Mains electricity

Alternating current

• current changes direction at a frequency of 50 cycles per second (50 Hertz)

positive → negative negative → positive

Direct current

• current flows in one direction around the circuit.

When is alternating current used? a.c.

mains electricity

(UK mains electricity supply is approximately 230 volts.)

When is direct current used? d.c.

supplied by a cell or battery

Explain the use of fuses in a domestic appliance

• Prevents a fire

• Contains a thin wire that heats up and breaks if too much current passes through it.

Experiment to calculate the melting point of a fuse

• To find the current that will melt the fuse…

  • use a variable power supply. Gradually increase current from zero and watch ammeter.

  • Reading will increase and suddenly fall to zero when fuse melts.

Variables that affect a fuse

The thinner the wire, the higher the resistance, so the fuse breaks at a lower current.

Explain the use of earthing in a domestic appliance

• The earth wire is a safety wire - it doesn’t carry a current.

  • If a fault develops, someone could get a shock from the metal case that is touching the wire.

• Current goes through the earth wire (low resistance) instead of person (high resistance.)

  Current will always flow through the pathway with the least resistance.

Name the 3 wires in a plug socket (which deliver mains electricity.)

• Live wire (brown) - electricity flows into live wire at high voltage

• Neutral wire (blue) - electricity flows out neutral wire

• Earth wire (green and yellow) - carries current if there’s a fault

Example of insulator and conductor

CONDUCTOR - metal (allows current to flow because metal ions carry a charge.)

INSULATOR - plastic (stops current flowing.)

Explain the use of double insulation in domestic appliances

Appliances made of plastic can be safely connected to mains electricity using ONLY the neutral and live wire…

Earth wire is not needed because the copper wires are INSULATED with plastic casing

Explain the use of circuit breakers in domestic appliances

• Device that disconnects the circuit when it detects that the current is NOT the same in the live and neutral wires.

  • This imbalance of current means that some current is leaking through the earth wire or person.

Explain how a circuit breaker works and its benefits

• Breaks circuit by opening switch when it detects a surge in current.

• Benefits of a circuit breaker compared to a fuse:

  • break the circuit as soon as there’s a surge in current - you don’t wait for it to heat up.

  • the circuit can be easily reset by flicking a switch on the device.

  • work for small current changes - more effective at protecting against electrocution.

Name the common type of circuit breakers

Residual Current Circuit Breaker (RCCB)

What is the function of a resistor?

To slow down the rate of flow of current in a circuit.

• When current flows through a resistor there is an electrical transfer of energy, and an increase in temperature.

Equation for electrical power

Electrical power = current x voltage

Watts = amps x volts

General equation for power

Power = energy/time

Equation for energy transferred

Energy transferred = current x voltage x time

Unit for charge

Coulomb C

Unit for energy

Joule J

Unit for resistance

Ohms

Unit for power

Watts

Equation for resistance

Voltage = current x resistance

Equation for charge

Charge = current x time