AQA GCSE Physics P5 Electricity in the home

Fuse

This component melts and breaks the circuit when too much current flows

Earth pin

The longest pin that connects first for safety, potential of 0V

neutral wire

This wire completes the circuit and carries away current (potential of 0V)

Live wire

This wire supplies the current, mains potential of 230V in UK

It is connected in the live wire so the supply is cut off if the fuse melts.

Explain where the fuse is located.

outer metal case

What is the Earth wire connected to? (not the pin!)

Blue

Colour of the neutral wire

Brown

Colour of the live wire

Green and yellow

Colour of the earth wire

alternating current (a.c.)

This current is constantly changing direction (supplied by the mains)

direct current (d.c.)

This current always flows in the same direction (supplied by cells and batteries)

Power

= p.d. x current (P=VI)

Energy

=Power x time (E=Pt)

p.d. x charge (E=QV)

electrical energy transferred = (equation)

Charge

= Current x time (Q=It)

p.d.

= current x resistance (V=IR)

current² x resistance

= Power (P=I²R)

Efficiency

= useful power output/total power input (x100%)

hii

All the equations

Ω

Units of resistance

Watts

units of power

Joules

units of energy

seconds

units of time

Amps

units of current

Volts

units of p.d.

% or ratio (decimal)

units of efficiency

Coulombs

units of charge

current in wires(or friction of moving parts) (This causes…)

This causes heat energy loss in appliances

A smaller proportion of energy is wasted, higher proportion of energy is usefully transferred

describe an appliance with a higher efficiency in terms of energy

If a fault causes the outer metal case to become live, high current flows down the low resistance earth causing the fuse in the live to melt, stopping the current from flowing.

How do the earth wire and fuse work together?

The appliance has insulation covering the wires and the outer case so no earth wire is needed as the case cannot become live.

Explain the meaning of double insulated ( in terms of a plug)

There is a p.d. between you (0V) and the appliance (230V) so a current will flow through you to earth.

WHY do you get a shock from a 'live' appliance?

230V

UK mains voltage

50Hz

UK mains frequency

Step-up transformer

Increases the voltage (so decreases the current) in the national grid.

Step-down transformer

Decreases the voltage (so increases the current) in the national grid.

For the same power, a high voltage means a low current. Low current means cables stay cooler so a smaller proportion of heat energy is wasted so the efficiency of the national grid is higher.

Why is energy transferred at a high voltage through the national grid?

It has live, neutral and earth wires

Describe 3-core cable

It has live and neutral wires only, no earth is needed as the appliance is double insulated

2-core cable

Brass: hard, strong and a good conductor

What are plug pins plug are made of & why?

Plastic: a good insulator so you don't get a shock

What is the plug outer case is made of & why?

Copper: very good conductor of electricity

What are cables and wires are made from & why?

A system of cables and transformers linking power stations to consumers.

What is the national grid?

For safety to reduce the chances of an electric shock being fatal

Why is the voltage stepped down to go into homes?