Hello and welcome to this video on the idea of transferring energy via work from the macroscopic scale to the microscopic scale. We have already seen that one way to transfer energy on a macroscopic scale is via work. In fact, our definition of energy is the ability to do work. We could flip this around and think of work as a way to transfer energy. It's really just a matter of our perspective. If we're thinking from a force perspective, then we think of energy as the ability to do work. If we're thinking from an energy perspective, then we think of work as a way to transfer energy. Let's think about the scales at which work can operate. The idea of work being a way to transfer energy applies over a large variety of distance scales. For example, when a star like the sun dies, it sheds its outer layers. These outer layers push against the tenuous gas between the stars, exerting a huge force over a distance of light years. We've also seen work in our everyday scales, people pushing object up hills giving them potential energy. Now let's think about work at the microscopic scale. Think about a fire extinguisher or one of those canned air devices for cleaning keyboards and the like. When the gas comes out of the cam, it's cold. Why is this? As the gas expands, it is exerting a force on the surrounding air over a distance. The gas is doing work on the surrounding air. In other words, energy is being transferred from the expanding gas to the air. According to conservation of energy, that energy has to come from somewhere, where does it come from? That energy comes from the kinetic energy of the expanding gas. Since we know that the kinetic energy is equal to three halves times the Boltzmann constant times the temperature, as the kinetic energy of the gas molecules of the expanding gas coming out of the canned drops, the temperature will drop. This is why the air coming out of these types of devices will feel cold. How does this really work though at a microscopic scale? Instead of thinking about the expansion of the gas, it's easier to visualize this in the reverse, where a gas is being compressed with, say, a piston. So here we have a piston filled with some sort of gas. We're going to follow a single molecule of the gas. Our piston is going to go down, while this particular gas molecule we are following is traveling straight up. So we push the piston and follow the molecule. Just like a tennis ball off of a tennis racket, the molecule is going to leave the collision, where the piston moving faster than it started. It will have gained kinetic energy. Again, since we know that temperature is a measure of the average kinetic energy of molecules, this gaining kinetic energy means that the temperature of the gas has gone up. Thus, we are looking at one way to transfer energy via work from the microscopic world we live in to the microscopic world. This concludes this video.