[填空题]
A pair of vertical conducting rails are placed in a uniform magnetic field of $0.40\,T$ directed out of the plane of the paper, as shown. A $30\,Ω$ resistor is placed on a $0.2\,m$ long conducting rod that can slide vertically downwards on the rails under the effect of gravity.
1.State and explain the direction of the current passing through the ammeter when the rod moves downwards.
2.The rod is released from rest and moves downwards along the rails. Explain how the induced current in the rod changes over time.
3.(1)At a given instant, the rod's velocity reaches $0.30\,ms^{−1}$. Calculate the magnitude of the induced emf in the loop formed by the rod and the rails.
(2)Calculate the induced current in the loop at this point in time. $I_{ind}$ = $\times10^{-3}$
