Solution

ID: past-exam-of-the-physics-b-course-of-the-university-of-cambridge/2021/paper-2/section-a/a5/solution

For charge sharing between distant connected spheres, use the standard approximation that the spheres are sufficiently far apart for mutual electrostatic effects to be negligible, and that the wire adds negligible capacitance. The capacitance of an isolated conducting sphere is . Thus
The initial charges are . Connecting the spheres allows charge to flow until their potentials are equal. Charge conservation gives
The initial and final electrostatic energies are and . Their difference becomes Joule heating in the resistive wire:
Numerically,
The resistance determines how fast this happens, not the final energy difference. Sufficient damping suppresses sustained charge-current oscillations; the calculation also neglects energy radiated away. At finite sphere separation the capacitance matrix would be needed, and the radii and initial potentials alone would not specify an exact correction.

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