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Chapter 23 Finding the Electric Field - II In this chapter we will introduce the following new concepts: The flux (symbol Φ ) of the electric field. Symmetry Gauss law We will then apply Gauss law and determine the electric field generated by: An infinite, uniformly charged insulating plane. An infinite, uniformly charged insulating rod. A uniformly charged spherical shell. A uniform spherical charge distribution. We will also apply Gauss law to determine the electric field inside and outside charged conductors. (23-1)

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(23-2)

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dA (23-5)

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e (23-6)

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(23-7)

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(23-8)

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S1S1 S2S2 S3S3 (23-9)

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Symmetry: We say that an object is symmetric under a particular mathematical operation (e.g. rotation, translation, …) if to an observer the object looks the same before and after the operation. Note: Symmetry is a primitive notion and as such is very powerful featureless sphere rotation axis observerRotational symmetry Example of spherical symmetry Consider a featureless beach ball that can be rotated about a vertical axis that passes through its center. The observer closes his eyes and we rotate the sphere. Then, when the observer opens his eyes, he cannot tell whether the sphere has been rotated or not. We conclude that the sphere has rotational symmetry about the rotation axis. (23-10)

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Featureless cylinder rotation axis observer Rotational symmetry A second example of rotational symmetry Consider a featureless cylinder that can rotate about its central axis as shown in the figure. The observer closes his eyes and we rotate the cylinder. Then, when he opens his eyes, he cannot tell whether the cylinder has been rotated or not. We conclude that the cylinder has rotational symmetry about the rotation axis (23-11)

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observer magic carpet infinite featureless plane Translational symmetry Example of translational symmetry: Consider an infinite featureless plane. An observer takes a trip on a magic carpet that flies above the plane. The observer closes his eyes and we move the carpet around. When he opens his eyes the observer cannot tell whether he has moved or not. We conclude that the plane has translational symmetry (23-12)

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Recipe for applying Gauss Law 1. Make a sketch of the charge distribution 2. Identify the symmetry of the distribution and its effect on the electric field 3. Gauss law is true for any closed surface S. Choose one that makes the calculation of the flux as easy as possible. 4. Use Gauss law to determine the electric field vector (23-13)

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S1S1 S2S2 S3S3 (23-14)

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S1S1 S2S2 S3S3 (23-15)

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S S'S' A A'A' (23-16)

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(23-17)

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S2S2 S1S1 (23-18)

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R r E O S2S2 S1S1 (23-19)

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Apply Gauss’s law 2. Choose Gaussian surface so that E can be taken out of integration: explore the symmetry of E Symmetry of EGaussian Surface Spherical.

Apply Gauss’s law 2. Choose Gaussian surface so that E can be taken out of integration: explore the symmetry of E Symmetry of EGaussian Surface Spherical.

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