In which of the following is the expansion of solids a disadvantage?

Analyze the question: We need the situation where thermal expansion of a solid is harmful (a disadvantage), not one where it is deliberately used.

Concept: Solids expand when heated. Engineers use this in some devices, but in others it causes errors that must be corrected.

Go through the options:

• A. Electric iron: The thermostat uses a bimetallic strip. Expansion is used to switch the current off at a set temperature, so it is an advantage.
• B. Fire alarm: A bimetallic strip bends when heated and completes a circuit to ring the bell, so it is an advantage.
• C. The balance wheel of a watch: When temperature changes, the metal wheel expands or contracts. This changes its moment of inertia and the period of oscillation, so the watch gains or loses time. This is a disadvantage, and it is why compensated balance wheels are made from bimetallic rims or special alloys such as invar.
• D. Bimetallic thermometer: It works by the unequal expansion of two metals, so it is an advantage.
• E. Fitting wheels in rims: The iron rim is heated so it expands, slipped over the wheel, and then cooled so it contracts and grips tightly. This is an advantage.

Answer: The balance wheel of a watch is the only case where expansion causes a problem (inaccurate timekeeping).

Tip: For "advantage or disadvantage of expansion" questions, ask whether the device uses expansion to work (thermostats, alarms, rim fitting, bimetallic thermometers) or whether it needs precision that expansion would spoil (watches, clocks, pendulums, measuring tapes). Precision instruments are usually the disadvantage cases.