In which of the following devices is the thermal expansion considered a disadvantage?

Analyze the question: We need the device where thermal expansion is a disadvantage (an unwanted effect), not one that deliberately uses expansion to work.

Concept: Thermal expansion is the increase in size of a material when heated. Many devices use it on purpose:

• Fire alarm: A bimetallic strip bends when heated and completes a circuit to ring the bell. Expansion is useful.
• Thermostat: A bimetallic strip bends to switch a heating or cooling circuit on or off. Expansion is useful.
• Bimetallic thermometer: The strip coils or bends in proportion to temperature, which moves the pointer. Expansion is useful.
• Balance wheel of a watch: The balance wheel controls the timing of the watch. If it expands when it gets warm, its size changes, which changes its moment of inertia and therefore its oscillation period. The watch then gains or loses time. Expansion here is harmful, so the wheel is made from special alloys (such as invar-type compensating balances) to reduce it.

Solution:

1. Devices A, B and C depend on expansion to function.
2. The balance wheel needs a constant size to keep accurate time, so any expansion causes timing errors.
3. Therefore thermal expansion is a disadvantage in the balance wheel of a watch.

Tip: Ask yourself, "Does the device use expansion to work?" Bimetallic strips (alarms, thermostats, thermometers) use it. Precision timekeepers and measuring instruments (watch balance wheels, pendulum clocks, steel tapes) need it avoided, which is why invar is used.