Home » Handbook of O Level Physics » UNIT 20: PRACTICAL ELECTRIC CIRCUITRY

# UNIT 20: PRACTICAL ELECTRIC CIRCUITRY

• Nichrome is a high resistivity material that heats up when current passes through it.

• Nichrome wire is used in electric irons, heaters, etc.

• A current carrying conductor in a magnetic field experiences a force.

• This phenomenon is used in fans, motors, etc.

• Lamps

• Filament lamps (also known as incandescent light-bulbs)

• The filament is made of a material called tungsten which has high resistivity and a high melting point.

• Filament is made thin to increase resistance. It is shaped like a coiled coil. This reduces convection.

• Bulb is filled with an inert gas to prevent tungsten from reacting with oxygen.

• Fluorescent lamps

• By passing current through mercury vapour, ultraviolet as well as visible light are produced.

• Fluorescent powder absorbs ultraviolet and releases more visible light.

Note: Fluorescent lamps are more efficient than filament lamps.

• Some formulas:

P = I V
P = I² R

• Measurement of electrical energy

E = P t

• Joule is a small unit so in practice electricity bills use a unit called kilowatt-hours (kWh) to measure electrical energy used.

• To get answer in kilowatt-hours, put value of “P” in kilowatts and put value of “t” in hours.

• Dangers of electricity

• Damaged insulation

• Touching the exposed live wire (with dangerously high potential) can result in electric shock.

• Overheating of cables can cause fire.

• Damp conditions

• When hands are wet the resistance of the skin is reduced greatly.

• Fuses and circuit breakers are used to stop excessive current flow.

• Fuses, circuit breakers and switches are always fitted on the live wire so that appliances are not live when the circuit is broken. Otherwise, for example, a person repairing an electrical appliance can receive a shock.

• Fuses with a rating slightly higher than the expected current should be used.

• Wiring a plug

• Colour code

• Brown – Live wire

• Blue – Neutral wire

• Green / Yellow – Earth wire

• Parts of a plug

• Terminals

• Cord grip

• Wrap-round screws

• Appliances with “double insulation” normally have a casing made of plastic (or other non-metal).

• Earthing

• If the casing is earthed:

• When live wire touches the casing, excessive current will flow through the earth wire.

• The fuse will melt and hence disconnect the appliance.

• User will be safe.

• If casing is not earthed:

• When live wire touches the casing, the person touching it will receive an electric shock.