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Lecture_09

# Lecture_09 - Potential Energy Introduced the concept of...

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Introduced the concept of electric field E to deal with forces Introduced electric potential V to deal with work and energy Electric potential: electric potential energy per unit charge Potential Energy Potential energy is associated with pairs of interacting objects A single particle has no electric potential energy

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Work Work done on a particle is the product of the force exerted on the particle times the displacement through which the force acts: W = F Δ l Units: Joules In Cartesian coordinates: W = F x Δ x + F y Δ y + F z Δ z
q 1 q 2 r 12 Δ E 1 + Δ E 2 W int = W ext Δ U el - W int Δ K system + Δ U el = W ext + Q Total energy of the system can be changed ( only ) by external forces. Work done by internal forces: Electric Potential Energy of Two Particles Energy of the system: 1. Energy of particle q 1 2. Energy of particle q 2 3. Interaction energy U el To change the energy of particles we have to perform work. Δ E 1 + Δ E 2 = W ext + W int + Q if Δ ( mc 2 ) = 0 : Energy Principle for a multiparticle system. Δ U el = W int = F int d l i f

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Example An electron traveling to the right enters capacitor through a small hole at A. Electric field strength is 2x10 3 N/C. What is the change in the electron’s potential energy in traveling from A to B? What is its change in kinetic energy? Δ ( AB)= 4mm = (1.6x10 -19 C)(2x10 3 N/C)(0.004m) =1.3x10 -18 J Δ K = Δ U electric = -1.3x10 -18 J x Δ U electric = F i Δ l = ( eE x ) Δ x = eE x Δ x
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Lecture_09 - Potential Energy Introduced the concept of...

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