A complete loop is required for an electrical circuit to work
A resistor can be used in a circuit to change the current
Electric current is the rate of flow of charge
In a circuit, electric current is due to the movement of electrons
The size of the current for a given circuit depends on the resistance. Less resistance in a circuit means greater current and vice-versa
A variable resistor can be used to change the resistance in a circuit and hence the current
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Circuit Components
Common circuit components include resistors, lamps and diodes:
The resistance of a resistor such as a metal wire does not change provided that there is no significant change in its temperature; a graph of current against voltage shows that the current is proportional to the voltage
The wire in a filament lamp becomes hotter as the current in the filament increases, causing an increase in its resistance
A diode only allows current to pass in one direction (shown by the direction of the arrow on its symbol)
The direction of the current is always shown as being from positive to negative
The resistance of some circuits depend on their surroundings; these components are often found in electronic circuits used for switches and maintaining constant environmental conditions in, for example, greenhouses and incubators:
The resistance of a light-dependent resistor (LDR) decreases with increasing light level
The resistance of a thermistor decreases with increasing temperature
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Safety
A mains cable has three coloured wires:
Brown – the live wire
Blue – the neural wire
Yellow/green – the earth wire
Charges can safely flow to the ground if a conductor is earthed
Metal appliances are earthed in order to protect you from accidental shocks
An earthed conductor cannot become live
Double-insulated appliances have plastic casing and do not need to be earthed
The fuse is a safety device. A large current will melt the fuse and cut off the supply
A fuse prevents large currents from starting a fire
Circuit breakers are safety devices and can be described as ‘re-settable fuses’
A fuse has to be replaced after a fault whereas circuit breakers just have to be reset
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