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?

Answer 1

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.