Fleming's left-hand rule gives the direction of the force on a current-carrying wire in a magnetic field. It works when the wire, the field and the force are all at right angles to each other.
Hold the thumb, first finger and second finger of your left hand at right angles to each other. The thumb points in the direction of the force (the motion). The first finger points in the direction of the magnetic field, from north to south. The second finger points in the direction of the current, from positive to negative.
If the current is reversed, the second finger points the other way, so the thumb points the opposite way and the force is reversed. The same happens if the field is reversed. If both are reversed together, the force stays the same.
For example, if the field points east and the current flows north, the force on the wire acts downwards.
