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gauss' law 2

# gauss' law 2 - Carl Friedrich Gauss 1777-1855 Last Time...

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1 1 Gauss’ Law – II Carl Friedrich Gauss 1777-1855 This lecture: HRW 23.6-23.9 For next time: HRW 24.1-24.7 I have changed my office hour on Friday to 4-5. 2 Last Time Electric Flux Amount of electric field crossing a surface Gauss’ Law … and applications Today More applications of Gauss’ Law: Conductors and insulators in different geometries r r enclosed 0 Closed Surface Q Φ = E dA = ε nothing moves 3 Field lines can only end at infinity or on negative charges, so the region must have negative charges. An electric field in some region has a constant direction, but decreases in strength in that direction. What can be concluded about the charge in the region? (Hint: sketch the field lines). Interactive Lecture Question #5.5 Challenge 4 Point charge +Q is located at the center of one of the faces of a cube, in the plane of the face, as shown. Cube of side L The flux thru both cubes = Q/ ε 0. Gauss’ Law: Example What is the total flux through the cube? Imagine placing an identical cube on top, as shown. Interactive Lecture Question #6.1 0 Q 2 ε By symmetry, the flux thru the original cube is half the total flux, What if the charge was just inside the cube? What if the charge was just above the cube? 0 Q ε 0

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2 5 Electric Fields Inside Isolated Conductors In a conductor, there are many electrons that are free to move. In electrostatic equilibrium, there is no electric field inside a conductor. ¼ E When an external electric field is applied, the electrons move opposite it. As they do, they build up on the surface and generate an opposing field.
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gauss' law 2 - Carl Friedrich Gauss 1777-1855 Last Time...

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