Imagine two parallel infinite sheets, carrying uniform surface charge +σ+σ (on the sheet z=d) and −σ−σ (at z=0). They are moving in the y direction at constant speed v. (a) What is the electromagnetic momentum in the region of area A? (b) Now suppose the top sheet moves slowly down (speed u) until it reaches the bottom sheet, so the fields dissapear. By calculating the (magnetic) force on the charge (q=σAq=σA), show that the impulse delivered to the sheet is equal to the momentum originally stored in the fields.