In hot summer after a bath, the body's (a) internal energy decreases (b) internal energy increases (c) heat de

I. Multiple Choice Questions · Q1

Q. In hot summer after a bath, the body's

(a) internal energy decreases

(b) internal energy increases

(d) no change in internal energy and heat

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Concept understanding — Latent Heat and Change of State

Latent Heat and Change of State

When a substance changes state — solid to liquid (melting) or liquid to gas (boiling) — it absorbs heat without any change in temperature. The heat that is exchanged during this constant-temperature phase change is called latent heat. For a mass $m$ that completely changes state,

Q = mL,

where $L$ is the specific latent heat of the substance for that transition: the latent heat of fusion $L_f$ for melting/freezing and the latent heat of vaporisation $L_v$ for boiling/condensation. The absorbed energy goes into breaking the bonds between molecules (increasing potential energy) rather than increasing their kinetic energy, which is why the temperature holds steady during the change.

Reading a heating curve. If heat is supplied at a steady rate to ice and the temperature is plotted against time, the graph has sloped segments and flat plateaus:

A sloped segment (e.g. ice warming, or water warming from $0\,^\circ$ C to $100\,^\circ$ C) is governed by specific heat, $Q = mc\,\Delta T$ .
A horizontal plateau at $0\,^\circ$ C is melting: ice and water coexist in equilibrium while latent heat of fusion is absorbed.
A horizontal plateau at $100\,^\circ$ C is boiling: water and steam coexist while latent heat of vaporisation is absorbed.

During each plateau the two phases are in thermal equilibrium at the transition temperature, and only after the change of state is complete does the temperature start to rise again. The length of a plateau (at fixed heating rate) is proportional to the latent heat, which is why the boiling plateau is much longer than the melting plateau for water.

Latent heat and the heating-curve concept are covered in the NCERT Class 11 Physics chapter on Thermal Properties of Matter, and "latent heat of fusion and vaporisation formula" along with "change of state class 11 important questions" are frequently searched by CBSE board and NEET physics students. Reading a heating curve correctly, as explained above, is a recurring NEET and JEE Main question type built on this exact NCERT Class 11 concept.

Step 1. After a bath, a thin film of water remains on the skin. This water evaporates into the surrounding air.

Step 2. Evaporation is a liquid-to-gas change of state, which requires latent heat of vaporization; that heat is drawn from whichever body is in contact with the water film -- here, the skin.

Step 3. Since heat leaves the body to supply this latent heat, and no compensating heat enters (no work is done either), the first law $\Delta U = Q - W$ with $Q<0$ gives $\Delta U<0$ : the body's internal energy decreases. This is exactly why you feel a chill right after stepping out of a bath, even in hot summer.

Step 4. Option (b) is the opposite of the physics; options (c) and (d) both misuse "heat" as though it were a stored quantity that can "decrease" or stay unchanged -- but there genuinely IS a change in internal energy here.

✓Final answer

(a) internal energy decreases

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Q.In hot summer after a bath, the body's

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