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Electric Potential

Electric Potential. Chapter 26. Review from ch 7. When positive work is done, the potential energy decreases. Total energy is conserved. Work done by E field. Using calculus and the coulomb force equation, we can come up with. Work done by E field. We can write using potential energy.

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Electric Potential

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  1. Electric Potential Chapter 26

  2. Review from ch 7 • When positive work is done, the potential energy decreases. • Total energy is conserved.

  3. Work done by E field • Using calculus and the coulomb force equation, we can come up with

  4. Work done by E field • We can write using potential energy • Thus

  5. Example • A small plastic ball with a mass of 2.0 g and an electric charge of 0.10 mC moves in the vicinity of a stationary metal ball with a charge of 2.0 mC. When the plastic ball is 0.10 m from the metal one, the plastic ball is moving directly away from the metal one with a speed of 5.0 m/s. • What is the speed of the plastic ball when the two balls are 0.20 m apart?

  6. You try • What if the plastic ball had a charge of -0.10 mC?

  7. Potential • Potential energy per unit charge • Measured in volts

  8. Potential from a group of charges

  9. Example • Pages 583-584

  10. Equipotential surfaces • A surface such that every point on the surface has the same potential. • Since the potential is the same, E does no work as a charge moves along an equipotential surface.

  11. Millikan oil-drop experiment • 1909-1913 in Chicago by Robert Millikan • Very small electrically charged oil drops sprayed into electric field • Field adjusted until the gravitational force down balanced the electric force up. • Figured out charge on the oil drops.

  12. Millikan oil-drop experiment • Figured out that the charge was always a whole number multiple of 1.6 x 10-19 • “discovered” the charge on the electron • Validation of atomic theory • Combined with earlier research of J.J. Thomson to determine mass of electron

  13. Mass of oil drops

  14. Electronvolt • Unit of energy (like J) • Useful in atomic and nuclear calculations • 1 eV = 1.6 x 10-19 J • The change in energy as 1 electron moves through a potential difference of 1 V

  15. Rest energy E0 = mc2 • For an electron, 81.87 x 10-15 J or 0.511 MeV • Mass of subatomic particles is often expressed in MeV/c2 to make the numbers easier to work with • Mass of electron = 0.511 MeV/c2

  16. Cathode ray tubes • Found in computer monitors and oscilloscopes • Similar to TV picture tubes • Use an electron beam – historically called a cathode ray • A near vacuum • Electrons sent through two sets of electrically charged plates • One to aim it horizontally • One to aim it vertically

  17. Equations • We did this kind of problem last chapter • There are more equations on pages 594 - 595

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