Why does air compressed to a high pressure create heat?
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The simplest answer is to think of taking all of the heat in a room full of air, and squeezing that heat into a small volume. That small volume will get very hot because it has all the heat from an entire room concentrated into a small volume.
If you let that small volume of highly compressed air cool off to room temperature, then expand it back to fill the room, it will get very cold. This is because the heat in a small volume has been spread out to a large volume and there isn’t much to go around.
This is really how a refrigerator works. The fluid captures the heat in the fridge and expands. It’s then compressed, concentrating that heat. That heat is dissipated through a radiator. When it’s cooled off, that fluid is expanded again, capturing the heat of the fridge and the cycle continues.
The trick is that after you concentrate the heat (by compressing the gas), you must dissipate the heat, then when it expands, it will be quite cold.
It doesn’t actually “create heat”, there is the same amount of thermal energy in the compressed substance as before. It is just that the relative concentration of thermal energy is increased so the temperature goes up.
Heat flows from areas of higher concentration to lower so heat flows out into the environment. Think about squeezing water out of a sponge, did your squeezing create water?
You’re adding energy to the system when you compress it. Imagine you have a piston in a cylinder with air in it. As you push the piston in, the air compressed. As it does, it pushes back on the piston. So you have a force applied over a distance. Guess what that is? Work. Aka energy. The energy has 2 places it can go. It can leave the system, or it can heat up. It’ll want to leave, but this takes time. If you compress the air fast enough, it’ll heat up.
Temperature of a gas also translates to average kinetic energy of that gas. The atoms and molecules in a large space can go so far before they bounce off one another and the wall. As you shrink the volume, you decrease that average distance they can go without bouncing, so the bouncing increases. If you stuck a thermometer in the to measure the temperature, you get more bounces on the thermometer transferring the energy so you read a higher temperature.
What bounces a ping pong ball back faster, a stationary wall or a mobile wall moving towards the ball? Of course the wall moving towards it since it imparts more energy to the ping pong ball.
That’s kinda like what compression does to the atoms being compressed. The walls squeeze around the atoms to impart extra energy on them. Since heating something up on the atomic scale is bouncing atoms around at higher speed, the extra speed results in higher temperature.
Because “temperature” is just another way of saying “how fast the particles of a substance are moving, on average”. For example, in hot water, the individual H2O’s are bouncing around off each other faster than they are in cold water.
When you compress a gas, imagine a room full of flying bouncy balls and suddenly the walls move inwards. The balls will end up bouncing around faster, right? That’s the same thing as ending up at a higher temperature, which means it “created heat”.
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