When an athlete perspires after running, he loses heat through

Step 1: Analyze the question The question asks how an athlete loses heat when he perspires (sweats). The key word is "perspires," so we need the heat-loss mechanism linked to sweat.

Step 2: Explain the concept Evaporation is the change of a liquid into vapour at a temperature below its boiling point. For a liquid to evaporate, its molecules need energy (latent heat of vaporisation). The fastest-moving, most energetic molecules escape from the surface. They take their energy with them, so the average kinetic energy of the remaining liquid falls. This means the liquid, and the surface it touches, gets cooler.

Step 3: Apply it to the athlete

• Running raises the body's temperature.
• The body releases sweat onto the skin.
• The sweat absorbs heat from the body to evaporate.
• This removes heat from the body and cools the athlete down.

Step 4: Check the other options

• B. Conduction: heat transfer by direct contact, mainly in solids. It isn't the process involved in sweat drying off the skin.
• C. Radiation: heat transfer by electromagnetic waves, needing no medium. The body does lose some heat this way, but it has nothing to do with sweating.
• D. Convection: heat transfer through the movement of fluids (liquids or gases). Air moving over the skin can help, but the cooling effect of sweat itself is not convection.

Step 5: Answer The correct option is A. Evaporation.

Exam tip: Link keywords to processes:

• Sweating or perspiring → evaporation (cooling)
• Hot metal spoon in tea → conduction
• Sun's heat reaching Earth → radiation
• Boiling water or sea breeze → convection

This is also why a wet cloth or a pot of water feels cool as it dries.