Question
Thu December 06, 2012 By: Meghana
 

what is the calcution we do to find the magnetic moment of a bar

Expert Reply
Mon December 10, 2012
The magnetic moment of a magnet is a quantity that determines the force that the magnet can exert on electric currents and the torque that a magnetic field will exert on it.
 
Each pole is the source of magnetic force which weakens with distance. Since magnetic poles always come in pairs, their forces partially cancel each other because while one pole pulls, the other repels. This cancellation is greatest when the poles are close to each other i.e. when the bar magnet is short. The magnetic force produced by a bar magnet, at a given point in space, therefore depends on two factors: on both the strength p of its poles, and on the vector ? separating them. The moment is defined as:
 
The pole strength of a bar magnet may be determined by measuring the force F exerted on one pole of the magnet by an external magnetic field B0. The pole strength is then defined by
p = F/B0
Note the similarity between this equation and q = F/E for electric charges.
In Experiment 3 we learned that the magnitude of the magnetic field, B, due to a single magnetic pole varies as the inverse square of the distance from the pole.
2rpkB?=
in which k' is defined to be 10-7 N/A2.
Consider a bar magnet with poles a distance 2x apart. Consider also a point P, located a distance r from the center of the magnet, along a straight line which passes from the center of the magnet through the North pole. Assume that r is much larger than x. The resultant magnetic field Bm at P due to the magnet is the vector sum of a field BN directed away from the North pole, and a field BS directed toward the South pole. The distances from P to the North and South poles are r - x and r + x, respectively. The magnitude of the resultant field at P is
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