(a) When key K is closed, current flows through the rod AB. Since AB carries current and is placed in a magnetic field, it experiences a force and gets displaced (moves/deflates upward or in a specific direction) due to the motor effect — the interaction between the magnetic field and the current-carrying conductor.
(b) Fleming's Left-Hand Rule determines the direction of force on the rod.
Statement: Stretch the thumb, forefinger, and middle finger of the left hand mutually perpendicular to each other. If the forefinger points in the direction of the magnetic field and the middle finger points in the direction of current, then the thumb points in the direction of force (motion) on the conductor.
---