MadSci Network: Physics

Re: Why does blowing hot stuff make it cooler?

Date: Tue Oct 20 01:20:17 1998
Posted By: Radu Grigore, Undergraduate, Electronics and Telecommunications, Politehnica University of Bucharest
Area of science: Physics
ID: 907650894.Ph

The short answer would be:
a) Because the speed of evaporation increases;
b) Yes, a bigger surface means quicker cooling.

But to understand this you must first know the answer of two more
(theoretical) questions:
1. What is termic motion and temperature?
2. What is evaporation?

So, let's start.
1. Every body has a discrete structure which means it is made of little
   particles which can be counted (as oposed to continous structure -
   you can't count the points on a line). This little particles (atoms,
   molecules) are always moving one related to another; this motion
   is named termic motion.
   Temperature is a measure of the amount of this motion. It is
   proportional with the mean kinetic energy of the particles.
2. Of course there will always be particles moving faster and particles
   moving slower. A particle in a liquid which is moving fast toward
   the surface will eventualy escape and became a part of the gas
   above. There are always vapors above a liquid. Some of the particles
   which escaped earlier might also fall back in.
   If the number of the particles which escape is greater than the
   number of particles which fall back we say the liquid evaporates
   (transforms to gas).
   If the faster molecules go into the gas above then into the liquid
   remain the slower ones which means lower temperature.

Hints: If you blow there will be less molecules which fall back.
       A bigger surface meanse more molecules escape compared to the
       molecules which fall back in absolute numbers.

If you want a more explicite answer don't hesitate to email me:

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