Imagine two parallel infinite sheets, carrying uniform surface charge +σ (on the sheet at z=d) and −σ (at z=0). They are moving in the y direction at constant speed v (as in Problem 5.17).
(a) What is the electromagnetic momentum in a region of area A?
(b) Now suppose the top sheet moves slowly down (speed u) until it reaches the bottom sheet, so the fields disappear. By calculating the total force on the charge (q=σA), show that the impulse delivered to the sheet is equal to the momentum originally stored in the fields. [Hint: As the upper plate passes by, the magnetic field drops to zero, inducing an electric field that delivers an impulse to the lower plate.]