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Unformatted text preview: 1/30/12 1 1/30/12 1 Chapter 23 • Electrostatic Potential Energy of a system of fixed point charges is equal to the work that must be done by an external agent to assemble the system, bringing each charge in from an infinite distance. q 1 Point 2 Point 1 1/30/12 2 Example: Electrostatic Potential Energy q 1 Point 2 q 2 Point 1 If q 1 & q 2 have the same sign, U is positive because positive work by an external agent must be done to push against their mutual repulsion. If q 1 & q 2 have opposite signs, U is negative because negative work by an external agent must be done to work against their mutual attraction. 1/30/12 3 Example: Three Point Charges q 1 Point 2 r 1,2 q 2 Point 1 r 1,3 r 2,3 Point 3 q 3 1/30/12 4 Electrostatic Potential Energy • We can conclude that the total work required to assemble the three charges is the electrostatic potential energy U of the system of three point charges: 3 , 2 2 3 3 , 1 1 3 2 , 1 1 2 3 2 r q kq r q kq r q kq W W W U total + + = + = = The electrostatic potential energy of a system of point charges is the work needed to bring the charges from an infinite separation to their final position 1/30/12 2 1/30/12 5 Calculate Electric Field from the Potential...
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