# ELI5. How/why does pressure generate heat?

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Pressure causes item to get hot (to the extreme being enough pressure creating a star).

But if heat is essentially the movement of atoms, why does more pressure cause more heat?

Ie. The focused pressure of an ice skate blade makes the ice under it to melt.

In: 2

Because you’re reducing the space the molecules have available to bounce, so it’s like they are hitting on the borders with more speed.

Air molecules have mass. Their quivering motion carries momentum and energy. The faster they quiver, the more energy they carry.

A volume of gas is like a gymnasium filled with tens of thousands of little rubber balls all bouncing around inside. Assuming negligible gravity and that the balls lose very little energy per bounce, the collective force of the bounces against the gym walls represents the gym’s pressure. The speed at which the balls travel between bouncing off the walls and each other is their temperature.

Now, decrease the size of the gym. The same balls with the same amount of energy now bounce off the walls more often, and the increase in collisions between them propels the balls faster as they bounce out of each others’ way.

Thus both pressure and temperature increase, but in a smaller volume.

It isnt the pressure that makes heat. It’s the process of compressing the gas that makes heat. You have to put WORK into the gas while compressing it, and that work show up as heat. At the atomic level, pushing in the piston to compress the gas literally bounces the atoms back with increased energy, and that adds up to higher temperature (identical to higher kinetic energy in the atoms)

When you hit a tennis ball with a racquet, you increase its speed. Similarly with a gas, if you compress it in a cylinder with a piston, say, the moving piston hits the molecules and increases their speeds. But it’s the speed of these molecules which defines their temperature: higher speeds = higher temperature. In the process the pressure is also increased, as there is less space for the same quantity of gas.

A less microscopic view is that by moving the piston against the gas pressure you expend some work = add energy to the gas = make it hotter. That’s conservation of energy.

When a gas cloud becomes a star, the incoming gas falls towards the center of gravity of the star. That is gravitational energy is converted to heat when the gas is compressed under its own weight, until fusion reactions set in and stop the condensation by generating heat, so also pressure which tends to fling the gas away. And you get an equilibrium between gravitational attraction and fusion generated pressure.

Pressure is directly related to the average velocity of the atoms or molecules. A “hot”
molecule is really just moving faster and bumping into more molecules. When you compress these molecules together you are taking all that movement and concentrating it. If you leave it there and the heat dissipates the pressure also drops because there is less movement in the now cooler molecules.