Imagine two parallel infinite sheets, carrying uniform surface charge $+$$($on the sheet atz$=$d$)$ and$-$$($at z$=0).$ They are moving in they direction at constant speed v $($as in Problem $5\mathrm{.}17).$

$($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$=$A$),$ show that the impulse delivered to the sheet is equal to the momentum originally stored in the fields.