IGCSE Physics / Mains electricity
Why a current heats a resistor
Edexcel International GCSE Physics (4PH1), chapter 6: Mains electricity, specification point 2.3. Free lesson.
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A current heats a resistor because moving electrons transfer energy to ions in the metal. This heating is useful in some appliances, but cables carrying large currents need to be thick to avoid overheating.
How collisions heat a resistor
A metal contains a regular pattern, or lattice, of positive ions and free electrons. The voltage pushes the electrons through the metal. Because the wire has resistance, the electrons collide with the ions and transfer some of their energy to them. The ions vibrate more, so the resistor’s temperature rises.
Describe the heating as an energy transfer
The correct energy description is that energy is transferred electrically to the thermal store of the resistor. The electrons are not used up. They keep flowing around the circuit, and the current coming out of the resistor is the same as the current going in. Say that energy is transferred to the resistor’s thermal store. Do not say that the current is used up or turns into heat.
Where the heating effect is useful
Appliances can be designed to transfer energy electrically to a thermal store on purpose. They use hot elements or wires to make a useful change. A kettle has a heating element, a coil of high-resistance wire, under the water. It boils the water. A toaster has thin wires that get hot enough to glow and make toast. An electric heater uses a hot element to warm a room. A hair dryer blows air over a hot coil to dry hair. A fuse has a thin wire that melts if the current is too big. It is in a plug.
Why high-power cables are thick
Heating can be unwanted. A thin cable has more resistance than a thick cable made of the same metal. With the same large current, the thin cable heats more and could overheat, melt its insulation and start a fire. A thick cable heats much less, so it is used for appliances such as kettles and cookers. A thin cable is suitable for a lamp, which draws a small current. Warm wires and chargers also show unwanted heating, and that energy is wasted in the surroundings. Thin cable: higher resistance, more heating for a large current, and a risk of overheating. Thick cable: lower resistance,…
Explain why a wire gets hot
A three-mark explanation follows the chain from collisions to a rise in temperature. Include each link so the cause of the heating is clear.