# Relationship between internal energy and PE and KE

1. Dec 25, 2012

### influx

Hi,

Firstly, I'd like to thank PhysicsForums and its members for helping me during my last couple of visits (around 7 months ago). With your help I managed to achieve an A at AS Physics (a UK exam)!

I am doing some basic concepts on internal energy etc and I came across the following:

''Heating is when energy is transferred between two objects due to a temperature difference. For example, if you put your cold hand on a warm radiator, energy is transferred from the radiator to your hand. This increases the internal kinetic energy of the molecules of your hand and so your hand warms up.''

So when you heat something up, the internal kinetic energy increases (and therefore the temperature does) but what happens to the internal potential energy? Could someone explain in basic terms?

Also:

''As 2 molecules in an ideal gas approach each other, the repulsive forces between the molecules slow them down until momentarily both are stationery. ''

So do the molecules actually collide with each other or not? Also, since the above describes what happens in an ideal gas, could someone explain what happens in a non-ideal gas (if there is such a thing?)

Thanks :)

2. Dec 25, 2012

### rude man

I have never heard of "internal potential energy" of atoms/molecules.

Not sure about the molecules colliding or not (looks like no), but yes there are non-ideal gases. Like, all of them! An ideal gas does not exist but gases at low pressure approximate the equation of state of an ideal gas very nearly, greatly simplifying computations.

3. Dec 25, 2012

### Staff: Mentor

In the case of an ideal gas, the molecules are far enough apart that the potential energy due to binary interactions (i.e, repulsive forces) between molecules is negligible, and the internal energy is dominated by kinetic energy. For non-ideal gases, the molecules are close enough together that the potential energy of the molecular interactions makes a significant contribution to the internal energy.

The repulsive force when two molecules get very close to each other is sufficiently high that they don't actually touch, even if they approach each other at high speed. Still, this is referred to as a "collision."

4. Dec 27, 2012

### MrWarlock616

From this video, molecules don't actually touch but get repelled with a large force and so it's a collision.