Ch 19 Electric Potential Energy and the Electric Potential

Ch 19 Electric Potential Energy and the Electric Potential...

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Unformatted text preview: Lecture Notes 19 lectric Potential Energy lectric Potential Energy Electric Potential Energy Electric Potential Energy and the and the Electric Potential Electric Potential oals Goals ■ To calculate electrical potential energy. To calculate electrical potential energy. ■ To define potential. To define potential. o study o study uipotential uipotential rfaces rfaces ■ To study To study equipotential equipotential surfaces surfaces ■ To examine capacitors and dielectrics. To examine capacitors and dielectrics. ■ To determine electrical field energy To determine electrical field energy Electric Potential Energy g E Gravity exerts a force, F = m g , on the asketball f ass ork one y Because of the electric field E , an electric rce = erted n ositive st basketball of mass m. Work is done by the gravitational force as the ball falls from A to B. force F q o E is exerted on a positive test charge + q o . Work is done by the electric force as the charge moves from A to B. ● F g = Gm 1 m 2 / r 2 and F e = kq 1 q 2 / r 2 . Both of these forces are conservative . ● It follows that a potential energy can be associated with them. 1. The Gravitational Case: W a → b = mgh a − mgh b = U a − U b = − Δ U The work-energy theorem states that the change in kinetic energy Δ K=K b − K a during any displacement is equal to the total work done on the particle. So if W a → b gives the total work, then K b − K a =U a − U b , which we usually write as: K a +U a =K b + U b More on the comparison between it ti l d l t i fi ld the gravitational and electric fields 2. The Electrical Case: e n present e ork = s = 'Es rms f otential We can represent the work W a → b Fs q Es in terms of a potential energy U , as we did for gravitational potential energy. The y component of force, F y = – q'E , is constant, and there is no x or z y component, so the work is independent of the path the particle takes from A to B, just as the potential energy for the gravitational force = g as = gy e otential ergy r e ectric eld F y – mg was U mgy , the potential energy for the electric-field force F y = – q'E is U = q'Ey . PE PE W a → b = U a − U b = − Δ U B A AB W EPE EPE = W a → b = q'Eh A − q'Eh B = U a − U b = − Δ U Work and energy changes ork is done on the charge by the field ● Work is done on the charge by the field. 19.2 The Electric Potential Difference PE PE o B o A o AB q q q W EPE EPE − = E The potential energy per unit charge is called the electric potential . 19.2 The Electric Potential Difference Definition of Electric Potential The electric potential at a given point is the electric potential energy of a small test charge divided by the charge itself: o q V EPE = SI Unit of Electric Potential: joule/coulomb = volt (V) o AB o A o B A B q W q q V V − = − = − EPE EPE ( ) AB q W q V − = Δ = Δ EPE o o 19.2 The Electric Potential Difference The electric potential V at any point in an electric field is the electric potential energy...
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This note was uploaded on 04/01/2011 for the course PHY 135 taught by Professor Wagihghobriel during the Spring '11 term at University of Toronto- Toronto.

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Ch 19 Electric Potential Energy and the Electric Potential...

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