Physics 212 Lecture 5 Todays Concept Electric Potential

Physics 212 Lecture 5 Today's Concept: Electric Potential Energy Defined as Minus Work Done by Electric Field Physics 212 Lecture 5, Slide 1

Main Point 1 First, the Coulomb force is a conservative force. By that, we mean that the work done by the Coulomb force on a charged particle as it is moved between any two points is independent of the path taken between those two points. Physics 212 Lecture 5, Slide 2

Main Point 2 Second, since the Coulomb force is a conservative force, we can define an electric potential energy associated with this force. In particular, we defined the change in electric potential energy of a charged particle moved from point A to point B to be minus the work done by the Coulomb force on that particle as it moves between the two points. Physics 212 Lecture 5, Slide 3

Main Point 3 Third, the electric potential energy of a system of fixed charged particles is just equal to the scalar sum of the electric potential energies due to each pair of particles. Physics 212 Lecture 5, Slide 4

Recall from physics 211: F dr W Object speeds up ( DK > 0 ) W Object slows down ( DK < 0 ) F dr or F dr W Constant speed ( DK = 0 ) Physics 212 Lecture 5, Slide 5

Example: Two Point Charges Calculate the change in potential energy for two point charges originally very far apart moved to a separation of “d” d q 1 q 2 For point charges often choose r=infinity as “zero” potential energy. Physics 212 Lecture 5, Slide 6

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Potential Energy of Many Charges Two charges are separated by a distance d. What is the change in potential energy when a third charge q is brought from far away to a distance d from the original two charges? Q 2 d d q Q 1 d Physics 212 Lecture 5, Slide 8

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Checkpoint 1 A B C D E 34 Physics 212 Lecture 5, Slide 10

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Checkpoint 2 A 31 B C D Physics 212 Lecture 5, Slide 12

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Checkpoint 3 34 Physics 212 Lecture 5, Slide 14

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Checkpoint 4 A charge is released from rest in a region of electric field. The charge will start to move A) in a direction that makes its potential energy increase B) in a direction that makes its potential energy decrease C) along a path of constant potential energy 34 Physics 212 Lecture 5, Slide 16

Example A positive charge q is placed at x=0 and a negative charge -2 q is placed at x=d. At how many different places along the x axis could another positive charge be placed without changing the total potential energy of the system? 40 q -2 q X=0 X=d x Physics 212 Lecture 5, Slide 17

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