P5 Electricity at home

Question 1

Alternating current

Answer 1

Electric current in a circuit that repeatedly reverses its direction

Question 2

Direct current

Answer 2

Electric current in a circuit in one direction only, created by a direct voltage

Question 3

Fuse

Answer 3

A fuse contains a thin wire that melts and cuts the current off if too much current passes through it- breaking the circuit

Question 4

Oscilloscope

Answer 4

A device used to display the shape of an electrical wave

Question 5

Plugs

Answer 5

A plug has an insulates case and is used to connect the cable from an appliance to a socket

Question 6

Step-down transformers

Answer 6

Electrical device used to step-down
the size of an alternating potential difference, at the local consumer, with the current increased by this transformer. These transformers have more turns on the primary coil than the secondary(higher potential difference across the primary coil than the secondary coil)

Question 7

Step-up transformers

Answer 7

Electrical device used to step-up
the size of an alternating potential difference. It has more turns on the secondary coul thanks the primary coil(higher potential difference across the secondary coil than the primary coil).As the potential difference is increased by the transformer, the current is decreased.

Question 8

Three-pin plug

Answer 8

A three-pin plug has a live pin, a neutral pin and an earth pin

Question 9

How are alternating currents produced

Answer 9

By alternating voltages in which the positive and negative ends keep alternating

Question 10

The UK mains supply

Answer 10

The alternating current supply of around 230V, responsible for the electricity in your home

Question 11

Frequency of alternating current mains supply

Answer 11

50HZ

Question 12

What supplies direct current?

Answer 12

Cells and batteries

Question 13

Three-core cables

Answer 13

With three wires inside of them, each with a core of copper and a coloured plastic coating.

Question 14

Each wire in a three-core cable

Answer 14

1) Brown live wire - provides the alternating potential difference(at about 230V) from the mains supply.
2) Neutral wire - blue. The neutral wire completes the circuit- when the appliance is operating normally, current flows through the live and neutral wires. It is around 0V.
3) Earth wire- green and yellow. It is for protecting the wiring and for safety - it stops the appliance casing from becoming live. It doesn’t usually carry a current - only when there’s a fault. It’s also at 0V.

Question 15

Why can a live wire give you an electric shock?

Answer 15

Your body(like the earth) is at 0V. This means if you touch the live wire, a large potentialdifference is produced across your body and a furrent flows through you. This causes a large electric shock which could injure or even kill you. Electric shocks can occur even if a plug socket or a light switch is turned off, here a current isn’t flowing, but there’s still a potential difference in the live wire and if you make contact with this, your body would provide a link between the supply and earth, so a current would flow through you.

Question 16

Why is any connection between live and earth dangerous?

Answer 16

If the link creates a low resistance path to earth, a huge current will flow, which could result in a fire.

Question 17

How is energy transferred between stores electrically?

Answer 17

By electrical appliances, designed to transfer energy to components in the circuit when a current flows.

Question 18

Examples of electrical appliances transferring energy

Answer 18

Kettles transfer energy electrically from the mains alternating current supply to the thermal energy store of the heating element inside the kettle.
Energy is transferred electrically from the battery of a handheld fan to the kinetic energy store of the fan’s motor.

Question 19

How does current affect the efficiency of an electrical appliance?

Answer 19

The higher the current, the more energy is transferred to the thermal energy stores of the components(and then surroundings), so no appliance transfers all energy completely usefully.

Question 20

Power rating

Answer 20

The maximum amount of energy transferred between stores per second when the appliance is in use

Question 21

How do power ratings affect running costs of appliances?

Answer 21

The lower the power rating, the less electricity an appliance uses in a given time, so the cheaper it is to run.

Question 22

How is energy transferred in potential difference?

Answer 22

1) When an electrical charge goes through a change in potential difference, energy’s transferred,
2) Energy is supplied to the charge at the power source to raise it through a potential.
3) The charge gives up this energy when it falls through any potential drop in components elsewhere in the circuit.

Question 23

National grid

Answer 23

A giant system of cables and transformers that covers the UK and connects power stations to consumers(anyone who’s using electricity).

Question 24

How do power stations cope with increased demand?

Answer 24

Power stations often run at well below their maximum power output, so there’s spare capacity to cope with a high demand.
Lots of smaller power stations are kept in standby just in case.

Question 25

How is the national grid an efficient way of transferring energy?

Answer 25

With a high current, lots of energy’s transferred to the thermal energy store of the surroundings and it’s much cheaper to boost the potential difference up really high and keep the current as low as possible.
For a given power, increasing the potential difference decreases the current, decreasing the energy lost by heating the wires and the surroundings.

Question 26

Composition of transformers

Answer 26

Transformers all have two coils, a primary coil and a secondary coil, joined with an iron core

Question 27

How efficient are transformers?

Answer 27

Transformers are nearly 100% efficient, so the power in primary coil = power in secondary coil

Question 28

Why is it better to use a cable with a low resistance than to use a cable with a high resistance?

Answer 28

Because this lowers the potential difference across the cable, making it more efficient

Question 29

Why would a homeowner choose one light bulb over another?

Answer 29

If one uses less power
If one has a greater lifetime than the other

Question 30

What effect does a current through a wire have on the wire?

Answer 30

Makes it hot/warm

Question 31

Which wire inside the cable is the fuse connected to?

Answer 31

The live wire

Question 32

Structure of plug

Answer 32

Brass pin - good electrical conductor
Plastic/rubber outer case, because it’s a good electrical insulator

Question 33

Earth wire

Answer 33

Connected to the longest pin of a plug, used to earth the metal case of a mains appliance

Question 34

How often does alternating current change cycle?

Answer 34

Twice every cycle

Question 35

How are earth wires good for safety?

Answer 35

The charge flows through the earth wire instead of the person, because of the resistance of the earth wire being less than a person’s resistance

Question 36

What will allowing the surface of the solar panels to be dirty do to the electricity generated?

Answer 36

Less electricity generated due to a lower light intensity