Purposes of Components in Electric Circuits

Fuse – the purpose of a fuse is to protect the appliance in which it is installed and the user. If there is a short circuit a large current blows the fuse, then there is no circuit and no current flows at all. If the casing of the appliance becomes live, a wire connected internally to the case completes the circuit. A large current flows and protects the circuit. This is very important, since if a user touched the live case they could be electrocuted.

Switch – the purpose of a switch is to control the supply of current to individual components in a circuit or to the whole circuit. Placing a switch next to each component in a parallel circuit may mean that components can be switched on and off one at a time.

Variable Resistor – to control the voltage or current supplied to a component in a circuit, or to the circuit as a whole. This may be used to operate a light dimmer for example or calibrate an ammeter or voltmeter.

Resistor – May be used to reduce the consequences of a short circuit by limiting the current.

Cell or battery – used to supply a constant dc voltage. The voltage supply is usually less than about 30V.

Diode – these allow current to flow only in one direction. They may be used to reciify ac current ie turn it into dc current.

Thermistor – suffers a fall in resistance if the temperature increases. Make be used to control temperature when used in a thermostat for example.

Light Dependent Resistor - suffers a fall in resistance if the light level increases. Make be used to control the switching on and off of streetlamps.

Lamp – may be used to indicate that a circuit is on or live, as when a piece of equipment is plugged in but not switched on – it is on standby.

Ammeter – Measures the current in a circuit.

Voltmeter – Measures the voltage across a component. If a battery is not connected to a circuit, measuring the voltage across the terminals gives the electromotive force (EMF). This is called open circuit.

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