The four fingers of the right hand point to the V direction, hold it in the B direction, and the thumb points to the VXB direction. This is the right hand spiral rule. You can't find it without direction B.

Methods and steps of solving dynamic electromotive force

1. Take out a line element dL from the moving conductor and mark the direction of its moving direction dL and B..

2. The right-hand spiral rule determines the direction of VXB and the electromotive force D ε = (VXB) XdL in DL.

3. Unify the integral variables, determine the upper and lower limits of the integral, and calculate the total electromotive force.