9.3.2Electricity and Circuits
Reducing Heating in Circuits — Edexcel GCSE Combined Science 1SC0 - Physics Revision Notes & Topical Questions
5 min read 8 parts Lesson 14 of 22
Part 1 of 6Why a wire heats up
1
A metal is a lattice of positive ions surrounded by delocalised electrons that are free to move.
2
When there is a current, the electrons drift through the lattice.
3
The electrons collide with the lattice ions, which resist their flow.
4
In each collision an electron gives some of its energy to the ion, which then vibrates more.
5
Greater vibration of the ions means the metal's internal energy and temperature increase.
Part 2 of 6Appliances that use the effect
Designed to heat
- Designed to heat Electric heaters, ovens, hobs, toasters and kettles all use a high-resistance element which becomes hot when a current flows through it.
Heat not wanted
- Heat not wanted In most other appliances, such as the cables carrying current to them, thermal energy is a waste and can be dangerous.
Part 3 of 6Reducing unwanted heating (HT only)
Less heating in wires
Reduce the current
- Reduce the current A smaller current means fewer electron-ion collisions each second, so less energy is transferred to the wire.
Use a low-resistance wire
- Use a low-resistance wire A wire with lower resistance transfers less energy at a given current.
- Thicker wires have lower resistance.
Choice of metal
- Choice of metal Copper is the usual choice: its resistance is low and it is fairly cheap.
- Metals with even lower resistance exist but are more expensive.
Part 4 of 6Linking cause and effect
| Change | Effect on collisions | Effect on heating |
|---|---|---|
| Larger current | More frequent collisions | More thermal energy |
| Higher resistance | Greater opposition to electron flow | More thermal energy at the same current |
| Thicker copper wire | Lower resistance | Less thermal energy |
Part 5 of 6Worked example: why thick cables are used
1The problem
- The problem A cooker needs a large current of about 30 A, and a thin wire carrying that current would become dangerously hot.
2Choose a thick wire
- Choose a thick wire A thicker copper wire gives the electrons more room to move,
- so there are fewer collisions with the ions and the resistance is lower.
3Lower heating
- Lower heating Less energy is transferred to the lattice each second, so the cable stays much cooler at the same current.
4Safe result
- Safe result This is why cookers and showers are connected with thick cables, while a lamp can use a thin one.
Part 6 of 6Other ways of limiting heating
Protective devices
- Protective devices Fuses and circuit breakers cut off the supply when the current is too large, which stops a wire overheating in a fault.
Cooling
- Cooling Appliances such as computers use fans and vents so that thermal energy is carried away and components do not overheat.
Exam link
- Exam link Always link a change to the number of collisions: fewer collisions per second means less energy passed to the ions and a smaller temperature rise.
Key terms
| Term | Meaning |
|---|---|
| Lattice | The regular arrangement of positive ions in a metal. |
| Delocalised electrons | Electrons in a metal that are free to move through the structure. |
| Current | The rate of flow of electric charge. |
| Thermal store | The energy store associated with the temperature of an object. |
| Resistance | The opposition of a component to the flow of current; a thicker wire has a lower resistance. |
Avoid these mistakes
1
Saying electrons heat up themselves and carry the heat - energy is passed to the ions in collisions.
2
Writing that the ions flow in a metal - the ions vibrate in fixed positions and only the electrons move.
3
Suggesting a higher resistance wire for household cables - that would increase heating.
4
Forgetting that copper is chosen for both low resistance and low cost.
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