Physics (P2) - Mains Electricity

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  • Created by: Roma
  • Created on: 14-12-13 10:41
What is a direct current?
When cells and batteries supply current that passes round the circuit in one direction.
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What is an alternating current?
When the current from the mains supply passes in one direction, then reverses and passes in the other direction.
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What is the frequency of the UKs mains supply?
50Hz, which means it changes direction 50 times each second. The voltage of the mains is 230V.
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What is the peak voltage of an alternating p.d.?
The maximum voltage measure from 0 volts.
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What is the live wire?
Alternatively positively and negative every cycle and neutral wire at 0 volts.
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How are most electrical appliances connected to the mains supply?
Through sockets using cables and a three pin plug.
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What is the outer cover of a three pin plug made of?
Rubber or plastic as they are both good electrical insulators.
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What are the pins of a three pin plug made of?
Brass as it is a good electrical conductor . It is hard and does not rust or oxidise.
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Which is the longest pin?
The earth wire.
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Why should the cable grip be fastened tightly over the cable.
So there are no bare wire showing inside the plus and the correct cables must be connected firmly to the terminal of the correct pin.
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Which colour wire is the live pin?
The brown wire.
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Which colour wire is the neutral pin?
The blue wire.
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What are the green and yellow wires?
They are connected to the earth pin.
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How and why must metal cases be earthed?
By the cases being attached to the earth wire. If there is a fault and the live wire touches the metal case, the case becomes live and could give a shock.
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Why don't appliances with plastic cases need to be earthed?
They are double insulated and are connected to a power supply with two core cables with just a live and neutral wire. They cannot be live.
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Why would a cable of double thickness be used?
If a lot of current needs to be carried.
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How is a fuse fitted?
In series with the live wire. This cuts the appliance off from the live wire if the fuse blows.
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What happens if a fault develops in an earthed appliance?
A large current flows to earth and melts the fuse, disconnecting the supply.
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What should the rating of the fuse be?
Slightly higher than the normal current of the appliance. If it is too high, it will not melt soon enough. If it is too low, it will melt as soon as the appliance is switched on.
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What is a circuit breaker?
An electromagnetic switch that can be used in place of a fuse. It opens and cut off the power supply if the charge is bigger than a certain value.
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What residual current circuit breaker (RCCB) do?
It cuts off the current in a live wire if it is different to the current in the neutral wire. It works faster than a fuse or normal circuit breaker.
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What is the power supplied to a device?
The energy transferred to it each second.
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What do you usually measure in an electric circuit?
The current through an appliance and the potential difference across it.
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What is an electric current?
The rate of flow of charge.
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What happens when charge flows through an appliance?
Electrical energy is transferred to other forms. In a resistor, electrical energy is transferred to the resistor so the resistor becomes hotter.
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Why can electrical faults be dangerous?
They can cause electric shocks and fires.
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How do you reduce the risk of potential hazards?
Get equipment checked regularly, don;t overload sockets, don't touch electrical equipment with wet hands.
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How should you choose an electrical appliance?
Power and efficiency rating should be considered as well as cost. Filament and halogen bulbs are less efficient than lower energy bulbs and do not last as long.
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Other cards in this set

Card 2

Front

What is an alternating current?

Back

When the current from the mains supply passes in one direction, then reverses and passes in the other direction.

Card 3

Front

What is the frequency of the UKs mains supply?

Back

Preview of the front of card 3

Card 4

Front

What is the peak voltage of an alternating p.d.?

Back

Preview of the front of card 4

Card 5

Front

What is the live wire?

Back

Preview of the front of card 5
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