We know that a current carrying conductor has a magnetic field arround it.
Now if it is placed in a uniform magnetic field of some other magnet, then there will be two magnetic fields, one due to cuurent in the conductor and the other due to magnet. The interaction of these two magnetic fields will result in a net force that will act on conductor. It can be explained as follow.
Let us consider that a uniform magnetic field exists in certain region in upward direction. A current carrying conductor is placed in it at right angles to the direction of magnetic field such that the current in the wire is directed out of plane. It can be shown with a sign of dot.
The magnetic lines of force due to current in the conductor will be in the plane of paper and can be shown by concentric dotted circles.
From the above figure it can be seen that on the right hand side of conductor the magnetic field due to current in the conductor is in the same direction as that of field in which it is placed so they will support one another. Hence the magnetic field on right side will be strong.
On the left hand of conductor, the two magnetic fields (because of current carrying conductor and magnet) are in the opposite direction so they cancel the effect of each other. So on the left side the magnetic field will be weak.
Thus on one side of the conductor, net magnetic field strong and where as on the other side net magnetic field is weak. As a result a force on conductor towards left side where the field is weak.
The above fact can be demonstrated by a simple experiment.
A brass rod is placed on two parallel brass rails which are connected across a battery. The two rails lie in the resgion between pole pieces of U- shaped magnet.
The brass rod R, the two rails and battery make a closed circuit. When current passes through the rod, it rolls along the rails.
Fleming’s left-hand rule:
Whenever a current carrying conductor is kept inside the magnetic field, it experiences force. The direction of force on conductor relative to direction of current and applied magnetic field can be described by Fleming’s left-hand rule which is as follows.
If forefinger, middle finger and thumb are stretched perpendicular to each other such that forefinger points in the direction of applied magnetic field and middle finger points in the direction of electric current then thumb will point in the direction of force on a conductor.
The magnitude of force on a current carrying conductor in a uniform magnetic field is directly proportional to the amount of current inside the conductor, length of the conductor inside the magnetic field and strength of magnetic field. It also depends on the angle between direction of current and that of magnetic field.
If conductor is placed at right angles to the direction of magnetic fields, then magnitude of force will be maximum and if it is placed along the direction of field, then this force will be zero.