# Q & A: heat and motion

Q:
I understand that any sort of heat will cause particles of matter to move (for example heat causing vibration of atoms in a solid). But I want to know, firstly WHY does this heat cause motion? Is motion a natural reaction of all forms of matter to heat energy being applied to it? Secondly, does heat cause only electrons to move, or does it also cause the movement of the nucleus? Thirdly, if only the electrons move, how does such movement cause the whole atom to move (as is evident in the transformation of a solid into a liquid, wherein atoms and molecules move more freely and independently of each other)? And finally, once in liquid form, how does the liquid hold itself together in spite of the extra energy causing the particles to pull it further apart (to use an example:how does water stay as water even though its molecules are excited by heat energy...what prevents these excited particles from simply pulling away completely from the water and turning into a more gas-like substance)?
- Marcus (age 23)
Ireland
A:
1. There really isn't a separate thing called heat energy. Given enough time, energy has a way of distributing around in a random-looking way among small-scale modes. These include the motions of atoms and molecules. When energy that's already gone into small modes drifts from one region to another, we say that heat is flowing. So the real question is not why heat gives motions of little things (true by definition) but why energy systematically flows from large-scale modes to small-scale ones. That follows from the second law of thermodynamics (entropy maximization), the tendency of nature to wander through all possible allowed quantum states. We don't yet know ultimately why the second law is true.

2. Thermal motions include all the parts. In a solid there are thermal sound waves running around, shaking the nuclei.

3. Your "if" clause isn't true, so I guess this question is moot.

4. The molecules in a liquid are held together by forces, most typically the vanderWaals forces. That means it takes some energy for one to pull away and enter the gas phase. Of course they do occasionally get that energy, so liquids do evaporate. The hotter they are, the more likely they are to have enough energy, so the faster they evaporate. If the molecules can escape into a huge volume, the liquid will entirely evaporate. If they're somewhat more confined, the rate of escape will be balanced by the rate of return, and the material will be part liquid and part gas.

How do you  calculate how much ends up in the liquid and how much in the gas? The ultimate principle is again the second law of thermodynamics. The molecules and their energy distribute in a way to maximize the net entropy.

Mike W.

(published on 06/01/2012)