# Does the kinetic energy of molecules of solid change during compression or extension?

I do know that the potential energy increases as you compress a solid but will there be a corresponding decrease in kinetic energy? SImilarly, will extension of a solid lead to decrease in potential energy and corresponding increase in kinetic energy?

I recently read that the change in volume changes the internal energy in solids unlike that of ideal gases (where internal energy depends only on temperature). It explained that the there is increase in the internal potential energy when volume of solid is reduce by compression. But I think that internal energy (i.e sum of internal kinetic and potential energy) shouldn't really changes if one form of energy converts to another. Maybe my interpretation is wrong but I can't see where.

• It could increase, it could decrease, or it could stay the same. How much work are you doing to compress or extend the solid? – probably_someone May 17 '18 at 1:07
• Doesn't the total internal energy remain the same assuming the conservation of energy? – suiz May 17 '18 at 1:13
• Does it? If you're doing work on it (i.e. adding energy to the system from an external source), then the answer is no. If the system is doing work on its environment (i.e. transferring its energy outside its boundaries to some external bath), then the answer is also no. – probably_someone May 17 '18 at 1:14
• Is that because the force applied may not be conservative? – suiz May 17 '18 at 1:15
• Even if it's conservative, if there's an external force, then the internal energy could increase or decrease. For example, if you have a piston full of gas, and then you put a weight on top of it, the gas in the piston is going to get hotter if you let the weight suddenly compress it (as gravity, a conservative force, is doing work on the gas). – probably_someone May 17 '18 at 1:17