Please explain the physics of a Cloud Chamber A friend of mine was telling me about building a cloud chamber while he was in graduate school.  As I understand it, this allows you to "see" interactions caused by high energy particles going through the cloud chamber.  This has fascinated me, and I would like to build one with my daughter, but I want to make sure I am able to explain it to her when the eventual questions come.  Can someone help me out please?  How would I explain a cloud chamber to someone who is a freshman in high school?
 A: Feynman used to say - if you can't explain something in simple words, such that a child could understand, then you don't understand it either. So here's my take:
A cloud chamber is nothing more than a box where mist is about to form, but not quite yet. There's vapors of stuff (either alcohol, or water, or something else) in it, and the temperature is such that the vapors are almost about to produce mist (or "clouds"). Imagine wetlands or marshes on a cold autumn morning, it's kind of like that - fill a box with that kind of "cold wet air".
Now a charged particle (such as Alpha radiation from a chunk of radioactive ore) zips through the chamber at high speed. It bumps into water (or alcohol) molecules and ionizes them - it creates a trail of ionized molecules marking its path.
Now, the vapors are such that they really want to produce mist; any tiny disturbance is enough to push them over the edge. The trail of ionized molecules is enough to do that - the ions attract a bunch of molecules, the resulting clumps attract even more, and before you know it a droplet of water is formed, then another, and another. Voila, a trail of mist follows the particle.
I could try to describe the construction, but this Instructables page will do it much better:
http://www.instructables.com/id/Make-a-Cloud-Chamber-using-Peltier-Coolers/
Basically, you evaporate some alcohol and let it run over a very cold area (cooled by the Peltier elements). Like breath coming out of your mouth in the cold air of winter, the alcohol vapors will tend to produce mist, so some vapors will turn to mist anyway. But the process happens a lot faster when a charged particle zips through the chamber - so, if you place a tiny bit of radioactive material nearby, tiny white trails will seem to come from it and traverse the chamber, because mist tends to form that much better around the ionized trails left by the radiation in its wake.
More designs:
http://www.lns.cornell.edu/~adf4/cloud.html
http://www.nothinglabs.com/cloudchamber/
http://www.bizarrelabs.com/cloud.htm
A: The cloud chamber works by producing a super-saturated vapor, as explained by florin. When a charged particle passes by, it ionizes the molecules of the liquid, and these ions become centers for droplets, which condense around the ionization trail. But why are ions such good seeds for droplet condensation?
The reason is just electrostatic-dipole interaction, the cloud-chamber fluids are all dipoles. Choosing unit of energy eV, unit of length 1A, and unit of charge 1e, the Coulomb constant k is 14.4 (eV A/e). The dipole moment of water and alcohol (two common vapors for a cloud chamber) are both about .4 eV A.  That means that you have to go out a distance of 13 Angstroms before the thermal energy is comparable to the maximum dipole energy.
Within this region, the statistical equilibrium requires that the first dipole that enters sits on top of the ion, because the potential well is essentially infinitely deep. The ion plus polar molecule will attract another ion, and so on, until a droplet is formed.
Ignoring interaction between the dipoles, the radius of a droplet stabilizes electrostatically approximately when the energy gain from being on the surface of the droplet is equal to about 6kT, the factor of 6 is the approximate entropy gain of being in gas vs. liquid. This happens relatively quickly, so you get a microscopic droplet. But in a supersaturated liquid, there are strong forces already between dipoles which mean that there is very little cost to forming bigger drops. The energy gain just from the inter-molecular forces already balances the entropy loss from leaving the gas phase. The only thing that doesn't balance is the surface tension cost.
The radius of the drop is then approximately determined by the place where the average energy gain for a dipole on the surface is equal to the free energy difference between surface and bulk fluid. When this radius exceeds the critical droplet size for the supersaturated liquid, you get nucleation. Since it is going to be about 10 A, it is already pretty big. There is no chance of producing a 10A drop, containing hundred of molecules, by thermal fluctuations.
