NEET Physics: Thermal Properties questions with solutions

Q1. A sphere, a cube and a thin circular disc all made of the same material, have same mass. Their initial temperature is \( 3 \times 10^{3}^{\circ} \mathrm{C} . \) The body which cools rapidly is :

1. sphere
2. cube
3. both sphere and cube
4. circular plate

Answer: circular plate

For bodies of the same material and mass, the one with the greatest surface area relative to its volume cools fastest because it can radiate heat more quickly. A thin circular disc has a much larger exposed area compared with its volume than a sphere or cube, so it cools most rapidly.

Q2. While measuring the thermal conductivity of liquids, the upper part is kept hot and lower one cool so that:

1. convectional flow is stopped
2. radiation is stopped
3. conduction is easier
4. it is easier to perform the experiment

Answer: convectional flow is stopped

In a liquid, if the lower part is hotter, the less dense warm liquid rises and convection currents form. By keeping the upper part hot and the lower part cool, the denser liquid stays below the lighter liquid, so convection is suppressed and heat transfer is mainly by conduction.

Q3. Which of the following is correct about Phase \( A \rightarrow \) Phase \( C ? \)

1. Sublimation
2. Deposition
3. Vaporization
4. condensation E. Melting

Answer: Sublimation

Sublimation is the process where a substance changes directly from solid to gas. Since Phase A to Phase C implies skipping the liquid phase, sublimation is the correct choice.