Using Right Hand Rule
Motion is up. Point thumb up in direction of motion indicating the way positive charges are moving.
Point Fingers from Left to Right. Field from N to S.
Palm is pointing into the page, so positives experience a force to End Y. So negatives experience a force towards end X.
As only electrons can move, they will move towards end X.
As there is no external circuit they will accumulate at end X making it negative.
This can be confirmed with Lenz's Law.
Thumb points in direction of positive charges, into Y.
Fingers point from N to S (Left to Right)
Palm points down confirming that the Force produced is opposing the motion.