There aren't any dumb questions, only dumb answers. Here are some of my dumb answers...
Solar heating of materials is not a "black and white" issue. You have to think about wavelengths. The input from the Sun (after atmospheric effects) is primarily in the visible wavelengths. (That's why our eyes are adapted for it.) So the heat energy absorbed by your aluminum plate depends on its absorption (low albedo) in the visible and near infrared range.
On the other side of the equation, how does the plate lose heat? If there is not much conduction or convection (low surface area, low wind, etc.), it can only lose by radiation. The peak radiation of a body in the range of few hundred degrees or less is in the far infrared. So, to stay cool, the infrared albedo needs to be low and the visible albedo to be high. Things that are reflective (white) can't radiate their own heat well. In other words, you want "black" in the far infrared and "white" in visible/near IR. (That's the greenhouse effect in reverse.)
The temperature rises until the heat power lost (proportional to temp**4 in a black body) equals the power received.
White paint doesn't always have good IR properties. We had to specify titanium dioxide paint for painting radio telescopes. It was expensive, but had the best properties. Everything stayed safe to touch even in the summer desert. (From a TowerTalk@contesting.com discussion.)
P (power) = E (volts) times I (current)
Why? The amount of work you do is proportional to the amount of charge (electrons) you move times the force that's pushing against you (that's voltage). Moving lots of charge against little force is little work; moving little charge against a strong force is also little work. Etc.
When you have a constant resistance, voltage and current are proportional to each other (E = I x R, Ohm's Law), so that's how you get
P = E x (E/R) = E x E / R = E**2 / R
Do it a different way, and you see that P = I**2 x R also.
A dB is a decibel, a tenth of a bel. But nobody uses "bels" -- don't ask why! Ten decibels or one bel is a ratio of 10, exactly. Ratio of what? Anything! But normally we use dB for power ratios.
A ratio of two (doubling) corresponds to 10 log 2 (base 10) = 3.0103 dB. That's not exactly 3 dB, but close enough for us!
A ratio of four is 6 dB, close enough. And a ratio of 4 in power (2 in voltage) is one S-unit, according to most folks. Why? It's just the way it was defined many years ago. We'd be better off nowadays to use dBm (dB relative to 1 milliWatt) like engineers normally do. The reference point that is most commonly cited is that S9 should be equivalent to 50 microVolts at the receiver input, but many (most?) receivers' S meters are not well calibrated. See some other net documents on S-Meters: here and here.
As for the "the impedance of free space", that comes from Maxwell's Equations. They describe how electric and magnetic fields can propagate through space (ie make "radio waves"). It turns out that in a vacuum, the magnetic and electric fields of a radio wave always occur in a particular ratio. The magnetic field corresponds to "current" and the electric field to "voltage", so the ratio of electric to magnetic field is like a "resistance" which has the value of 120 x pi = 377 ohms, more or less. That's a theoretical number, but if you can figure out how to measure the electric and magnetic fields, you could find the ratio experimentally. It's not something you could do with a multimeter very well.
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QRU for now!
-Martin
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