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 many 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. Telling them apart takes a single question, asked about the current rather than about anything else: does it always pass the same way round the circuit, or does it keep turning round? Everything after that is protection. You will follow one fault through a metal-cased appliance, from the loose live wire that makes the case dangerous, through the earth wire that gives the current somewhere harmless to go, to the fuse that melts a fraction of a second later, and you will 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.
- 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 socket in a classroom wall D. The supply feeding a street lamp Answer: A
- Which statement describes an alternating current? A. A current that is always the same size B. A current that is repeatedly changing direction C. A current that always passes in the same direction D. A current that flows only when a fault develops 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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Lehrbuchabschnitt
Electrical Safety: Systems and Devices
Electricity has two hazards. A thermal hazard occurs when there is electrical overheating. A shock hazard occurs when electric current passes through a person. Both hazards have already been discussed. Here we will concentrate on systems and devices that prevent electrical hazards. Figure 23.31 show
"College Physics: Electrical Safety: Systems and Devices" by OpenStax, OpenStax, Rice University, licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Source: https://openstax.org/books/college-physics/pages/23-8-electrical-safety-systems-and-devices. Access for free at https://openstax.org/books/college-physics/pages/1-introduction
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Lehrbuchabschnitt
Alternating Current versus Direct Current
Most of the examples dealt with so far, and particularly those utilizing batteries, have constant voltage sources. Once the current is established, it is thus also a constant. Direct current (DC) is the flow of electric charge in only one direction. It is the steady state of a constant-voltage circu
"College Physics: Alternating Current versus Direct Current" by OpenStax, OpenStax, Rice University, licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Source: https://openstax.org/books/college-physics/pages/20-5-alternating-current-versus-direct-current. Access for free at https://openstax.org/books/college-physics/pages/1-introduction
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