Lorentz force and linear motors

[rishi kesh]

I have a picture below please have a look at it. The guy is using two conducting rods placed parallel to each other and placed below it are magnets. When battey is connected to rods and a conductor is placed over them it starts to move.That works according to flemmings left hand rule. I was wondering why does it not work with permanent magnets? Why do we need electric current only?

 

[sophiecentaur]

There is no reason why motion can't be produced, using two permanent magnets. But, once the magnets have reached the lowest possible potential energy condition (i.e. as close together or as far apart as they can get), there is no more net motion in the wanted direction. An electric motor, using some arrangement of reversing the electric current, can maintain motion in one direction for ever.