Lecture06_Kim

# Lecture06_Kim - Lecture 6-1 Lecture 6-2 Electric Potential...

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1 Lecture 6-1 Electric Potential Energy of a Charge in Electric Field • Coulomb force is conservative => Work done by the Coulomb force is path independent . • Associate potential energy to charge q 0 at any point r in space. ( ) U r h It’s energy! A scalar measured in J (Joules) dl a 0 dW q E dl = ca a i Work done by E field 0 dU dW q E dl = - = - ca a i Potential energy change of the charge q 0 Lecture 6-2 Electric Potential Energy of a Charge (continued) 0 dW q E dl = ca a i 0 ( ) ( ) r i U U r U i q E dl = - = - h h ch h i 0 dU dW q E dl = - = - ca a i i is “the” reference point . Choice of reference point (or point of zero potential energy ) is arbitrary. 0 dl a i is often chosen to be infinitely far ( ) Lecture 6-3 Gravitational vs Electrostatic Potential Energy ( ) ( ) b a U U b U a d F l = - = - i a a c a b qE ca mg ca Gravity Coulomb Work done by <gravity/Coulomb> force is the decrease in potential energy. mg l - qE l - (if g, E uniform) Lecture 6-4 Potential Energy in the Field due to a Point Charge q 0 0 2 0 2 0 0 ( ) P P r r U r q E dl q q k r dl l q q k dl l q q q q k k l r ∞→ ∞→ = - = - = - = = ca a i a ɵ i From This is also called the potential energy of the two-charge configuration of q and q 0 . What is the work required to separate the two charges to ?

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2 Lecture 6-5 Potential Energy of a Multiple-Charge Configuration (b) (a) 1 2 / kqq d 1 3 1 3 2 2 2 q q q q q k k k d d q d + + (c) 2 3 1 3 3 4 1 2 2 4 1 4 2 2 q q q q q q q q k k k k d d q q q q k k d d d d + + + + 2 d i j ij i j i j i j q q U U k r r < < = = - a a Lecture 6-6 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 a a • [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 a a 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! Lecture 6-7
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## This note was uploaded on 02/05/2012 for the course PHYS 241 taught by Professor Wei during the Spring '08 term at Purdue.

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Lecture06_Kim - Lecture 6-1 Lecture 6-2 Electric Potential...

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