A bimetallic strip consists of metals A and B. It is mounted rigidly as shown. The metal A has higher coefficient of expansion compared to that of metal B. When the bimetallic strip is placed in a cold bath, it will :

Solution

Physics sir · 4 steps

On cooling, the metal that expands more also shrinks more, so the strip curls towards A, which is on the left.

1. Read the figure. The strip stands upright with its lower end fixed in the base. Metal A is on the left and metal B is on the right, bonded along their full length.
2. Cooling shrinks both. The change in length is Δ L = Lα Δ T. In a cold bath Δ T is negative, so both metals shorten. Since α_A > α_B, A shortens more than B.
3. Why it bends. The two metals are joined, so they cannot slide past each other. The strip has to curve, with the shorter metal on the inside of the curve and the longer metal on the outside.
4. So. A is on the inside, so the free top end swings towards A's side. The strip bends towards the left, which is option (b).

The idea to keep. A bimetallic strip always curves with the shorter metal on the inside. On heating that is the low-α metal; on cooling it is the high-α metal. Find which one ends up shorter, then bend towards it.

Answer from the NTA final answer key. Solution by PrepWiser.