[long_answer] Tungsten is used as the filament in electric bulbs, while alloys such as nichrome are preferred for the heating coils of devices like toasters and electric irons. Both applications exploit the heating effect of electric current. (i) Explain why tungsten is chosen for a bulb filament rather than for a heating coil. (ii) Why is an alloy preferred over a pure metal for the heating element of a toaster or electric iron?
Generated by claude-sonnet-4-6 · 2026-06-26 01:12 · grounding rag
Model Answer
(i) Why tungsten is used for a bulb filament (not a heating coil):
In an electric bulb, the filament must become extremely hot so that it emits light. Tungsten has a very high melting point (3380°C), so it does not melt even at the high temperatures needed to produce light. It is also a strong metal that can retain the heat generated. However, tungsten is not preferred for heating coils because it would oxidise easily at high temperatures when exposed to air, making it less suitable for open heating elements.
(ii) Why an alloy is preferred over a pure metal for heating elements:
Alloys such as nichrome are preferred for heating coils because:
- They have higher resistivity than pure metals, so they produce more heat for the same current.
- They have a high melting point, so they do not melt or deform easily at high operating temperatures.
- They do not oxidise (corrode) readily at high temperatures, giving them a longer working life.
Source: Chapter 11, Section 11.7.1 (Practical Applications of Heating Effect of Electric Current)
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Explanation
- The textbook (Exercise 18a & 18b) directly asks these two questions, so examiners expect answers rooted in the three properties: melting point, resistivity, and oxidation resistance.
- For (i), the key contrast is: tungsten's high melting point suits the extreme temperatures needed for light emission in a sealed bulb; but heating coils need a material that also resists oxidation in open air — which tungsten doesn't do as well as nichrome.
- For (ii), always mention all three alloy advantages (high resistivity, high melting point, resistance to oxidation) — each is worth marks separately.
- Avoid writing vague statements like "alloys are better" without specifying why.