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This is a good question - one that I remember pondering when I was in high school, also. The situation can best be described by thinking about equilibrium. When water is in a sealed container, for example, there is always water evaporating into the air in the container and also water condensing out of the air. If you leave the container untouched for awhile in a room with constant temperature, the water and air will come to equilibrium and their will be no net evaporation.

Water in a puddle outside is different. Out in the weather, it is almost impossible for the puddle and the air around it to come into equilibrium (the area outside is much bigger than the area in the sealed container), so while there may be water condensing into the puddle, there will always be more evaporating into the air.

So, in the end, you don't need water to boil for it to evaporate.

Lauren Scott
(February 2004)

Why don't protons and neutrons, with so much potential energy, try to disorder themselves and thus do some useful work wtih it? It seems to be that entropy doesn't apply to them.

I don't know that I fully understand the intent of the question, but here goes.

It's not right to say that protons and neutrons have potential energy. Any individual isolated proton or neutron can't do any work, which is one way to define potential energy. If such a particle has kinetic energy, it could interact and you could think of that as work, but that's not potential energy.

To be more precise, the entropy of a system is the number of accessible "states" of a system within a small energy range. If we want to describe protons and neutrons themselves, we use quantum mechanics (we can describe large ensembles of protons and neutrons - i.e. solids, gases, and liquids, with classical mechanics). In quantum mechanics, the states of a system are described by various quantum numbers (spin, angular momentum, etc.) and quantum mechanics tells us the number of states available to a system within any energy range. When all the quantum numbers are their lowest values (the so-called ground state) there is no other available accessible state and the entropy is zero. Note that this is basically the third law of thermodynamics.

Also note that the second law of thermodynamics (the entropy of an isolated system can not spontaneously decrease) deals only with an ISOLATED SYSTEM and with a CHANGE in entropy. It says nothing about the entropy of a non-isolated system or the absolute value of the entropy.

Dr. Louis Barbier
(May 2004)

If you drive down a street after a snow and all the houses on one side are facing south and the other facing north, you will easily see that the Sun can very easily melt snow. There are about 1300 watts per square meter of area of energy from the Sun. Given that reasonable aluminum mirrors can reflect upwards of 90% of sunlight in the infrared region, I see no reason why you can't arrange 1 or 2 mirrors to melt snow off of something in your yard.

Dr. Louis Barbier
(November 2001)

No, absolute zero (0 Kelvin) is defined as where all molecular motion stops. The electrons still orbit the nucleus, the nucleons still spin, etc.

Dr. Eric Christian

Is there a law that prevents the attainment of absolute zero? Or is it that humans don't have the technology to reach it?

In order to reach a temperature of absolute zero, a "heat engine" would have to be built that is 100% efficient. This is not possible. We will never build such a device.

Dr. Louis Barbier

A little more information on the experiment you did to measure the temperature of red light vs. blue light would be helpful. It's not an easy measurement. If you used something colored red and colored blue, it is important to remember that something red REFLECTS red light and absorbs blue (and green and yellow and violet, etc.). So you would get the temperature of all the colors except the one that is reflected.

A more correct way to do this experiment is to have a red filter (red plastic for example) and a blue filter that only allow those colors through and then measure the temperature behind the filter. But even this experiment has several problems. The main problem is that you are really measuring the AMOUNT of light (more precisely the amount of energy in the light), not the temperature of the light. Because the atmosphere of the Earth scatters blue light (that's why the sky is blue), there is less blue light hitting your experiment than red light (plus the Sun doesn't really generate the same amount of red and blue light, and the filters have different amounts of light that they let through).

A better experiment would be to get a light bulb (a non-halogen flashlight bulb with clear glass will work) and a way to adjust the voltage (batteries and a variable resistor, also called a potentiometer or better still a variable voltage supply). You can easily see that as you turn up the voltage, the filament in the light bulb will first be dull red, then brighter red, then orange, yellow, and white (if you supply too much voltage, the filament will probably burn out before you get blue-white). This is exactly the same effect that causes the colors of stars. As the filament gets hotter (with more voltage) the color changes (and more total light is given off). If you put a thermometer (one that can go up to high temperatures) on the glass of the bulb, you should be able to see the increase in temperature as the voltage increases. However the color of the filament (and of a star) is where the most light is being generated, but some light is being generated all the way from infrared to ultraviolet.

Dr. Eric Christian
(October 2005)

Yes, it is possible (and relatively easy) to change the wavelength of electromagnetic waves. As you have pointed out, visible light from the Sun can heat matter and that matter then radiates as infrared waves, which our eyes cannot see but our bodies can sense as "heat".

Converting heat waves to light waves is a bit more difficult for the same reason it is more difficult to go from the bottom of a building to the top than the other way around. This is because it is easier to go from a higher energy state (the top of a building, or visual light) to a lower energy state (the bottom of a building, or heat radiation) than from a lower to a higher state. To convert heat waves into a higher-energy wave (like visible light or even X-rays) you need to store them up until you have the energy of the wave you want to produce. For example, you could charge a battery using heat and then power a flashlight with the charged battery.

See the IMAGERS website for more information on electromagnetic radiation.

Dr. Ed Tedesco
(December 2004)