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1、Equilibrium requires that all forces are cancelled.,Problem 15.1,Assume one of these charges, say q, moves a small D,not restoring force. Unstable.,Earshaws Theorem: A charge particle cannot be held in a stable equilibrium by electrostatic forces alone.,Problem 15.3 Directly use the result of dipole
2、,Problem 15.3,NB,Define,Problem 15.6,Problem 15.9,An infinite sheet of dx at x contributes a field,Outside the sheet, E = 0,left to x,So the field due to other part than dA is,outward.,The contribution from dA (very close to the surface) is,at both sides.,Total field is,*Problem 15.12,dA,total,The f
3、orce on dq is then,Problem 15.15,(a),(c) In (a) VP(0, y) . For Ex we need V(x, y),(b),Problem 16.1,in series,before,after,copper,Problem 16.3,Problem 16.6,Known V,Problem 16.6 Gauss law reads,The potential difference is,The capacity in unit length is,Problem 16.4,A tiny bump on outside wall is equiv
4、alent a little larger R, so C will be smaller a little bit.,(a),Problem 16.9 In the case q = const. double separation means half the capacitance and double energy.,Problem 16.9,(b),(a),(b),Problem 16.4 A dielectric slab of thickness b and dielectric constant is placed between the plates of a parallel-plate capacitor. Known,Problem 16.10,(d),(e),or 87.5% and 12.5%,(c)In the gap,
