Magnetism And Matter

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Common Misconceptions

3 min read

- Let's bust some of the common misconceptions

What is pole strength of a bar magnet? Is it the same as the dipole moment?

We know that a bar magnet when placed in a magnetic field

B

experiences a torque given by

τ = M \times B

where M is the magnetic dipole moment

The moment of a couple with arm length d is given by

τ = d \times F

Equating the above expressions we can see that,

F d s i n θ = M B s i n θ

F = \frac{M}{d} B

Let us say,

pole strength,

q_{m} = \frac{M}{d}

On comparing the above expression to the force on a charge due to electric field E,

q E

, we can say,

with analogy from electrostatics,pole strength is the equivalent of electric charge. Pole strength is NOT the same as dipole moment.

Where are magnetic poles situated in a bar magnet?

The effective magnetic poles of a bar magnet are not at the edges of a bar magnet as you might assume. The poles are located somewhat inside the edges and the same can be verified experimentally. The length of the bar magnet is called geometrical length and the length between two magnetic poles in a bar magnet is

called magnetic length.

Magnetic length is always smaller than geometrical length with a ratio of,

\frac{5}{6}

What happens when you break a magnet?
You must've learnt in smaller grades that, poles exist in pairs, and each broken piece of magnet has pair of North and South Poles. But what happens to the pole strength and the dipole moment associated with a bar magnet when it is broken to pieces. Let us demystify how the pole strength and the dipole moment changes when we break a bar magnet.