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Physics 212 Lecture 5

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

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Physics 212 Lecture 5

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

  2. 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.

  3. 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.

  4. 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.

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

  6. 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 q1 q2 For point charges often choose r=infinity as “zero” potential energy.

  7. Potential Energy of Many Charges d q d 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? Q2 d Q1

  8. Checkpoint 1 A B C D E 34

  9. Checkpoint 2 A B C D 31

  10. Checkpoint 3 34

  11. 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 increaseB) in a direction that makes its potential energy decreaseC) along a path of constant potential energy 34

  12. Example A positive charge q is placed at x=0 and a negative charge -2q 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? -2q q x X=0 X=d 40

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