關於電場的問題 (TO天同)

I suppose the plot is along the line joining the centre of the charged sphere and the longitudinal axis of the earthed conductor.

The earthed condctor, in fact, acts as a shield to the electric field produced by the charged sphere. As such, the field lines coming out from the charged sphere in the direction of the conductor end at the induced -ve charge on the left hand side of the conductor. The is no field lines after the conductor. This leads to a zero potential througout.

In case the conductor were NOT earthed, then electric field lines would resume after the conductor. These field lines are produced by the induced +ve charge at the far end of the conductor.