A parallel plate condenser with a dielectric constant K between the plates has a capacity C and is charged to a potential V volt. The dielectric slab is slowly removed from between the plates and then reinserted. The net work done by the system in this process is

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(1) zero. The slab is moved out and then brought back, so the capacitor ends in exactly the state it started in: the same charge, the same capacitance C and the same potential V. Energy is a function of state, so Δ U=U_final-Uᵢₙᵢₜᵢₐₗ=0. The process is carried out slowly, so there is no kinetic energy left over and no dissipation. Hence the net work done by the system over the complete cycle is zero: whatever work is done against the attractive pull while pulling the slab out is recovered when it is drawn back in.

A parallel plate condenser with a dielectric constant K between the plates has a capacity C and is charged to a potential V volt. The dielectric slab is slowly removed from between the plates and then reinserted. The net work done by the system in this process is

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