Using Electricity In The Home
Übersicht
A torch and a table lamp both light up, and the supplies behind them could hardly be less alike. The torch cell pushes charge around its circuit one way and keeps pushing that way until it is flat. The lamp is fed by a supply that reverses itself a hundred times a second, is powerful enough to kill, and is trusted anyway because three wires, a strip of thin metal and a switch that flicks itself off stand between it and the person holding the plug.
This lesson is about telling those two supplies apart and about the arrangements that make the dangerous one safe to live with. You will learn to read a supply straight off an oscilloscope screen: how many volts the crest of a wave is worth, how long one cycle lasts, and how to turn that time into a frequency in your head. Then you will follow a single fault through a metal-cased appliance, from the loose live wire that makes the case dangerous to the fuse that melts a fraction of a second later, and see why every one of those protective devices belongs in the same one of the three wires.
Ziele
- Cells and batteries supply current that always passes in the same direction. This is called direct current (dc).
- An alternating current (ac) is one that is repeatedly changing direction. Students should be able to determine the period, and hence the frequency, of a supply from diagrams. They should be able to compare and calculate potential differences of dc supplies and the peak potential differences of ac supplies from diagrams.
- Mains electricity is an ac supply, which has a set frequency and voltage. Knowledge of root mean square (rms) measurements and values are not required.
- There are a number of safety features that can be incorporated in electrical systems and appliances. One of these is earthing: if the metal body of an appliance becomes live through a fault, the current is harmlessly conducted away.
- If an electrical fault causes too great a current to flow, a fuse or a circuit breaker in the live wire disconnects the circuit. The current will cause the fuse wire to overheat and melt or the circuit breaker to switch off (‘trip’). A circuit breaker operates much faster than a fuse and can be reset.
- Appliances with metal cases are usually earthed. If a fault develops a large current flows from the live wire to earth. This melts the fuse and disconnects the live wire. Students should be aware that some appliances are double insulated and therefore have no earth wire connection.
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- Which of these supplies a current that always passes in the same direction? A. A battery B. The mains supply C. A signal generator set to 50 Hz D. The output of an alternator Answer: A
- One complete cycle of an alternating supply occupies 4 divisions on an oscilloscope screen. The time base is set to 5 ms per division. What is the frequency of the supply? A. 20 Hz B. 50 Hz C. 200 Hz D. 250 Hz Answer: B
- In which wire of a mains circuit must a fuse or a circuit breaker be fitted? A. The live wire B. The neutral wire C. The earth wire D. Either the live wire or the neutral wire Answer: A
- Which statement gives an advantage of a circuit breaker over a fuse? A. It is cheaper to make B. It operates much faster and can be reset C. It allows a larger current to flow before acting D. It does not need to be connected to the live wire Answer: B
- Why does a double insulated appliance not need an earth wire connection? A. It is supplied with direct current B. It draws only a very small current C. Its casing does not conduct, so no part the user touches can become live D. Its fuse is fitted in the neutral wire instead Answer: C
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