Physics - 5 Electricity in the Home

Direct current (d.c.)

electric current that flows in only one direction

Alternating current (a.c.)

a flow of electric charge that regularly reverses its direction.

Frequency of alternating current

the number of cycles it passes through per second

Mains frequency UK

50 Hz

Live wire

brown wire that carries the power from the source to a switch or appliance

Neutral wire

a blue wire that carries current away from the component

Earth wire

the green and yellow wire in a mains cable used to connect the metal case of an appliance to earth

Step-up transformer

a transformer that increases voltage for transfer to the National Grid

Step-down transformer

a transformer that decreases voltage for use in a home

What are the outer casings of plugs, sockets and cables made from?

hard-wearing electrical insulators

Three-pin plug

has a live pin, a neutral pin and an earth pin to connect to mains

Longest pin

this is the earth pin so that it connects first for safety reasons

What are plug pins made from and why? [3]

brass because it is:
- harder than copper
- doesn't rust or oxidise
- good conductor

Where is the fuse in a plug?

between the live pin and live wire

Why is there a fuse in a plug?

to prevent too much current from passing through

What colour is the earth wire?

yellow and green

What colour is the neutral wire?

blue

What colour is the live wire?

brown

What are mains appliance wires made from?

copper

Why is copper used for wires? [2]

- good conductor
- bends easily

What are two-core cables used for?

appliances with plastic cases

Short circuit

when the live and neutral wire touch and a big current passes between them

Energy transferred (in an electrical circuit) =

power x time

What forces electrons round a circuit?

work done by the power supply

The current through an electrical appliance is:

the charge that flows through it each second

The potential difference across an electrical appliance is:

the energy transferred to the appliance by each coulomb of charge that passes through it

The power supplied to an electrical appliance is:

the energy transferred to it each second

Power supplied =

current x potential difference

How should you choose a fuse?

choose one that is over the normal operating potential difference of the appliance

power =

current² x resistance

The power supplied to a resistor is proportional to...

the square of the current

Charge flow =

current x time

Why does a resistor get hotter? [5]

- electrons are pushed round the circuit
- they bump into metal ions in the resistor
- this causes the ions to gain kinetic energy
- they vibrate
- the resistor gets hotter

A resistor becomes hotter when:

charge flows through the resistor, transferring energy to it

Coulomb

unit of charge

The energy supplied to an appliance depends on... [2]

- how long it has been switched on
- the power supplied to it

Efficiency values can be expressed as [2]:

- ratio
- percentage

Why do electrical appliances waste energy? [2]

- the heating effect of current in wires and components
- friction causes heating in appliances with moving parts