is double insulation?

Answer 13

As well as the wiring inside the appliance wing insulated the appliance itself will have a casting made out of an insulating material, usually plastic.

Question 14

Do you need double insulation and an earth wire?

Answer 14

No, if there’s nothing metal on the exterior or the appliance that can be pushed there is no need for earth connection.

Question 15

What are some of the main uses of electricity in homes?

Answer 15

Heating and cooking (Lighting)

Question 16

What is the equation for power?

Answer 16

Power=current x voltage

P=I x V

Question 17

What is power measured in?

Answer 17

Watts (W)

Question 18

What is current measured in?

Answer 18

Amps

Question 19

What is voltage measured in?

Answer 19

Volts

Question 20

What is the equation for energy transferred?

Answer 20

Energy transferred = current x voltage x time

E = I x V x t

Question 21

What is energy measured In?

Answer 21

Joules (J)

Question 22

How do you work out how much energy has been converted in kW hours?

Answer 22

Multiply the power of the appliance in kW by the time it was operating in hours.

Question 23

How much do electric companies charge per kW hour?

Answer 23

12p

Question 24

What is direct current?

Answer 24

Current that’s flow is always in the same direction, though it does not have to have the same value all the time. This is supplied in batteries.

Question 25

What is alternating current?

Answer 25

The current flows in one direction and then another. This is what the mains supply is, at 50 Hz.

Question 26

Why are frayed wires dangerous?

Answer 26

You could touch the live wire, which is exposed and become electrocuted as it is no longer insulated.

Question 27

Why is water near electrical components dangerous?

Answer 27

Water is a good conductor of electricity so if it comes in contact with electricity and you come in contact with the water, you could recieve an electrc shock

Question 28

What does an RCCB (circuit breaker) do?

Answer 28

They are designed to cut the circuit when the current exceeds a certain value

They make use of either the heating effect or the magnetic effect of a current to control a switch that will break the circuit. They have an advantage over fuses as they are re-settable

Question 29

What is a common conversion of electrical energy and when is it useful or not?

Answer 29

The conversion of electrical energy into heat. This is widely used in our homes (e.g. irons, cookers, kettles, hairdryers etc.) but it is not useful when it occurs in cables that carry electricity as this is when energy is thought to be ‘wasted’.

Question 30

How are electrical cables in homes designed to waste as little energy as possible? What precautions also need to be made however?

Answer 30

The cables have very low resistance as this is where energy would be lost as heat, so it is as small as possible. However it is important to realise that these cables still have some resistace, and, if the current flowing through them is too big, the cables themselves will overheat and may even cause a fire. It is for this reason that fuses are used - to stop cables overheating - and why it is essential to have the right fuse or circuit breaker for each circuit or individual appliance.

Question 31

Why is having overloaded cables (too many plugged into one electricity supply) dangerous?

Answer 31

The current can get too large and can cause a fire

Question 32

How can trip hazards of cables be prevented?

Answer 32

Do not use extension cables and provide sufficeint sockets

Question 33

How can having an electrical device with a heating element be dangerous?

Answer 33

Risk of burns and insulation could melt and cause a fire

Question 34

How can danger from misusing equipment be prevented?

Answer 34

Use proper insulated tools, use of ‘blanks’ to cover sockets that children can reach

Question 35

What is important to do with fuses and circuit breakers?

Answer 35

It is vital that they are always connected in the live side of circuits. Otherwise, though the circuit will be disconnected and no current will flow, parts of it (e.g. the contacts in a bulb holder) will still be live

Question 36

How does an electrical heater work?

Answer 36

It is simply a resistor designed to transfer energy from the electricity supply into heat energy.

As the heater warms up, it transfers energy to the air or water around it by conduction, and the warm fluid then rises to produce a convection current

If the heating element gets so hot that it begins to glow, then heat transfer by means of infrared radiation becomes predominant e.g. in toasters and grills

Question 37

How does a current in a resistor result in the electrical transfer of energy and an increase in temperature?

Answer 37

The passage of an electrical current through a conductor produces a heating effect - this is because electrons have kinetic energy when they move and some of this energy is passed on to the atoms in the conductor as collisions occur, which increases the amplitude of vibration of the atoms, meaning the temperature of the conductor as a whole is raised

A larger resistance generally means more collisions and thus a greater heating effect - this effect is exploited in devices such as filament light bulbs and the elements in electric kettles

Question 38

What is 1kWh equal to?

Answer 38

1000W x 3600s = 3,600,000 J or 3.6 MJ

Question 39

Explain why the fuse ampage has to be higher than the max ampage / can’t be lower

Answer 39

the current can’t exceed the value of the fuse ampage because otherwise the fuse will blow and cut the circuit