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Lesson Notes:

MAGNETIC FORCE :

(a) Force on a Current-Carrying Conductor in a Magnetic Field :
Immediately after Oersted's discovery of electric currents producing magnetic fields and exerting
forces on magnets, Ampere suggested that magnet must also exert equal and opposite force on a

current-carrying conductor. When a current carrying conductor is kept in a magnetic field (not
parallel to it), a force acts on it. This force is created due to the interaction of magnetic field of the

current in the conductor and the external magnetic field on the conductor. As a result of this
superposition, the resultant magnetic field on one side of conductor is weaker than on the other

side. Hence the conductor experiences a resultant force in one direction.
Take a small aluminium rod AB. Suspend it horizontally by means of two connecting wires from a
stand. Now, place a strong horseshoe magnet in such a way that the rod is between the two poles

with the field directed upwards. If a current is now passed in the rod from B to A, we will observe
that the rod gets displaced. This displacement is caused by the force acting on the current-carrying

rod. The magnet exerts a force on the rod directed towards the right, with the result the rod will get
deflected to the right. If we reverse the current or interchange the poles of the magnet, the

deflection of the rod will reverse, indicating thereby that the direction of the force acting on it gets
reversed. This shows that there is a relationship among the directions of the current, the field and

the motion of the conductor.

(a)The direction of force is obtained by the Fleming's left hand rule.

Fleming's left hand rule :