Two large parallel metal plates are oriented horizontally and separated by a distance 3$d$ . A grounded conducting wire joins them, and initially each plate carries no charge. Now a third identical plate carrying charge $Q$ is inserted between the two plates, parallel to them and located a distance $d$ from the upper plate, as in Figure $\mathrm{P} 26.57$ (a) What induced charge appears on each of the two original plates? (b) What potential difference appears between the middle plate and each of the other plates? Each plate has area $A .$