Safe Electrics

Circuits and Symbols, Resistance, Circuit Components, Safety

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• Created by: tom
• Created on: 14-04-11 11:55

Circuits

• 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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