Answer
The following are the four methods in which heat is lost from the body-- conduction, radiation, convection, and evaporation.
Conduction: In this process heat at the body's surface is conducted away when it comes in contact with cooler surfaces (or air), and this results in cooling of the body
Radiation involves the loss of heat as infrared radiation. When we are warmer than the environment, we give off heat to the environment in the form of heat IR) rays. This has the effect of lowering the body temperature. Convection is the transfer of heat to moving fluids like water, air or blood. When molecules of a fluid become heated, they move faster than cooler molecules. Thus warm blood carries heat from core areas to the periphery of the body. When the skin become heated by warm blood some of this heat is picked up by the surrounding air which becomes more energetic and rises as it becomes lighter. The surrounding cooler air flows in to take its place. As this convection cycle continues, more and more heat is lost from the skin, and the body cools.
Evaporation is the process by which liquid is transformed to gas. In the body this happens when sweat on the surface of the skin is transformed into the gas, water vapor. It takes a great deal of heat to convert liquid water (H2O) to water vapor (H2O), a fact that is usually stated by saying, "the latent heat of vaporization is high"
At a comfortable environmental temperature of 21 deg. C (69.8 F) the body loses heat in the following proportion -60%, 18%, and 22% by radiation, conduction, and evaporation , respectively.
Work Step by Step
Values for heat of vaporization about 0.58 kcal/gal. ( or 540 cal/gram).
