## Archive for the 'DIY science' Category

### Stay! Staaaay!

I’ve been growing salt crystals. Unlike a pet, salt crystals won’t tear up the furniture, do their business on the carpet or need to be walked in the rain. Really I was testing to see if I could do this as a time-lapse project and wanted to test how long it would take. And it has fulfilled Hofstadter’s law: it always takes longer than you expect, even when you take into account Hofstadter’s law.

I took the standard approach of heating some water and dissolving a bunch of (uniodized) salt in it, letting it cool and pouring it into a beaker. And I waited. And waited. Finally, after a few weeks:

One thing I should have anticipated is how long it takes. There was some salt left in the pot when I poured the solution into the beaker, so I thought the crystallizing would begin quickly, but it didn’t. Plus, the evaporation was slow. I knew that boiling point elevation and freezing point depression are colligative properties (they depend on the number of dissolved atoms) so I reasoned that evaporation rate should be as well. And it is — there is Raoult’s law

The vapour pressure of an ideal solution is dependent on the vapour pressure of each chemical component and the mole fraction of the component present in the solution.

The vapor pressure of salt is very low, so as its concentration rises the total vapor pressure of the solution drops, and so does the evaporation rate. Rather than evaporating fully, one would expect it to reach an equilibrium with the atmosphere which would depend on the humidity. In fact, if the salt concentration were high enough, one might expect it to dehumidify the air, which is precisely what some people do. Salt concentrations are used in dehumidifiers — you expose the solution to the air and let is “grab” some water, then heat it up (often solar, for a completely passive system) to let the excess water evaporate, and cool it again in a cycle. Or you can have a solution with some solute left in the container, and as you “grab” the water, you dissolve more of the salt, so it can continue doing its job as long as there is more salt that can dissolve.

Another unexpected event in all of this is that I was getting salt crystallizing on the surface of the water. A small “raft” would float there until it grew massive enough that it would sink (or someone poked it). I had thought the crystallization would just build on any crystal that started up, but there are lots of small cubes rather than just a few large ones. Still, the biggest cubes are perhaps 10-20x larger on a side than the original grains.

### The Main Event of the Evening: Reflection vs. Fluorescence

Cool Things You Can Do With a Blue Laser: Reflection vs. Fluorescence

[W]hat is going on here? This isn’t just reflection, this is something else. How do I know? If it were just reflection, the only color would be green (same as the incident light). This is an example of fluorescence. Basically, in fluorescence, the light doesn’t just oscillate the electrons. The light excites the electrons to a higher energy level.

### Physics for a Girl and a Boy

What happens if you hold a slinky at the top end, let it extend, and then drop it?

Two videos: set up and solution, but in the second video another question is asked. As with a previous video in this series, you can click on the answer in the second video to load the answer to the question.

### You Shall Not Pass!

In the first comment we find the following question

Does car window glass block IR?

I like the idea of testing this with a piece of glass and a heater. You could probably do it with a toaster or an electric stove and a Pyrex baking dish (don’t put the dish directly on the burner, though, because they can explode that way)

Here you go. I had a beaker rather than a baking dish, and I used the IR thermometer I demoed a few months ago

You can see that the filaments are heating up, but when the beaker is put in place, the temperature drops to ambient. So it blocks basically all of the IR in the region of sensitivity of the device.

### Osmosis is in the Kitchen with Dinah

Suggestion for anyone who replicates this: Try it with food coloring (something that will give contrast)

### Not a Viable Source of Alternative Energy

Back when I was playing with a strong magnet dropping through a coil of wire I wondered how much energy I could extract from the dropped magnet and if I could do anything with it. The coil I was using was at least 15 cm in diameter, which means that I wasn’t capturing all of the flux lines from the magnet — the field of a dipole drops off as 1/r^3, so a smaller diameter would be much better and the slowing of the magnet could be noticeable, as we’ve seen before with someone dropping a magnet down a copper tube.

Since I’m a physicist, I wanted to quantify this. I didn’t have a copper tube handy, but I do have a roll of aluminum foil which is on a roll with an inner diameter of about 3.8 cm (1.5 in), which is a reasonably tight fit for my strong magnet. I set up my slow-motion camera and my ipod in stopwatch mode to double-check the timing (yes, it was shooting at a rate of 210 frames per second)

I exported the video to individual frames to make it easier to analyze, and counted frames. The free drop takes about 0.25 seconds, give or take (it’s hard to tell exactly which frame represents release) and I estimate the distance as being about 32 cm (a foot-long roll = 30 cm, with the start just above and stop just below). The drop through the aluminum foil roll takes about 0.38 seconds. The freefall drop is easy to analyze: v = gt, and to double-check for g, just rearrange the familiar kinematics equation and solve. The drop time implies a speed of about 2.45 m/s at the exit. For g we get 10.2 m/s^2, so my little experiment seems good to 10% or better.

For the drop through the tube, we don’t know exactly what’s going on. There’s a damping force that varies with speed and eventually we would expect the magnet to reach terminal velocity. To get an estimate, though, let’s first assume it’s a uniformly lower acceleration. That would give us a value of 4.4 m/s^2 for the acceleration and an exit speed of 1.67 m/s. If we assume it hits terminal speed immediately then the speed would be 0.84 m/s. The truth is somewhere in the middle. There are probably several ways I could test this further, but the ones I can think of either require dropping the magnet from a distance above the tube, and it’s a tight fit, so it probably means lots of trials before I got lucky and got the magnet to drop in, or using a longer tube. I know aluminum foil comes in different lengths, but I only have the one. Since I want an idea of the energy extracted, let’s use the worst case value of 1.67 m/s.

I found the mass of the magnet using a small electronic scale and a plastic cup to keep the magnet away from the metal pan (where it might also be attracted to the interior or the case and mess up the measurement) and subtracted the mass of the cup. 60 grams.

Which means the magnet lost about 0.1 Joules of kinetic energy in the foil, in less than 0.38 seconds, or an average power of just over a quarter of a Watt, in that worst-case scenario. The best-case is 50% higher. And this is using aluminum — copper will give is a better result. Recall that Faraday’s law is
$$V = -frac{dphi}{dt}$$
Copper’s resistivity is about 2/3 of aluminum’s, so a given potential will drive about 50% more current and boost the resistive force owing to the larger field from the additional current. In other words, we can expect copper to be more efficient at converting the mechanical energy to electrical. It will more closely approximate the terminal-speed-quickly scenario, and it should have a smaller terminal speed.

What I want to do in the near future is wind a coil on one of these cardboard tubes and see if I can light up a little light bulb.