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Week of September 17th

Week of September 17th - Class notes for for week of...

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V + I - R1 2.4k R2 3.3k R3 6.8k R4 3.6k R5 7.2k R6 4.2k I1 I2 I3 V5 I4 Class notes for for week of September 17 th Ohm’s law Last week we discussed the nature of conductivity, and we described three classes of materials, conductors, semi-conductors, and insulators. As you might expect, things are not that simple. For example not all the metals are better conductors than others. This is due to the probability of the electron being scattered by collisions while traveling through the metal and is very different for the different metals. Similarly, insulators are not perfect, and a few electrons can slip through them, helped along by impurities or unlikely probabilities. So we need to describe conductivity a little more precisely. Georg Ohm published in 1827 an empirical rule that defines conductivity. Much later with the discovery of quantum mechanics, it was found that his rule depends on the statistical probabilities of electron motion. Despite this Ohm’s law works for different materials that exhibit conductivities that vary over a range of 25 orders of magnitude(10 25 ). We will use Ohms law in the following form: where V is the voltage across the device of interest, I is the current through the device and R is the resistance of the device. Resistance is thus defined to be: We find that resistance has units of volt per amp which is the same as the derived unit ohm . In electrical engineering, it is more common to discuss resistivity as opposed to conductivity. The two quantities are related with
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