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A Closer Look: Why do We Need Heated Towel Racks?
In this video, weather forecaster Bill Babcock tells us that, when we
step out of the shower, it takes energy from our skin to turn liquid water
into water vapor via evaporation. To understand this process better, let's
take a closer look at what happens at the microscopic level.
Recall that
earlier in the session we looked at what happens when energy is transferred
as heat from a hot mug of tea to your hand:
Essentially,
the process we are about to describe is the reverse of that process:
heat flows from your skin to the water.
In both your skin and the water
on your skin, the particles are moving with a distribution of speeds
(i.e., some move faster than others) but
the average energy of their motion is related to the temperature of
your skin and the temperature of the water. However, the particles in the
water that are moving faster than average may be moving fast enough
to
overcome
the pull they feel from their neighbors and break away from the surface
of the liquid water into the air. By doing this, the remaining water
is at a lower temperature because the average energy of motion of all
the
particles has gone down. See the side illustration for clarification.
Since there is now a temperature difference between your skin and the
water on it, heat flows to the water. On a microscopic level, your skin
particles collide with the water molecules, transferring some energy of
motion. As a result, the water molecules move faster and your skin particles
move slower. This leaves the temperature of your skin lower and you feel
colder, while the temperature of the water goes up and the faster-moving
molecules escape. The process keeps repeating until all the water is gone.
This is why it doesn't feel as cold if you towel off quickly.
This same
process happens at the surface of any liquid that is evaporating. Try
observing what happens in the microscopic world when heat is transferred
in the Session
3 Virtual Particle Lab.
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prev:
three scales of temperature
