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A parallel-plate capacitor has charge q and plate area A. (a) By finding the work needed to increase the plate separation from x to x + dx, determine the force between the plates. (Hint: See Eq. 8-22.) (b) Then show that the force per unit area (the electrostatic stress) acting on either plate is equal to the energy density ⌃0E2/2 between the plates.
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