Thursday, December 10, 2009

Black Body Radiation and Global Warming

Here's a quiz:

You hear about a new band called "Black Body Radiation" and you think:
a) Just another rock band with meaningless long names like "Red Hot Chili Peppers", "Bare Naked Ladies", "Stone Temple Pilot", "Nine Inch Nails"
b) A racially motivated name
c) A name based on some obscure sexual innuendo
d) A name created by science nerds

The answer of course would be d). Any other answer would be an amazing coincidence.

I remember learning about "Black Body Radiation" and scratching my head at the name. Surprising I ended up spending 10 years of my career focused primarily on this subject. So what is it?

First, I need to explain how things heat up.
  1. Touch something hot (also called "conduction"). The heat moves from the hot thing to the thing touching it.
  2. Blow hot air (also called "convection"). This is where the convection oven gets its name.
  3. Stand by the radiator (also called "radiation"). Invisible light called thermal or "infrared" radiation is put off by warm objects. This is where "Black Body Radiation" comes in.
The hotter an object, the more thermal radiation it puts out. You can test this by putting your hand close to your face with your eyes closed. Without touching your face, you can feel the heat from your hand. This is the radiation part of "Black Body Radiation".

If I take two pieces of metal and polish one to a mirror finish and paint the other with black paint, the black piece will feel hotter if I put my hand near it (without touching). This is the "Black Body" part since its black and any thing solid is called a "body". The reason scientists like "black bodies" is because then they can use mathematical formulas to figure out exactly how much energy is being radiated.

Here's an online calculator (Not the best).

Another interesting thing that happens as things heat up is that the "peak" of the thermal energy gets closer to visible light. The first color is a red, thus the term "red hot". As it gets hotter, it gets orange, then yellow. Here's an online viewer for the color of hot objects. As you slide the temperature back and forth you can see how the color goes from black (invisible radiation) to various colors. The temperature input is in Kelvin. If you want to convert it, type "convert 500 K to F" in google and it will do it for you).

A 1 sq. ft plate (black of course) at room temperature (70 F) emits (puts out) about 12.5 Watts.

To double the energy emitted from the plate to 25 Watts, the temperature has to be 170 F (40 degrees below boiling).

To equal a 100 Watt light bulb, you would have to heat the 1 sq ft plate to 430 F and it will start to glow a faint red. This is just below the 451 F required to burn books! (Just ask Ray Bradbury).

A wall (at room temperature) 8 ft tall and 10 feet long would put out 1000 Watts. In a room 10 ft by 10 ft (4 walls, floor and ceiling), 8500 Watts are being emitted! That's equal to 85 100 Watt light bulbs.

Now for Global Warming. The earth emits from its surface roughly 100,000,000,000,000,000 (100,000 trillion) Watts. If the earth heats up just 1 degree F, it will emit an additional 600 trillion Watts. What that means is the earth is losing this energy, or in other words the cooling power is almost the same as the CO2 greenhouse effect.

More on this later.

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