According to kinetic theory a liquid has molecules moving at many speeds. The av
ID: 583906 • Letter: A
Question
According to kinetic theory a liquid has molecules moving at many speeds. The average kinetic energy of the molecules is proportional to the temperature of the liquid. When an individual molecule just happens to have enough kinetic energy to overcome the forces holding one molecule near its neighbors it "boils off" or "evaporates". When the energetic molecule leaves the scene the remaining molecules have less energy on the average and the liquid cools a bit. This process is called cooling by evaporation. What effect does the evaporation of water from your skin have on how hot or cold you feel? During evaporation, heat energy is transferred. What is it transferred from and what does it transfer to?Explanation / Answer
a) We feel cold on evaporation of water from our skin .
The human body makes use of evaporative cooling byperspiration to give off energy even when surrounded by a temperature higher than body temperature. The cooling process is an example of the approach to thermal equillibrium.
Even when one is unaware of perspiration, physiology texts quote an amount of about 600 grams per day of "insensate loss" of moisture from the skin.
The cooling effect of perspiration evaporation makes use of the very large heat of vaporization of water. This heat of vaporization is 540 calories/gm at the boiling point, but is even larger, 580 cal/gm, at the normal skin temperature.
Q / t = 600 (gms/day) * 580 cal/grm * 4.186 J/cal * 1 day/24 hrs * 1 hr / 3600 sec = 17 watts
b) The heat is transferred from surroundings to body when the ambient temperature is above the body temperature.
It is transferred by conduction , convection and radiation into our bodies.
In evaporation process the water molecules absorb energy from body and cools the body.
heat transfer by vapourisation ;
If part of a liquid evaporates, it cools the liquid remaining behind because it must extract the necessary heat of vaporization from that liquid in order to make the phase change to the gaseous state. It is therefore an important means of heat transfer in certain circumstances, such as the cooling of the human body when it is subjected to ambient temperatures above the normal body temperature
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