Potential Energy of a Liquid - Why is it Negative for Small Molar Volumes?

[Ryker]

I was just reading a book on low-temperature physics and stumbled upon a graph that shows the energy of liquid Helium-4 in relation to its molar volume. The graph includes both zero-point energy and the potential energy of the liquid, and the latter goes steeply (basically a vertical line) from being positive for the smallest of molar volumes to being negative, and then gradually (non-linearly) increases as molar volume increases.

So I was wondering what exactly is this potential energy and why is it negative? I've found something online that suggest this potential energy represents pressure. Since I just finished my first year of undergrad, I'm not aware of all the forces between atoms, so I can't really explain why this energy would be more negative with smaller molar volumes. Do they repel each other, and repulsion as opposed to attraction is taken to be negative?

Thanks in advance for any help.

 

[Drakkith]

Any chance you could get us a copy of the graph? Maybe scanning it or something? It would be much easier to understand then.

 

[Ryker]

Sure, here it is (as you've undoubtedly noticed, it's a crappy MS Paint replication due to lack of scanner, and the lines aren't supposed to be as wobbly ):

 

[Drakkith]

Hrmm. I don't know. I wish I could help you.

 

[PJC01]

I can provide some insight into the potential energy of a liquid and why it may be negative for small molar volumes. The potential energy of a liquid refers to the energy associated with the interactions between the molecules in the liquid. This energy can be either positive or negative, depending on the strength and type of interactions between the molecules.

In the case of liquid Helium-4, the potential energy becomes more negative as the molar volume decreases. This is due to the fact that at smaller molar volumes, the molecules are closer together and the interactions between them become stronger. In this case, the repulsive forces between the molecules dominate, leading to a negative potential energy. This is because repulsive forces are often considered as negative in physics, as opposed to attractive forces which are positive.

Furthermore, at smaller molar volumes, the molecules have less space to move around and therefore have less kinetic energy. This results in a decrease in the total energy of the system, which is reflected in the negative potential energy.

It is important to note that the potential energy of a liquid is just one aspect of its overall energy. The total energy also includes the zero-point energy, which is the minimum energy that a system can have even at absolute zero temperature.

In conclusion, the negative potential energy of a liquid at small molar volumes is a result of the strong repulsive forces between the molecules. This is due to the close proximity of the molecules, which leads to a decrease in the total energy of the system. I hope this helps to clarify the concept of potential energy in liquids.