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Ch. 20 Electric Potential and Electric Potential Energy

Ch. 20 Electric Potential and Electric Potential Energy. Electric Potential Energy.

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Ch. 20 Electric Potential and Electric Potential Energy

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  1. Ch. 20 Electric Potential and Electric Potential Energy

  2. 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. • The unit is the Joule.

  3. Equation • DU = - W = q0Ed • q0 – test charge • E – Electric field • d - distance

  4. Electrical potential energy increaseswhen charges are brought into lessfavorable configurations

  5. Electrical potential energy decreases when charges are brought into more favorable configurations.

  6. Work must be done on the charge to increase the electric potential energy

  7. For a positive test charge to be moved upward a distance d, the electric force does negative work. • The electric potential energy has increased and U is positive (U2 > U1)

  8. If a negative charge is moved upward a distance d, the electric force does positive work. • The change in the electric potential energy U is negative (U2 < U1)

  9. Electric Potential (V) Electric potential is hard to understand, but easy to measure. • We commonly call it “voltage”, and its unit is the Volt. • 1 V = 1 J/C • Electric potential is easily related to both the electric potential energy, and to the electric field.

  10. The change in potential energy is directly related to the change in voltage. DU = qDV DV = DU/q • DU: change in electrical potential energy (J) • q: charge moved (C) • DV: potential difference (V) • All charges will spontaneously go to lower potential energies if they are allowed to move.

  11. Since all charges try to decrease UE, and DUE = qDV, this means that spontaneous movement of charges result in negative DU. • DV = DU / q • 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. Sample Problem: A 3.0 μC charge is moved through a potential difference of 640 V. What is its potential energy change?

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

  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: 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?

  17. 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?

  18. Sample Problem: A proton at rest is released in a uniform electric field. What potential difference must it move through in order to acquire a speed of 0.20 c?

  19. Electric Potential Energy forSpherical 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.

  20. Absolute 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)

  21. Electric Field and Electric Potential E = - V / d Two things about E and V: • The electric field points in the direction of decreasing electric potential. • The electric field is always perpendicular to the equipotential surface.

  22. 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?

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