This came up in another forum area but my query is more apropos here.
Yeah, I am trying to get a handle on why (not "if", that is very well established ) compression raises temperature. Obviously it has to do with the energy input required to push molecules of a gas closer together, increasing the density of the gas.
I am simply putting out a few thoughts on this and absolutely welcome informed explanation and corrections. It's een decades since I took physics classes.
In the bow shock wave of an object falling very fast there is no physical wall. Instead it is the gas itself that "can't get out of the way fast enough".
Why can it not get out of the way fast enough? Does it have to do with the inertia of the gas molecules? Does it have to do with bonds between molecules?
Then there is friction. Why does friction cause an increase in temperature of the surfaces involved? Again obviously there is the input of energy due to the relative velocities of the objects. One explanation I see on the internet is that molecules on each surface form a bond that gets broken as they move apart thus increasing the velocity or vibration of those molecules.
Friction is usually demonstrated with two solids in contact moving by each other. It could just as well be two fluids, or a fluid and a solid. The biggest difference I can see is that the fluid will form layers moving at different speeds. When the fluid is moving slowly there may be fewer, thinner layers meaning most of the friction is between solid and fluid. If the fluid is much faster the layer nearest the solid does not have the same freedom of movement and cannot form another layer. It must deal with the solid/fluid interface, ie friction, therefore it cannot move as fast as layers further from the solid. Essentially a limit on the friction between a solid and fluid. Layers further from the solid surface then are compressed against the contact layer. Most of the heat generated then is in this compression compared to the friction layer in contact with the solid.
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Originally Posted by phunk (Post 11556506)
I have never seen an explanation of the pressure/temperature relationship in a gas that attributed it to friction.
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I am simply putting out a few thoughts on this and absolutely welcome informed explanation and corrections. It's een decades since I took physics classes.
In the bow shock wave of an object falling very fast there is no physical wall. Instead it is the gas itself that "can't get out of the way fast enough".
Why can it not get out of the way fast enough? Does it have to do with the inertia of the gas molecules? Does it have to do with bonds between molecules?
Then there is friction. Why does friction cause an increase in temperature of the surfaces involved? Again obviously there is the input of energy due to the relative velocities of the objects. One explanation I see on the internet is that molecules on each surface form a bond that gets broken as they move apart thus increasing the velocity or vibration of those molecules.
Friction is usually demonstrated with two solids in contact moving by each other. It could just as well be two fluids, or a fluid and a solid. The biggest difference I can see is that the fluid will form layers moving at different speeds. When the fluid is moving slowly there may be fewer, thinner layers meaning most of the friction is between solid and fluid. If the fluid is much faster the layer nearest the solid does not have the same freedom of movement and cannot form another layer. It must deal with the solid/fluid interface, ie friction, therefore it cannot move as fast as layers further from the solid. Essentially a limit on the friction between a solid and fluid. Layers further from the solid surface then are compressed against the contact layer. Most of the heat generated then is in this compression compared to the friction layer in contact with the solid.
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