Question on Topic 3: Thermal Physics

[Guest 4lantheman]

Hello All,

 

I have two questions: 

 

1. I am finding difficulty understanding the explanation for the 'gain' of potential energy when a object undergoes phase change. i.e. In terms of molecules and energy, how can we explain the difference between P.E. in solids, liquids and gases 

 

This means:

 

U.E. (l) > U.E. (s) 

 

Right?

 

Please let me know if there is an alternative explanation, relating to work done -> force * distance // attraction force between molecules // distance between molecules. 

 

2. What happens on a microscopic level, when thermal energy is transferred to an object? An explanation in terms of molecules, relating to the heating curve would be great! 

 

Thanks in advance! 

Edited November 24, 2015 by 4lantheman

[Guest SNJERIN]

You are absolutely correct. The potential energy of the molecules or particles is defined as the energy associated with the intermolecular forces that exist between the molecules. So during a phase change in the system you are only increasing the separation between the particles (and not their temperature) and that requires work to be done since there are forces of attraction between particles. 

 

Now to your second question. Adding heat to a system ( assuming it is a non-ideal gas) will increase it's internal energy. That is, particles will vibrate with higher average speeds. Hence their average Ek will increase and this would appear macroscopically as an increase in temperature of the system. I don't think you really need to understand the details of how this thermal energy is distributed between the paretics. All you need to know is that: thermal energy–––> internal energy = increase in the potential energy and/or the average kinetic energy.  

 

Hope this helps.