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Question
When a dielectric slab is gradually inserted between the plates of an isolated parallel-plate capacitor, the energy of the system decreases. What can you conclude about the force on the slab exerted by the electric field?
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Solution
As the energy of the system decreases, the change in the energy is negative. Force is defined as a negative rate of change of energy with respect to distance.
`F = -(∂U)/(∂x)`
So, as the energy decreases, the force due to the electric field of the capacitor increases when the dielectric is dragged into the capacitor.
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