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Unformatted text preview: 1 2 Chapter 23 Electric Potential 3 231 Electric Potential and Potential Difference 4 Definition of Electric Potential V A U A q ≡ Potential energy at point A Potential at point A 5 Electric Potential • Electric potential is a scalar quantity. • The SI units of electric potential are joules per coulomb. • The unit of potential is the volt. • A VOLT is defined: 1 V 1 J/C ≡ 6 Potential Difference and Electric Potential The potential difference between points A and B , V B – V A , is defined as the change in potential energy (final value minus initial value ) of a charge q moved from A to B divided by the size of the charge. ∆ V V B – V A = ≡ ∆ U q Final point Initial point Important! 7 Conservative Forces • A force is conservative if the work it does on a particle moving between any two points is independent of the path taken by the particle. • The work done by a conservative force exerted on a particle moving through any closed path is zero. • Gravitational (Newton’s law of gravity) and Electrical (Coulomb’s law of electrical force) are both conservative forces. • Since electrostatic force is conservative, electrostatic phenomena can be described in terms of electrical potential energy. 8 Work W AB =  ∆ U =  q ( V B – V A ) Initial point Final point Final point Initial point Equation is true if the only force is the conservative electrostatic force. That is, there are no nonconservative forces acting on the system. 9 Work Again • Electrostatic force is conservative. • As a charge moves from some initial point A to some final point B under the influence of the electric force of magnitude qE exerted on it, the work done by the electric field on the charge is positive. W AB = Fd = ( qE ) d Initial point Final point Important! 10 Change in Potential Energy • As the electric field accelerates the charge, the charge gains kinetic energy. • As the charged particle gains kinetic energy, it loses an equal amount of potential energy. ∆ K =  ∆ U • By definition, the work done by a conservative force equals the negative change in potential energy, ∆ U. ∆ U =  W AB =  qEd • This equation is valid only for a uniform electric field. 11 Potential Energy ∆ U = U b – U a = qV ba Final point Initial point Important! 12 Conceptual Example 231 A negative charge. Suppose a negative charge, such as an electron, is placed at point b in the figure. If the electron is free to move, will its electric potential energy increase or decrease? How will the electric potential change? 13 Why is side “a” the high potential side? 14 The positive side is always the “high” potential side, regardless of the sign of the charge....
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 Fall '09
 Electrostatics, Potential Energy, Electric charge, uniform electric field

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