6.3 Domestic uses and safety — Practise with help

1. $I = \dfrac{300}{60} = 5.0\,\text{A}$.
2. $R_{total} = 5.0 + 7.0 = 12\,\Omega$.
3. A voltmeter.

If question 1 went wrong, check that you divided the charge by the time. If question 2 went wrong, remember that resistances add in series.

Think about the direction in which the charge flows in each kind of supply.

Give the two numbers for the UK supply first. Then say which kind of supply changes direction and which one does not.

The UK domestic supply has a frequency of 50 Hz and a potential difference of about 230 V.

A direct current flows in one direction only, so the potential difference keeps the same direction. A battery is a direct supply. An alternating current keeps changing direction, so the potential difference swaps ends many times each second. The mains supply is alternating.

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The equation $Q = It$ uses time in seconds. Change the time before you substitute.

Work out $1.5 \times 60 = 90\,\text{s}$, then multiply by the current.

One wire carries the supply, one completes the circuit, and one is there only for safety.

Match each job to a wire. Remember which wire is at 0 V in normal use.

The live wire carries the alternating potential difference from the supply. It is at about 230 V relative to earth.

The neutral wire completes the circuit. It is at, or close to, earth potential, which is 0 V.

The earth wire is a safety wire. It is at 0 V and carries a current only if there is a fault. It stops the metal case of an appliance becoming live.

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An open switch breaks the circuit, but it does not remove the supply from every wire.

Say where the switch might be placed, and which wire stays connected to the supply even when the switch is open.

The switch might be in the neutral wire, or between the appliance and the live wire. The wire that goes back to the supply is still connected to the live supply, so it stays at about 230 V. If a person touches it, a current can flow through the body to earth. That is why the live wire is dangerous even when the appliance appears to be switched off.

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Imagine the live wire coming loose inside the appliance and touching the metal case.

Say what would happen to the case without an earth wire, then what the earth wire does instead.

Without an earth wire, a loose live wire touching the metal case would make the case live at about 230 V. A person touching the case could receive a dangerous current.

With the case joined to the earth wire, the fault current has a low-resistance path to earth. A large current flows to earth, and the case does not stay at 230 V, so it is much safer to touch.

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The lamp and the two wires form a single closed loop.

Use the idea from Section 6.1 that the current is the same at every point in a single closed loop.

The lamp, the live wire and the neutral wire form a single closed loop. The charge that flows into the lamp must flow out of it again, because charge is not used up. The rate of flow of charge is the current, so the current in the live wire equals the current in the neutral wire. Only the energy carried by the charge is transferred to the lamp.

This answer is a model answer for you to compare with your own wording.