When a copper rod is moved in magnetic field, an emf is induced in such a way that current induced will try to oppose the motion of the rod. As we know, e∝
dϕ
dt
⇒iR∝
dϕ
dt
(∵e=iR) ⇒
dQ
dt
×R∝
dϕ
dt
(∵i=
dQ
dt
) ⇒
dQ
dt
∝
dϕ
dt
Hence, charge developed across the ends of the rod will be proportional to rate of change of magnetic flux.