25-electric potentials

# 25-electric potentials - Physics 4B Lecture Notes Chapter...

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Physics 4B Lecture Notes 25-1 Chapter 25 - Electric Potential Problem Set #4 - due: Ch 25 - 6, 7, 12, 17, 22, 28, 34, 36, 41, 50, 56, 63 The Law of Conservation of Energy is the single most useful law of physics. In this chapter we will develop the insights needed to make use of energy conservation to understand the ideas associated electrical phenomena. Lecture Outline 1. The Potential Energy of Charges in Electric Fields 2. The Definition of Electric Potential 3. The Potential Due to Point Charges 4. The Potential Due to Continuous Charge Distributions 5. Finding the Electric Field from the Potential 1. The Potential Energy of Charges in Electric Fields m q E g ds ds Recall the definition of potential energy as the negative of the work done by a conservative force, U ≡ - W c . Using the definition of work, U = - r F d r s . Recall how this went with gravity, U g = - r F g d r s = - m r g d r s = - m r g d r s in general. For a constant gravitational field, g, U g =-mgh. It goes the same way with electricity, U e = - r F e d r s = - q r E d r s = - q r E d r s . Electric Potential Energy U = - q r E d r s Example 1: Find the change in electric potential energy when two protons initially 0.100nm apart are completely separated. The electric potential energy is, U = - q r E d r s . The field is due a point charge and the motion is along the same direction, ˆ r . U = - e k e r 2 dr R = - ke 2 - 1 r R = - k e 2 R U = - 9x10 9 1.6x10 - 19 ( 29 2 0.100x10 - 9 = - 2.3x10 - 18 J q=e E + + q=e ds = dr

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Physics 4B Lecture Notes 25-2 Note that U = U( ) - U(R). It is customary to set U( ) 0so we can say that the original potential energy of the protons wasU(R) = + 2.3x10 - 18 J . These protons originally had a lot of potential energy when close together and are quite happy to fly apart when given the chance.
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## This note was uploaded on 07/01/2010 for the course PHYSICS 055 taught by Professor Basu during the Spring '10 term at UC Davis.

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25-electric potentials - Physics 4B Lecture Notes Chapter...

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