Lecture18_Feb27_2017 (1).pptx

# Lecture18_Feb27_2017 (1).pptx - Electricity and Magnetism...

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Electricity and Magnetism Physics 208 Dr. Tatiana Erukhimova Lecture 18

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Current, Ohm’s Law, Etc. dt dQ i Ohm ' s Law : R V i ; R Const ( independent of V ) A l R
t Q i ave dt dQ t Q i t 0 lim

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where R is resistance Resistance does not vary with the applied voltage resistor Const i V Law s Ohm : ' , R i V Ampere Volts R

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Experimentally it is found that R depends on the material the wire is made of and its dimensions. Does not depend on the shape. In a wire of uniform resistivity and cross sectional area, the electric field is a constant for constant currents. A l R area the is A length is l , y resistivit is ty conductivi is 1
+ - V i l VA l VA R V i 1 i

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Exercise 1 Given the resistivity of copper, about Ohm-m, what length of 0.5 cm diameter wire will yield a resistance of 10 Ohms? 8 10
Current Density Consider current flowing in a homogeneous wire with cross sectional area A.
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