Download Physics II Recall electric field,  • Gauss’ law/theorem

Physics II
•Recall electric field, Gauss’ law/theorem
•consequences
•Potential and potential energy
•consequences
Physics II
• q1 = q4 = Q; q2 = q3 = q
• what is Q/q so that the force on 1 is zero
– All charges same sign. There is no solution.
– Q and q different sign. There is a possible solution.
ƒ Is the answer the same if the force on 3 is to be zero
Physics II
• What is the force on the central chlorine ion?
• Suppose that we put in an electron that sits on a cesium site and neutralizes it, which way does the Chlorine move?
Physics II
• What is the force on the central chlorine ion?
• Suppose that we put in an electron that sits on a cesium site and neutralizes it, which way does the Chlorine move?
Physics II
• Gaussian (imaginary) surfaces
• Flux = Φ = ∑ E.dA
• E.dA for a cube is easy to visualize.
• Let calculus do it for a sphere or any other shape.
• Φc = qtotal /εo
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S1: Φc = q/ εo
S2: Φc = ‐q/ εo
S3: Φc = 0
S4: Φc = 0
•For a cylinder with axial field, no contribution to flux from the side walls.
•From the far top, >0
•Near (bottom) , <0
Physics II
• No force/field from the charge outside
• On inside circle E.4πr2=q/εo Coulomb’s law
• Very outside, E = k 5q/r2
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Physics II
• The electric field in a metal is zero
• Charge +q on inside surface.
• Because it is neutral, outside surface must be ‐q.
Physics II
• Electric field is radial.
• E.A = 0 for the top and bottom surfaces.
• Sideways:
E 2πrh = λh/ε0
E = λ/ 2πrε0
Physics II
• Plane
• 2EA = σA/εo
E = σ/2εo
Physics II
• E = σ/εo inside