Let, v1 and v2 be the incoming and outgoing velocities of electron into the capacitor and out of the capacitor, respectively. Since, electric field is along X-axis, hence electric force on electron along Y-axis, (Fy)=0 ∴ Change in momentum along Y-axis, ∆py=0 i.e. p1=p2 (along Y-axis) ⇒m1v1cosα=m2v2cosβ ⇒v1∕v2=cosβ∕cosα ∵ Kinetic energy (K)=1∕2mv2 If mass is same, K∝v2 ∴