# My Third Post: Change of Intermolecular Distance?

• B
• Weam Abou Hamdan
In summary, the conversation discussed the concept of equilibrium in a rod composed of two identical molecules, A and B. It was mentioned that when equal external forces are applied in opposite directions, an attractive intermolecular force is exerted between A and B to maintain equilibrium. This is consistent with the Lennard-Jones model, which explains the intermolecular force as being attractive when the distance between molecules exceeds the equilibrium distance. The same result is observed when the forces are directed inwards. However, when no external forces are applied, the distance between the molecules follows Hookes law and is dependent on the van der Waals forces, which are very weak interactions between atoms or molecules. The applicability of this concept depends on whether the questionf

#### Weam Abou Hamdan

Hello,

Let us imagine a rod resting on a frictionless horizontal surface such that the rod is made up of 2 identical molecules. Let these molecules be named A and B from left to right. If we pull A to the left and B to the right with forces of equal magnitude, the rod must remain at equilibrium according to Newton's First Law of Motion. This means that A and B must remain at equilibrium as well. Then, when A and B are pulled in opposite directions, an attractive intermolecular force will be exerted between A and B to cancel out the externally applied forces. This is consistent with the Lennard-Jones model since the intermolecular force was attractive when the distance between the molecules exceeded the equilibrium distance. The rod remains at equilibrium, but it becomes larger in length after the outward forces are applied.

A similar result would be obtained if the forces were directed inwards instead. In this case, the intermolecular force would be repulsive to cancel out the externally applied forces. Also, it would be consistent with the Lennard-Jones model since the distance between the molecules became less than the equilibrium distance. The rod remains at equilibrium, but it becomes shorter in length after the inward forces are applied.

Is this true?

Weam Abou Hamdan
Thursday, August 2, 2018

In each case, what do you think it will happens when the external forces are suppressed?

This is basically just Hookes law, which we have discussed previously

This is basically just Hookes law, which we have discussed previously
Is there a generalized formula for the distance between two molecules in the case where no forces are being applied at all?