Seneca - Electricity And Circuits

Question 1

Sources of direct current

Answer 1

Current flow in one way
Cells and batteries

Question 2

Source of alternating current

Answer 2

(The electric current repeatedly changes direction).
In the UK, the electricity from the plug sockets is a source of alternating current.This mains electricity supply has a frequency of 50 Hz and is about 230 V.

Question 3

Earth wire details

Answer 3

-The earth wire has green and yellow stripes.
-It has a 0V potential and only carries a current if there is a fault.
-It is a safety wire to stop the appliance becoming live - transfer energy to the earth instead of human.

Question 4

Live wire details

Answer 4

-The live wire is brown and carries a 230V potential.
-It provides the alternating potential difference from the mains electricity supply.

Question 5

Neutral wire details

Answer 5

-The neutral wire is blue.It is at or close to earth potential (which is 0V).
-It completes the circuit, and carries current away from the appliance - back to mains supply e.g. plug. At which will carry the current of the live wire.

Question 6

How are most electrical devices connected to mains electricity.

Answer 6

using a three-core cable (a cable containing 3 wires). The wires are colour-coded. The colour tells you the wire’s purpose.

Question 7

How to calculate power of EU appliances?

Answer 7

Using a power label - on the side of appliances (power rating)
We can use the equation
Power = potential difference x current
230v is the mains voltage in the EU.

Question 8

How to calculate work done of EU appliances ?

Answer 8

Using power label
Energy transferred = power x time
E.gm 100W lamp 100J per second.

Question 9

What is the UK national grid ?

Answer 9

the network of cables and transformers that connect power stations with houses and shops in the United Kingdom.

Question 10

Energy route to our homes - power stations

Answer 10

Power stations create an electric current, which is an energy carrier

Power stations produce an electricity voltage of 25 000 volts

Question 11

Why are large Power stations more efficient

Answer 11

Large power stations are more efficient than smaller power stations because they can burn lots of fuel at very high temperatures. This creates very hot steam that can turn the turbines.

Steam turbines are more efficient at higher temperatures.

Question 12

Energy route to our homes - step up power transformers

Answer 12

Step-up transformers then increase the potential difference to 400,000 Volts.

This reduces the current and this reduces how much heat is lost when it’s being transported across the country.

This increases the efficiency of the National Grid system.

Question 13

Energy route to our homes - step down transformers.

Answer 13

To get electricity to a safer voltage (230 Volts) in our homes, step-down transformers reduce the voltage to a lower value.

Question 14

What is the purpose of fuses and circuit breakers

Answer 14

Fuses and circuit breakers protect us from electrical appliances by stopping the flow of electric current flow if it gets too high. This stops the appliance overheating.

Question 15

How do fuses stop the flow of electricity?

Answer 15

Fuses contain a thin wire that melts if the current flowing through an appliance is too high.

This breaks the circuit and stops the flow of electric current.

Question 16

How do circuit breakers stop the flow of electricity and why are they better than fuses ?

Answer 16

Circuit breakers are special devices that switch off the flow of current if the current is higher than a certain value.

Circuit breakers do the same job as fuses but can be used lots of times.

Question 17

What are fuse ratings ?

Answer 17

Fuse ratings tell us the level where a fuse will begin to melt. The fuse chosen should have a value just above the normal current that an electrical appliance uses.

For example, if the operating current is 3.9 A, and fuses with values of 3 A, 5 A or 13 A are available, the most suitable fuse is the 5 A fuse

Fuses and circuit breakers must be chosen correctly to allow for the correct function to be provided.

Question 18

What are the advantages of circuit breakers over fuses?

Answer 18

Fuses only work once. Once blown, they must be replaced. We can reset circuit breakers. Circuit breakers also break circuits faster than fuses. This reduces the risk of getting an electric shock even more.

Question 19

What are the problems with metal casing ?

Answer 19

Lots of electrical appliances, such as toasters and microwave ovens, have metal casings.

If there is a fault in the appliance, one of the wires may touch the metal case. Metals conduct electricity, so anyone touching the case could then receive an electric shock.

Question 20

Why should we ‘earth’ a metal case ?

Answer 20

An earth wire is a low resistance wire that connects the metal case to the ground.

If the casing is touched by a wire inside the appliance, the current will flow through the earth wire.

Because the resistance of the earth wire is low, a very high current will flow through it, breaking the fuse in the device. This will stop the flow of current and make the appliance safe.

Question 21

Why should we NEVER connect the earth and live wire ?

Answer 21

The live wire and the earth wire can never be in contact because this would remove the safety feature of the earth wire.

This would also take a huge amount of current, which would overpower the fuse.

The fuse would melt and could explode.

Question 22

Why are live wires still dangerous when turned off ?

Answer 22

A live wire can still be dangerous when the mains switch is open (i.e. the plug socket or light switch is turned off).

This is because it might still hold some potential difference.

If you touched the live wire, your body would supply a link between the supply and the ground.

This is an electric shock.

Question 23

Why are electric shocks dangerous

Answer 23

An electric shock from the mains electricity supply could be fatal (could kill you).