Unit M - Electrical Energy & Supply

2b (2.2-6), 2c (2.20-21)

Plugs

• Have three wires - live, neutral, earth

• Only live and neutral wires usually needed, but earth wire stops you getting hurt if something goes wrong

• LIVE WIRE alternates between HIGH +VE AND -VE VOLTAGE of 230V

• NEUTRAL WIRE always at 0V

• Electricity normally flows in through live and neutral wire

• EARTH WIRE and fuse are just for safety

Double insulation

If appliance has plastic casing and no metal parts showing, it’s said to be double insulated

Plastic is insulator, so stops current flowing - meaning you can’t get a shock
Anything with double insulation doesn’t need earth wire

Earthing and fuses

• If fault develops in which live touches metal case, then because case is earthed, big current flows through live wire, case and earth wire

• Surge in current melts the fuse, cutting off the live supply

• This isolates the whole appliance, making it impossible to get electric shock from case
  Also prevents risk of fire caused by heating effect of large current

Circuit breakers

• Protect circuit from damage if too much current flows

• When circuit breakers detect surge in current, they break circuit by opening a switch

• Circuit breaker can easily be reset by flicking a switch on the device
  This makes them more convenient than fuses (have to be replaced once melted)

Heating effect in resistors

When there is electrical current in resistor, there is energy transfer which heats resistor

Because electrons collide with ions in lattice that make up resistor as they flow through it
This gives ions energy, causing them to vibrate and heat up

Heating effect increases resistor’s resistance - so less current flows

Heating effect can cause components in circuit to melt - so circuit stops working

Use of heating effect in resistors

Toasters contain coil of wire with very high resistance

When current passes through coil, temp increases so much that it glows and emits IR (heat) radiation which cooks bread

Equation: Power, Current and Voltage

P = IV

Power = Current x Voltage

[W] = [A] x [V]

Fuse ratings

• Fuses have current ratings and should be rated as near as possible but just higher than the normal operating current

• To work out the fuse needed, you need to work out the current that item normally uses

Equation: Energy, Current, Voltage and Time

E = IVt

Energy transferred = Current x Voltage x Time

[J] = [A] x [V] x [s]

Alternating current (a.c.)

Current is constantly changing direction

Used for mains supply, e.g. UK mains supply is approx. 230V

Direct current (d.c.)

Current keeps flowing in same direction

Used in cells and batteries

Voltage

Energy transferred per unit charge passed

One volt = one joule per coulomb

Equation: Energy, Charge and Voltage

E = QV

Energy transferred = Charge x Voltage

[J] = [C] x [V]