In a D.C. motor, when there is a current in the conductor a magnetic force arises on the conductor. If there is no load on the D.C. motor the magnetic force will move the conductor. With the motion of the conductor a changing magnetic field will create a back E.M.F. in the conductor in the direction that would cause a current in the opposite direction of the original current.
My question, then, is without a load on the D.C. motor would the magnetic force be directly proportional to the back E.M.F.?
Furthermore, if a load was introduced, The magnetic force would remain the same, given no change in current. But as a result of the more slowly changing magnetic field the back E.M.F. would decrease in magnitude. Given the smaller back E.M.F. and therefore smaller energy loss, since the original current has less opposing potential, Does this mean that a D.C. motor is more efficient if it runs more slowly?