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 = $- \frac{\partial U}{\partial x}$
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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