AIIMS 2006 Physics Heat Transfer Assertion Reason Question

To analyze the given question regarding the assertion and reason about perspiration and cooling the human body, let’s break down each component.

Assertion:

Perspiration from the human body helps in cooling the body.

This assertion is true. When the body heats up, it produces sweat (perspiration) that evaporates from the skin surface. The process of evaporation is endothermic, meaning it absorbs heat from the body, thus cooling it down. The cooling effect can be understood using the concept of latent heat of vaporization.

The heat absorbed during the evaporation can be quantified as:

$Q = m \cdot L_v$

where:

• $Q$ = heat absorbed,
• $m$ = mass of the sweat evaporated,
• $L_v$ = latent heat of vaporization of water (approximately $2260 \, \text{kJ/kg}$ at 100°C).

Reason:

A thin layer of water on the skin enhances its emissivity.

This reason is false in the context of explaining the cooling effect from perspiration. While it is true that a layer of water can affect the emissivity and thermal properties of the skin, the cooling effect from perspiration primarily arises from the evaporation process rather than emissivity. Emissivity is a measure of an object's ability to emit thermal radiation, and while water does have a high emissivity, this does not directly contribute to the cooling mechanism of sweat.

Conclusion:

Since the assertion is true and the reason is false, the correct answer is C) Assertion is true but Reason is false.

Why Other Options are Incorrect:

• Option A: Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

  • Incorrect, because while the assertion is true, the reason is false and does not explain the assertion.

• Option B: Both Assertion and Reason are true but Reason is not the correct explanation of Assertion.

  • Incorrect, as the reason is not true; it does not accurately describe the process that leads to cooling through perspiration.

• Option D: Both Assertion and Reason are false.

  • Incorrect, since the assertion is true.

Relevant Concepts:

• Evaporation: The process where liquid water turns to vapor, absorbing heat, which leads to cooling.
• Latent Heat of Vaporization: The amount of heat required to convert a unit mass of a liquid into vapor without a change in temperature.

In conclusion, perspiration cools the body primarily through the evaporation of sweat, and while water does have high emissivity, this property does not explain the cooling effect in question.