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11 Cards in this Set
- Front
- Back
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What is direct current (dc)?
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Current in one direction only. Batteries are direct current.
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What is alternating current (ac)?
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Alternating current, like mains electricity, repeatedly reverses its direction. Its frequency is the number of cycles is passes through each second.
In the UK, mains frequency is 50Hz. |
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How do you work out the frequency of ac supply?
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1/the time taken for one cycle = the frequency of ac supply
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Why don't appliances with plastic cases have an earth wire connection?
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They are double insulated since they have a plastic case (plastic is a good electrical insulator).
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What adaptations does a three-pin plug have?
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The pins are made from brass since brass is a good conductor and doesn't rust or oxidise. Copper isn't as hard as brass even though it conducts better.
The case material is an electrical insulator. The inside of the case is shaped so the wires and pins cannot touch each other when the plug is sealed. The plug contains a fuse between the live pin and the live wire. If too much current passes through the wire in the fuse, it melts and cuts the live wire off. |
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Which colours correspond to which wires in a three-pin plug?
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Earth is green and yellow.
Neutral is blue. Live is brown. |
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What are the adaptations of a cable?
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Copper is used for the wires since it is a good electrical conductor and bends easily.
Plastic is a good insulator and prevents anyone touching the cable from receiving an electric shock. Two-core cables are used for appliances that have plastic cases. Cables of different thicknesses are used for different purposes. Cables joining the wall sockets in the house are thicker then the ones joining the light fittings. This is because more current passes along wall socket cables. This stops the heating effect of the current making the wires too hot. |
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How does a fuse work?
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A fuse contains a thin wire that heats up and melts if too much current passes through it. We say the fuse 'blows'. The fuse should always be in series with the live wire between the live wire and the application.
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What is a circuit breaker?
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An electromagnet in series with a switch that opens when too much current passes through it. They work faster than fuses and can be reset more quickly.
A Residual Current Circuit Breaker (RCCB) works faster since it cuts off the current in the live wire when it is different from the current in the live wire. It is more sensitive than either a fuse or ordinary circuit breaker and can be used where there is no earth connection. |
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What is the unit of energy supplied by mains electricity?
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The kilowatt-hour.
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Why does a step-up transformer raise the voltage?
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Less current is needed to transfer the same amount of power so less energy is lost through heat.
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