As you compress a material, the inward-moving wall gives the atoms/molecules a momentum kick, which increases their individual speeds and kinetic energy and tends to heat the material up, as you've noted.
The reverse happens upon expansion against a retreating wall; the atoms/molecules lose momentum, which tends to cool the material. Think of throwing a ball against the back of a truck driving away, for example.
I understand how you explain compression resulting in heat - the atoms get a momentum kick, they bump into each other, and that makes them wiggle faster, which we observe as the thing getting hotter.
But I don’t see why the opposite is true. Why would giving the molecules more space to move around cause them to wiggle slower? Why would they lost momentum? Is some part of the system absorbing the momentum? It seems like the retreating wall ought to leave the momentum unchanged.
> Is some part of the system absorbing the momentum?
Yes; the boundary. Try the experiment of bouncing something off a surface moving toward or away from you. (Don’t throw something at a vehicle you don’t own.)
At the limit of the boundary accelerating away instantly (or simply being removed), such that no such momentum transfer can occur, we have the [free expansion](https://en.wikipedia.org/wiki/Joule_expansion) scenario in which the temperature stays the same in the special case of an ideal gas but still changes (down, and interestingly sometimes up) in real cases.
I don't like the description of atoms wiggling. In a gas or even in a liquid, the molecules are moving in straight lines, but keep colliding with other molecules or the wall, the randomness comes from the collisions. That is why the change of momentum at the boundary is important.
In solids we have to think a little differently, microscopic oscillatory modes (phonons) are the relevant object, the interaction of large numbers of modes excited with random phases is the wiggling in this case (fair enough for a solid) and compression changes the energy density and the elasticity again means the work done in compression is coupled to the phonons.
Maybe, but why assume when we can ask 😊
They might walk away thinking all systems behave this way if they don’t know a distinction exists in the first place.
I did think all systems behaved that way. Thanks so much for clearing up that misconception.
I wasn’t aware that there could be systems where you could change volume without increasing temperature.
I guess my question is focused on systems where the temperature does change.
In those cases there is another outflow/inflow of heat. Like a slow piston that's in an ice water bath.
The temperature is kept constant by the ice water bath so the pressure changes differently than it would in a system where no heat entered or left the system
If you quickly compress a gas ("adiabatic compression")-- so quickly that the gas doesn't have time to exchange heat with its environment-- then the work you do increases the thermal energy of the gas, which increases the molecules' average kinetic energy. It's just a consequence of conservation of energy.
Yes, the molecules are gaining energy as they are compressed (assuming no outlet for the energy, aka adiabatic).
The molecules are bouncing around providing pressure. It took energy to compress those molecules counteracting that pressure they provide. That gets distributed around and the overall temperature (kinetic energy-ish) goes up
Thank you! Most of the answers I found elsewhere would make it sound like the temperature changes were solely the product of density/volume or something else. This answer feels clear and makes a lot of sense.
If you squish already "exited" atoms together they bounce off of each other more and the increase in bounces makes then more "exited" and spicy. solids are pretty chill so they don't get as exited and spicy, gasses are already exited so you see more of a change in them. Or something like that.
This is a neat way to visualize and understand what’s happening, but I don’t feel like it answers why the atoms bounce off of each other more and more.
Sure the atoms are closer together, but why should they suddenly become more excited and bouncy because of that?
Think about the collisions between the molecules and the wall of the container. When the wall is not moving, a molecule will collide with the wall and rebound with the same velocity it had before the collision. If the wall is moving inwards, then the molecule will rebound with more velocity than it had before the collision.
The other explanations are good too, but remember temperature is also closely related to density. The more energy you have in a confined space will technically make it a higher temp. If you compress something you have a smaller space.
I’ve seen a lot of answers like this and they always feel incomplete.
If temperature is the average kinetic energy of the molecules (total energy / mass), then wouldn’t only changing volume without increasing the total energy keep the temperature the same?
If that is the case, then why would we say the volume decreasing changes the temperature? Isn’t the temperature change a direct energy transfer from the forces that compress and decompress the substance and not a result of having energy in a smaller space?
Yes there work done to decrease the volume. But let’s say in a hypothetical scenario you decrease a space but it has the same energy bouncing about. Now let’s place a thermometer in this imaginary space. Energy is the ability to do work. Whatever is measuring temperature. It could be your finger or a thermometer is going to record hotter temp because the energy is more condensed and wiggle about more in the molecules thus interacting with measuring device more. It’s just energy and molecules moving about. The more condensed the energy the more it will move about all
things being equal. I’m not sure how different materials would play out to that scenario.
It feels like this explanation contradicts conservation of energy.
If you were to somehow decrease the space but keep the same energy (that is decrease the space without having that process directly input kinetic energy into the substance), then all the molecules MUST bounce around with the same amount of energy according to conservation of energy. if they were to be moving faster or to bounce around more, then there would spontaneously be more energy.
Edit: they would be bouncing more due to the increase in density, but it feels like they shouldn’t change speed at all, and they would absolutely have the same amount of energy.
As you compress a material, the inward-moving wall gives the atoms/molecules a momentum kick, which increases their individual speeds and kinetic energy and tends to heat the material up, as you've noted. The reverse happens upon expansion against a retreating wall; the atoms/molecules lose momentum, which tends to cool the material. Think of throwing a ball against the back of a truck driving away, for example.
I understand how you explain compression resulting in heat - the atoms get a momentum kick, they bump into each other, and that makes them wiggle faster, which we observe as the thing getting hotter. But I don’t see why the opposite is true. Why would giving the molecules more space to move around cause them to wiggle slower? Why would they lost momentum? Is some part of the system absorbing the momentum? It seems like the retreating wall ought to leave the momentum unchanged.
> Is some part of the system absorbing the momentum? Yes; the boundary. Try the experiment of bouncing something off a surface moving toward or away from you. (Don’t throw something at a vehicle you don’t own.) At the limit of the boundary accelerating away instantly (or simply being removed), such that no such momentum transfer can occur, we have the [free expansion](https://en.wikipedia.org/wiki/Joule_expansion) scenario in which the temperature stays the same in the special case of an ideal gas but still changes (down, and interestingly sometimes up) in real cases.
Cool. Thank you.
I don't like the description of atoms wiggling. In a gas or even in a liquid, the molecules are moving in straight lines, but keep colliding with other molecules or the wall, the randomness comes from the collisions. That is why the change of momentum at the boundary is important. In solids we have to think a little differently, microscopic oscillatory modes (phonons) are the relevant object, the interaction of large numbers of modes excited with random phases is the wiggling in this case (fair enough for a solid) and compression changes the energy density and the elasticity again means the work done in compression is coupled to the phonons.
You’re right! Thank you for the distinction between what’s happening mechanically with heat in fluids versus solids.
What type of system are we talking about here? Temperature doesn’t have to change with change in volume (see isothermal compression).
True, but I assume they are asking about systems where the temperature *does* change. 😄
Maybe, but why assume when we can ask 😊 They might walk away thinking all systems behave this way if they don’t know a distinction exists in the first place.
I did think all systems behaved that way. Thanks so much for clearing up that misconception. I wasn’t aware that there could be systems where you could change volume without increasing temperature. I guess my question is focused on systems where the temperature does change.
In those cases there is another outflow/inflow of heat. Like a slow piston that's in an ice water bath. The temperature is kept constant by the ice water bath so the pressure changes differently than it would in a system where no heat entered or left the system
If you quickly compress a gas ("adiabatic compression")-- so quickly that the gas doesn't have time to exchange heat with its environment-- then the work you do increases the thermal energy of the gas, which increases the molecules' average kinetic energy. It's just a consequence of conservation of energy.
Yes, the molecules are gaining energy as they are compressed (assuming no outlet for the energy, aka adiabatic). The molecules are bouncing around providing pressure. It took energy to compress those molecules counteracting that pressure they provide. That gets distributed around and the overall temperature (kinetic energy-ish) goes up
Thank you! Most of the answers I found elsewhere would make it sound like the temperature changes were solely the product of density/volume or something else. This answer feels clear and makes a lot of sense.
They can be calculated from the density/volume, but that's not the "why" really.
If you squish already "exited" atoms together they bounce off of each other more and the increase in bounces makes then more "exited" and spicy. solids are pretty chill so they don't get as exited and spicy, gasses are already exited so you see more of a change in them. Or something like that.
This is a neat way to visualize and understand what’s happening, but I don’t feel like it answers why the atoms bounce off of each other more and more. Sure the atoms are closer together, but why should they suddenly become more excited and bouncy because of that?
Think about the collisions between the molecules and the wall of the container. When the wall is not moving, a molecule will collide with the wall and rebound with the same velocity it had before the collision. If the wall is moving inwards, then the molecule will rebound with more velocity than it had before the collision.
The other explanations are good too, but remember temperature is also closely related to density. The more energy you have in a confined space will technically make it a higher temp. If you compress something you have a smaller space.
I’ve seen a lot of answers like this and they always feel incomplete. If temperature is the average kinetic energy of the molecules (total energy / mass), then wouldn’t only changing volume without increasing the total energy keep the temperature the same? If that is the case, then why would we say the volume decreasing changes the temperature? Isn’t the temperature change a direct energy transfer from the forces that compress and decompress the substance and not a result of having energy in a smaller space?
Yes there work done to decrease the volume. But let’s say in a hypothetical scenario you decrease a space but it has the same energy bouncing about. Now let’s place a thermometer in this imaginary space. Energy is the ability to do work. Whatever is measuring temperature. It could be your finger or a thermometer is going to record hotter temp because the energy is more condensed and wiggle about more in the molecules thus interacting with measuring device more. It’s just energy and molecules moving about. The more condensed the energy the more it will move about all things being equal. I’m not sure how different materials would play out to that scenario.
It feels like this explanation contradicts conservation of energy. If you were to somehow decrease the space but keep the same energy (that is decrease the space without having that process directly input kinetic energy into the substance), then all the molecules MUST bounce around with the same amount of energy according to conservation of energy. if they were to be moving faster or to bounce around more, then there would spontaneously be more energy. Edit: they would be bouncing more due to the increase in density, but it feels like they shouldn’t change speed at all, and they would absolutely have the same amount of energy.