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Does heat affect heat or sunlight absorption? Can the outside coloring of an object affect the temperature inside?
Question Date: 2006-12-07
Answer 1:

Yes, the heat of an object affects the amount of energy (heat, sunlight) that it can absorb. At some level of energy input, an object will reach an equilibrium temperature. At this point, the object is giving off exactly as much energy as it is absorbing. Beyond this point, the object's temperature will not rise: the only energy absorbed is exactly what is needed to maintain this equilibrium temperature (and thus replace the energy that the object is loosing to the environment). Energy input beyond this will be not being absorbed. Think of a hot stove burner: if you put it on "high", it will continue to heat up (absorb energy) until it reaches a maximum temperature. This maximum temperature is determined by the material that the stove burner is made of. Some materials don't absorb much energy and don't heat up much; others absorb a lot of energy and heat up quite a bit. Once the stove burner reaches its equilibrium temperature, it will not heat up any more, no matter how much energy you add. So a cold stove burner will absorb a lot of energy but a hot stove burner will absorb much less because it's closer to its equilibrium temperature. At its equilibrium temperature, the stove burner is giving off exactly as much energy as it is absorbing, so to maintain this equilibrium temperature, it does absorb energy, but not as much as at lower temperatures. If the stove burner was perfectly insulated, it would not absorb any energy at all, because it wouldn't be loosing energy to the environment.

It's impossible to perfectly insulate anything, and for a stove burner, the whole point is to transfer the heat to a cooking pot, so you would never want to insulate it anyway. But a solar water heater cell you would want to insulate, because you want to transfer the heat of sunlight to water running through the cell and not to the environment. Like a stove, solar water heater cells will absorb the energy in sunlight until they reach an equilibrium temperature, but this temperature is not fixed. Why? Because the solar water heater cell still looses some energy to the environment, and this amount of energy is determined by temperature difference between the solar water cell and the air outside it. So solar water heater cells can reach some characteristic temperature ABOVE THEIR SURROUNDINGS, and then will not heat up any more. This is their equilibrium temperature. If the maximum temperature increase of a solar water cell is 100 degrees Fahrenheit, and the outside temperature is 40 degrees Fahrenheit, then the inside of the solar water cell will reach 140 F at its equilibrium temperature and will not heat up any more, no matter how much sunlight is hitting it. If the outside temperature is 100 F, then the inside temperature of the SAME solar water cell will reach 200 F.

So solar water cells work best in sunny, warm environments, not just sunny environments.

And yes, the equilibrium temperature of an object is partly determined by its color. Think of two cars sitting in the sun with their windows closed. If one car has black paint and the other car has white paint, the inside of the black car will heat up faster, since black colors absorb the most sunlight and white colors absorb the least, AND the equilibrium temperature inside the black car will actually be higher than the equilibrium temperature inside the white car. Why do black colors absorb the most sunlight? When it comes to color, light is either being reflected or absorbed. Black things look black because they reflect almost 0% of visible light, which means they absorb almost all of the sunlight that hits them. White things look white because they reflect almost 100% of visible light, so they absorb very little sunlight that hits them.

Answer 2:

Color is the tendency of an object to preferentially emit/absorb certain wavelengths of light more than others. And object that absorbs more wavelengths will heat up more rapidly and have a higher equilibrium temperature when exposed to light. This certainly applies to the inside of the object: a dark car will warm up inside more rapidly than a light car (barring air-conditioning, size of windows being different, and other factors that is).

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