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# Quiz 4 - design in order to increase its resistance without...

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ECE3025, Summer 2011, Quiz 4 June 17, 2010 1. (2 points) Name a condition permits an electric ﬁeld to exist in a metal. 2. (4 points) Derive the formula for the resistance of a material with length L , cross-sectional area A , and conductivity σ . 3. (2 points) Under what conditions do we assume that we have a surface current density and under what conditions do we assume we have a volume current density? 4. (2 points) If your resistor had a ﬁxed length due to a design constraint, how would you modify your
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Unformatted text preview: design in order to increase its resistance without changing the material you are using? 5. (15 points) A long, round wire of radius a and conductivity σ is coated with a material of conductivity . 1 σ . a) what must be the thickness of the coating so that the resistance per unit length of the total new coated wire is 25% less than the resistance of the uncoated wire% ? b) Assuming a total current I in the coated wire, ﬁnd-→ J and-→ E in the wire. 1...
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