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lecture 8

# lecture 8 - Chapter 29 Electric Potential Reading Chapter...

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1 Electric Potential Reading: Chapter 29 Chapter 29

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2 If is an infinitesimal displacement of test charge, then the work done by electric force during the motion of the charge is given by Electrical Potential Energy s r When a test charge is placed in an electric field, it experiences a force E r q F qE = r r s r sF qsE = r r r r
3 Electrical Potential Energy E r q F qE = r r s r sF qsE = r r r r This is the work done by electric field. In this case work is positive. Because the positive work is done, the potential energy of charge-field system should decrease. So the change of potential energy is U sF qsE = - = - r r r r sign minus F mg = r r g r s r This is very similar to gravitational force: the work done by force is sF mgs = r r rr The change of potential energy is U sF mgs = - = - r r rr

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4 Electrical Potential Energy: Example E r q F qE = r r s r - = - = - = - r r r r cos B A U U sF qsE qsEα α A B = - cos B A U U qsEα E r q F qE = r r 1 s r α A B 2 s r s r 1 2 1 B A U U s F s F qs E - = - - = - r r r r 2 0 s E = r r = - cos B A U U qsEα = 1 cos s The change of potential energy does not depend on the path: The electric force is conservative
5 Electrical Potential Energy E r q F qE = r r s r α A B cos B A U U qsEα = - The electric force is conservative For all paths: B B A A U U U q E ds = - = - r r d s is oriented tangent to a path through space ds r ds r

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6 Electric Potential Electric potential is the potential energy per unit charge, The potential is independent of the value of q . The potential has a value at every point in an electric field Only the difference in potential is the meaningful quantity. E r q F qE = r r s r - = - = - = - = - r r r r cos B A B A U U sF V V sE sEα q q q α A B U V q =
7 Electric Potential To find the potential at every point 1. we assume that the potential is equal to 0 at some point, for example at point A , 2. we find the potential at any point B from the expression E r s r B B B A A A V V Eds Eds = - = - r r r r α A B 0 A V = = - cos B V sEα

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lecture 8 - Chapter 29 Electric Potential Reading Chapter...

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