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Nichrome is a high resistivity material that heats up when current passes through it.
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Nichrome wire is used in electric irons, heaters, etc.
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A current carrying conductor in a magnetic field experiences a force.
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This phenomenon is used in fans, motors, etc.
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Lamps
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Filament lamps (also known as incandescent light-bulbs)
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The filament is made of a material called tungsten which has high resistivity and a high melting point.
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Filament is made thin to increase resistance. It is shaped like a coiled coil. This reduces convection.
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Bulb is filled with an inert gas to prevent tungsten from reacting with oxygen.
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Fluorescent lamps
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By passing current through mercury vapour, ultraviolet as well as visible light are produced.
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Fluorescent powder absorbs ultraviolet and releases more visible light.
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Note: Fluorescent lamps are more efficient than filament lamps.
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Some formulas:
P = I V
P = I² R
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Measurement of electrical energy
E = P t
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Joule is a small unit so in practice electricity bills use a unit called kilowatt-hours (kWh) to measure electrical energy used.
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To get answer in kilowatt-hours, put value of “P” in kilowatts and put value of “t” in hours.
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Dangers of electricity
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Damaged insulation
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Touching the exposed live wire (with dangerously high potential) can result in electric shock.
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Overheating of cables can cause fire.
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Overheating can happen due to short-circuit or overloading.
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Damp conditions
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When hands are wet the resistance of the skin is reduced greatly.
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Fuses and circuit breakers are used to stop excessive current flow.
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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.
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Fuses with a rating slightly higher than the expected current should be used.
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Wiring a plug
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Colour code
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Brown – Live wire
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Blue – Neutral wire
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Green / Yellow – Earth wire
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Parts of a plug
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Terminals
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Cord grip
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Wrap-round screws
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Appliances with “double insulation” normally have a casing made of plastic (or other non-metal).
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Earthing
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If the casing is earthed:
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When live wire touches the casing, excessive current will flow through the earth wire.
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The fuse will melt and hence disconnect the appliance.
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User will be safe.
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If casing is not earthed:
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When live wire touches the casing, the person touching it will receive an electric shock.
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