If the charges in some material take, say, 5 ns to approach rough equilibrium (s

Question

If the charges in some material take, say, 5 ns to approach rough equilibrium (so = 5 ns), any process that takes significantly longer than 5 ns will preserve the static nature of the charge distribution, and so you’ll be able to make various electrostatic approximations. Electromagnetic radiation comes in a variety of frequencies, and thus has oscillations with a variety of periods. Figure out the radiation frequencies corresponding to the relaxation times for copper and germanium, and identify which parts of the EM spectrum they come from (Xray, radio, etc). This sort of thing can really matter if you go to do materials science work with radiation – radiation that has periods well above the relaxation time for a material can interact quite differently with the material than radiation with periods well below it.

Explanation / Answer

relaxation time for copper T = 1.5 x 10-19 s ==> frequency = 1/T = 6 .67x 1018 Hz [X- ray]

elaxation time for copper Germanium = 1.3 x 10-12 s ==> frequency = 1/T = 7.7x 1011 Hz [microwave]

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