The right hand rule to find the direction of the magnetic field for a falling bar is:
The magnetic force is given by the vector product of the velocity and the magnetic field.
F = q v x B
Where the bolds indicate vectors, F is the force, q the charge on the particle, v the velocity and B the magnetic field.
In the vector product, the vectors are perpendicular, which is why the right-hand rule has been established, see attached:
They indicate that the bar is dropped, therefore its speed is vertical and downwards, it moves to the left therefore this is the direction of the force, we use the right hand rule, the magnetic field must be horizontal, we have two possibilities:
In conclusion using the right hand rule we can find the direction of the magnetic field for a falling bar is:
Learn more about the right hand rule here: brainly.com/question/12847190