Home » Handbook of O Level Physics » 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.

        • Overheating can happen due to short-circuit or overloading.

      • 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.

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