All Problems Electric Capacitance, Energy of an Electric Field
Problem 3.142
Inside a parallel-plate capacitor there is a plate parallel to the outer plates, whose thickness is equal to \(\eta=0.60\) of the gap width. When the plate is absent the capacitor capacitance equals \(c=20 \mathrm{nF}\). First, the capacitor was connected in parallel to a constant voltage source producing \(V=200 \mathrm{~V},\) then it was disconnected from it, after which the plate was slowly removed from the gap. Find the work performed during the removal, if the plate is (a) made of metal; (b) made of glass.
3.142. (a) \(A=1 / 2 C V^{2} \eta /(1-\eta)^{2}=1.5 \quad \mathrm{~mJ}\) (b) \(A=1 / 2 C V^{2} \eta \varepsilon(\varepsilon-1) /[\varepsilon-\eta(\varepsilon-1)]^{2}=0.8 \mathrm{~mJ}\)