Internal Energy

Internal Energy

If a cup of tea is hot, we think that it has heat energy. If it is at room temperature, does it have energy?

Yes, because the molecules in the cup and the water would still vibrate or move around, attractting or repelling each other. They have kinetic energy because of the movement. They also have potential energy because of the attraction, in the same way that a stone has potential energy because it is attracted by the earth.

This energy hidden in an object is called internal energy. When the object is hotter, the molecules vibrate or move with more kinetic energies. Then it has more internal energy.

So the proper name for energy we normally associate with a hot object is internal energy. In physics, the word "heat" refers to the energy that is transferred to the object from a hotter one. After the transfer, heat becomes the internal energy. We do not call it heat after that.

If internal energy is just the kinetic and potential energies of the molecules in an object, we do we not just call it kinetic and potential energies?

There are two reasons. One is that the molecules are extremely small. The other is that the movements are random. Because of these reasons, we cannot see the movements at all, and the object can appear completely at rest.

So it is more convenient to reserve the names kinetic and potential energies for the movement of the object as a whole.

What if the movements of the molecules are not random? For instance, if we have just one molecule moving along a straight line, then we would still use the terms kinetic and potential energies for this molecule.

This idea of internal energy also helps us to understand why when a stone hits the ground, the kinetic energy can be converted to heat. The impact can produce a lot of vibration in the atoms of the stone and the ground. The regular vibrations lead to atoms colliding and would eventually become random - it becomes internal energy.

The Physics Notes