# Lec07-1 - ture 7-1 ture Electric Potential Energy of a...

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cture 7- cture 7- 1 Electric Potential Energy of a Charge (continued) 0 ( ) ( ) r i U U r U i q E dl = - = - v v u vv g i is “the” reference point . Choice of reference point (or point of zero potential energy ) is arbitrary. 0 dl r i is often chosen to be infinitely far away (∞) = l d E q dW 0 - = - = l d E q dW dU 0 - = r i l d E q 0

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cture 7- cture 7- 2 Electric Potential • So U( r )/q 0 is independent of q 0 , allowing us to introduce electric potential V independent of q 0 . 0 ( ) ( ) U r V r q r r [Electric potential] = [energy]/[charge] SI units: J/C = V (volts) U( r ) of a test charge q 0 in electric field generated by other source charges is proportional to q 0 . 0 ( ) ( ) U r V r q r r taking the same reference point Potential energy difference when 1 C of charge is moved between points of potential difference 1 V 1 J = Scalar!
cture 7- cture 7- 3 Electric Potential Energy and Electric Potential positive charge High U (potential energy) Low U negative charge High U Low U High V (potential) Low V Electric field direction High V Low V Electric field direction

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cture 7- cture 7- 4 E from V x V E x = - y V E y = - z V E z = - Expressed as a vector, E is the negative gradient of V V E - = r r We can obtain the electric field E from the potential V by inverting the integral that computes V from E : E always points from high V to low V. (But high V is high U only for a positive charge.) + + - = - = r z y x r dz E dy E dx E l d E r V ) ( ) (
cture 7- 5 READING QUIZ 1 A conducting sphere A with radius R A = 1 meter is connected via a conducting wire to a conducting sphere B with radius R B = ½ meter. The spheres have a total charge Q = Q A + Q B . The radial electric fields at surface of the spheres are E A and E B respectively. Which of the following statements is correct? A|

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## This note was uploaded on 12/07/2011 for the course PHYS 241 taught by Professor Wei during the Fall '08 term at Purdue University-West Lafayette.

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Lec07-1 - ture 7-1 ture Electric Potential Energy of a...

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