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