Physical Science: Session 7
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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