At, x=0,(KE)1=0 and x=0.5m,(KE)2=0.12J From work-energy theorem, work done = change in kinetic energy ⇒ Force × displacement =ΔKE QE×d=KE2−KE1 ⇒ Q×300×0.5=0.12 or Q=
0.12
300×0.5
=800µC The charge Q is moving in the direction of electric field. Hence, it is positive.