Friday, June 8, 2012
Note: This program first aired on June 9, 2012.
Phase change. It’s a term that refers to the change of a state of matter. When a substance changes from solid to liquid, or liquid to gas it is said to change phase. What makes phase change interesting is that there is a set energetic pattern associated with it, and both the natural world, and human industry have incorporated that energy in useful ways.
When a substance, any substance, changes from a solid to a liquid, or a liquid to a gas, it requires energy. Energy must be absorbed by the substance for this to happen. We know this. It takes energy for ice to melt right? Or liquid water to boil away to steam. As the molecules of the substance absorb more energy, they move more. The more and further they move, the ‘looser ‘ substance’s structure becomes. At a certain point, the molecules absorb enough energy that they are behaving differently enough to appear as a different state of matter.
One wonderfully reassuring facet of nature is its symmetry. In this case, if energy is absorbed in one direction of phase change, the opposite happens when we reverse the phase change. When a substance goes from liquid to solid, or gas to liquid, energy is released. The molecules must release all that extra energy in order to slow down enough to get close enough to each other to go into a denser “lower energy’ state of matter. And of course, we understand this too; we have to cool water off to freeze it solid.
If you keep this concept in mind, you will recognize it in many places. Its why your refrigerator works, why the propane tank gets cold when you run it, why sweating works to cool us off, why panting helps dogs. Sweating, and panting are just operations that utilize the physics of evaporation. Our (and our dogs’) bodies are producing a liquid and taking advantage of the fact that turning it into a gas will remove energy from the system, in this case, our bodies. That is why sweating cools us off, evaporating that water off our skin absorbs energy, the extra heat we are trying to get rid of.
A refrigerator is an elegant example of how we use phase change in both directions to our advantage. We can construct a simple model of a refrigerator by imagining a tube in a closed loop. Half of it is inside a box, and half is outside. We put a substance in the closed loop that wants to be gas at room temperature (or even a little cooler). This means that left to its own devices, it at room temperature it will readily absorb energy and boil, changing from liquid to gas. Humans have figured out that if they compress that gas into a liquid, it will release energy. A refrigerator is simply a system that takes that substance, compresses it to a liquid form, pumps it into the space to be cooled, allows it to evaporate in the tube (hence cooling the surrounding area), pumps it out of the space to be cooled, compresses it back to a liquid (thus releasing the heat) and back in and back out, and around and around it goes. Evaporation happens inside the refrigerator absorbing energy, condensation happens outside the refrigerator, releasing energy. That’s why its warm behind the fridge—its not because of the motor, its because the heat that was in the stuff inside the fridge has been absorbed and moved out, via the substance in the tube (or coils—lots of tubes). And what is this substance? In most domestic refrigerators modern refrigerant gasses similar to the chloroflourocarbons of ozone layer fame are used, rendered less dangerous by having the chlorine removed.
What is remarkable to me is the simplicity and consistency of this pattern. It doesn’t change, when things condense, they release heat. Period. When things evaporate, they absorb energy, no questions asked. With frenetic pace of life today and the unprecedented opportunities and distractions we face in every moment, I find these indisputable facts infinitely reassuring.