Heating with thermal expansion

Jul 2013
11
0
If a cup of milk is heated from 30'C to 60'C with thermal expansion, is it correct to say the kinetic energy and potential energy of the molecules of milk both increase?
 
Apr 2015
1,035
223
Somerset, England
No work is done during a free thermal expansion.

You mentioned a cup of milk so all the thermal energy goes to increase the kinetic energy of the particles. That is the average speed increases and the simple kinetic theory says there is not particle interaction.

If youv had mentioned a solid then it would be slightly more complicated as the thermal energy is carried as vibrational energy, which is a constant exchange between kinetic and potential energy. So if the KE increases then so must the PE, though not at the same time.
 
Jul 2013
11
0
if there is no any phase change, heating will only increase the kinetic energy of the molecules in a system, having no/little effect on the potential energy, right?
 

MBW

Apr 2008
668
21
Bedford, England
You could argue that as it is heated the milk will expand and so the top of the milk will rise higher in the cup and so will have increased potential energy.
But this effect is trivial and usually ignored.
 
Jul 2013
11
0
That's why most of the exercises I did at my level would assume that the thermal expansion for liquid/solid is neglected, or the gas is heated in a rigid container to avoid the complexity of the matter