Chapter 10 – Electricity
Consider the following statements and click to reveal the answer.
1. Give two reasons why fuse wire has a higher resistance than flex.
It has a higher resistance because it is very thin and is made from a metal which does not conduct as well as copper.
2. What is likely to happen if an extension cable made from thin flex is used to supply a heater?
The wire will resist the electricity and become hot and eventually melt. Thick wires should always be used on extension flexes.
3. Why is resistance a problem for electrical supply companies?
The miles of wire underground or hanging from pylons present a substantial resistance to the flow of electricity. Removing the heat produced is difficult, especially where wires are buried underground.
4. Would you expect two wires side by side to conduct electricity better than a single wire? Explain your answer.
Yes. They will act like one thicker wire resulting in less resistance.
5. Why does a variable resistor get warm?
It is resisting electricity and converting some into heat.
6. Predict the brightness of the bulbs in the circuits below, if a 3 volt battery is used and the bulbs are as follows:
Bulb (a) 6 volts/0.2 amps ... this bulb has a resistance of 30 Ω
Bulb (b) 2.5 volts/0.2 amps ... this bulb has a resistance of 12.5 Ω
Bulb (c) 2.5 volts/0.1 amps ... this bulb has a resistance of 25 Ω
- A series circuit with two Bulbs (b);
- A series circuit with Bulb (b) and Bulb (c);
- A parallel circuit with Bulb (a) in one pathway and Bulb (b) in the other.
- Equally bright (equal resistance).
- Bulb (c) is brighter than (b) (the electricity has to work harder to get through the higher resistance bulb, therefore more energy is converted in this bulb).
- Bulb (b) is brighter than (a) because Bulb (b) has less resistance so more electricity goes down this pathway than down the pathway controlled by high-resistance Bulb (a).
7. Look back at the Practical task in the section on series and parallel circuits. If the second pathway does not have any chair in it, why does this act like a short circuit?
The short circuit occurs when there is no resistance in one of the pathways of a parallel circuit.
8. Calculate the time that it would take:
- A 100W bulb to use 1kW hour of electricity;
- A 20W fluorescent tube to use 1kW hour of electricity.
- 100W bulb: 10 hours.
- 20W fluorescent tube: 50 hours.
- How many joules of energy will an 8kW shower use in 10 minutes?
- Will it use less energy than heating a 3kW kettle for an hour?
- 8kW shower: 4 800 000J.
- Yes. The kettle will use 10 800 000J.
10. Why do bulbs in series circuits glow less brightly than the same bulbs in a parallel circuit?
In the series circuit the resistance of both bulbs is combined and the flow is strongly resisted. There is less resistance to electrical flow when there is only one bulb (resistor) in the circuit.
11. Which has the higher resistance: a 100W bulb or a kettle? Explain your answer.
The bulb has the higher resistance because the kettle allows more electricity to pass through it.
12. Explain why the wire flex leading to a kettle is thicker than the flex leading to a table lamp.
The flex leading to the kettle has to carry a greater flow of electricity. If the kettle’s flex was thin, it would get hot because of the high resistance of the thin wire.