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Announcements

Announcements. Use this power point to complete your notes for this unit. Work on the questions that we did not do for the last homework quiz. From your textbook, P491 #1,3,4,11,12,17,21 Please, be responsible and do this! Go to the following website and try some of the activities!

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Announcements

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  1. Announcements • Use this power point to complete your notes for this unit. • Work on the questions that we did not do for the last homework quiz. • From your textbook, P491 #1,3,4,11,12,17,21 • Please, be responsible and do this! • Go to the following website and try some of the activities! • http://www.learnapphysics.com/apphysicsb/index.html

  2. Electric Potential and Potential Energy

  3. Electric Potential Energy • Electrical potential energy is the energy contained in a configuration of charges. Like all potential energies, when it goes up the configuration is less stable; when it goes down, the configuration is more stable. • Electrical potential energy increases when charges are brought into less favorable configurations (ex:, like-sign charges getting closer together, or unlike-sign charges farther apart). • Electrical potential energy decreases when charges are brought into more favorable configurations. • The unit is the Joule.

  4. Electrical Potential Energy • Charges separated by a distance have a force between them • If released, the force causes the charges to move, giving the charges kinetic energy. • KE comes from the store potential energy in the arrangement of the charges.

  5. Calculating Electrical PE

  6. Electrical Potential Energy • Electrical Potential Energy is a scalar. • The sum of all electrical potential energies among an array of charges gives the net electrical potential energy of the system.

  7. Work and Electrical Potential Energy • Electrical force is a conservative force, the work is also a conservative force: • Wo=-∆U • Work done by electric force is opposite to change in electric potential energy • When a positive charge moves in the same direction as the electric field, the work done BY the field is positive and the charge loses electrical potential energy.

  8. Electric Potential • Electric potential is hard to understand, but easy to measure. • We commonly call it “voltage”, and its unit is the Volt. • Electric potential is easily related to both the electric potential energy, and to the electric field.

  9. Electrical Potential • The amount of energy that is available per charge at any given location from a source charge, Q

  10. Electrical Potential and Potential Energy • The change in potential energy is directly related to the change in voltage. • U = qV • U: change in electrical potential energy (J) • q: charge moved (C) • V: potential difference (V) • All charges will spontaneously go to lower potential energies if they are allowed to move.

  11. Electrical Potential and Potential Energy • Since all charges try to decrease potential energy, and DU = qDV, this means that spontaneous movement of charges result in negative DU. • Positive charges like to DECREASE their potential (DV < 0) • Negative charges like to INCREASE their potential. (DV > 0)

  12. Sample Problem A 3.0 μC charge is moved through a potential difference of 640 V. What is its potential energy change?

  13. Electrical Potential in Uniform Electric Fields • The electric potential is related in a simple way to a uniform electric field. • V = -Ed • V: change in electrical potential (V) • E: Constant electric field strength (N/m or V/m) • d: distance moved (m) d E DV

  14. Sample Problem An electric field is parallel to the x-axis. What is its magnitude and direction if the potential difference between x =1.0 m and x = 2.5 m is found to be +900 V?

  15. Sample Problem An electric field is parallel to the x-axis. What is its magnitude and direction if the potential difference between x =1.0 m and x = 2.5 m is found to be +900 V?

  16. Sample Problem What is the voltmeter reading between A and B? Between A and C? Assume that the electric field has a magnitude of 400 N/m. y(m) C 1.0 A B 1.0 2.0 x(m)

  17. Sample Problem How much work would be done BY THE ELECTRIC FIELD in moving a 2 mC charge from A to C? From A to B? from B to C?. How much work would be done my an external force in each case? y(m) C 1.0 A B 1.0 2.0 x(m)

  18. Energy Conservation in Electric Fields

  19. Announcements

  20. Conservation of Energy Review • In a conservative system, energy changes from one form of mechanical energy to another. • When only the conservative electrostatic force is involved, a charged particle released from rest in an electric field will move so as to lose potential energy and gain an equivalent amount of kinetic energy. • The change in electrical potential energy can be calculated by • DUE = qDV.

  21. Sample Problem If a proton is accelerated through a potential difference of 2,000 V, what is its change in potential energy? How fast will this proton be moving if it started at rest?

  22. Sample Problem If a proton is accelerated through a potential difference of 2,000 V, what is its change in potential energy? How fast will this proton be moving if it started at rest?

  23. Sample Problem A proton at rest is released in a uniform electric field. How fast is it moving after it travels through a potential difference of -1200 V? How far has it moved?

  24. Sample Problem A proton at rest is released in a uniform electric field. How fast is it moving after it travels through a potential difference of -1200 V? How far has it moved?

  25. Bonus Demo Lab • Charge two Mylar or Plastic beads by touching a charged rubber rod. Describe what happens, using what you know about electrostatics. • Calculate the number of excess electrons on each bead.

  26. Bonus Demo Lab • Charge two Mylar or Plastic beads by touching a charged rubber rod. Describe what happens, using what you know about electrostatics. • Calculate the number of excess electrons on each bead.

  27. Bonus Demo Lab • Charge two Mylar or Plastic beads by touching a charged rubber rod. Describe what happens, using what you know about electrostatics. • Calculate the number of excess electrons on each bead.

  28. Potential and Potential Energy of Configurations of Point Charges

  29. Announcements

  30. Electric Potential Energy for Spherical Charges • Electric potential energy is a scalar, like all forms of energy. • U = kq1q2/r • U: electrical potential energy (J) • k: 8.99  109 N m2 / C2 • q1, q2 : charges (C) • r: distance between centers (m) • This formula only works for spherical charges or point charges.

  31. Sample Problem What is the potential energy of the configuration shown below? y (m) 2.0 1.0 2 mC 4 mC x (m) 1.0 2.0

  32. Sample Problem How much work was done in assembling the charge configuration shown below? y (m) 2.0 -3 mC 1.0 2 mC 4 mC x (m) 1.0 2.0

  33. Electric Potential (spherical) • For a spherical or point charge, the electric potential can be calculated by the following formula • V = kq/r • V: potential (V) • k: 8.99 x 109 N m2/C2 • q: charge (C) • r: distance from the charge (m) • Remember, k = 1/(4peo)

  34. Sample Problem What is the electric potential at (2,2)? y (m) 2.0 -3 mC 1.0 2 mC 4 mC x (m) 1.0 2.0

  35. highest high medium low lowest Equipotential surfaces positive negative

  36. High potential Low potential Equipotential surfaces

  37. Question • What can you say about the intersection between field lines and equipotential surfaces?

  38. Sample Problem Draw field lines for the charge configuration below. The field is 600 V/m, and the plates are 2 m apart. Label each plate with its proper potential, and draw and label 3 equipotential surfaces between the plates. You may ignore edge effects. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

  39. Sample Problem Draw a negative point charge of -Q and its associated electric field. Draw 4 equipotential surfaces such that DV is the same between the surfaces, and draw them at the correct relative locations. What do you observe about the spacing between the equipotential surfaces?

  40. Fill in the following table for spherical charges

  41. What is magnitude and direction of electric field?b) What is shortest distance one can go to undergo a change of 5.00 V?

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