Question

The unit of pole strength is
A.) $A{{m}^{2}}$
B.) $A{{m}^{-1}}$
C.) $Am$
D.) $A{{m}^{-2}}$

Answer 1

Hint: The pole strength of a magnet is the amount of magnetic force experienced by a unit magnetic pole at a certain distance from the magnet. It is the magnetic dipole moment of a magnet per unit length of the magnet. It is a measure of how strong the magnet is. A magnet with greater pole strength is a stronger magnet.

Formula used:
$\text{Magnetic pole strength (of a bar magnet) = }\dfrac{\text{Magnetic dipole moment (of the bar magnet)}}{\text{Length (of the bar magnet)}}$

Complete step by step answer:
The pole strength of a magnet is a measure of the magnetic strength of the magnet. Greater the magnetic pole strength, greater is the magnetic strength of the magnet. Mathematically, the pole strength of a bar magnet is the dipole moment per unit length of the bar magnet. Hence,

$\text{Magnetic pole strength (of a bar magnet) = }\dfrac{\text{Magnetic dipole moment (of the bar magnet)}}{\text{Length (of the bar magnet)}}$ --(1)

From equation (1), we can find the SI unit of the magnetic pole strength.

The SI unit of magnetic dipole moment is Ampere – metre squared $\left( A{{m}^{2}} \right)$ and the SI unit of length is metre $\left( m \right)$.

Therefore, putting these units in (1), we get the SI unit of magnetic pole strength as
$\dfrac{A{{m}^{2}}}{m}=Am$

Hence, the SI unit of magnetic pole strength is Ampere – metre $\left( Am \right)$.

Therefore, the correct option is C) $Am$.

Note: Students must remember that magnetic monopoles do not exist in reality and hence there is nothing called a single monopole which can have a magnetic pole strength. A body always has two opposite poles with two opposite pole strengths of equal magnitude but opposite signs. A bar magnet also has a north pole and a south pole. It is impossible to isolate the north pole of a magnet. In fact, this theoretical concept of a magnetic monopole and its pole strength was formulated to draw up an analogy between electric fields and electric charges.
Students also should not get confused between magnetic pole strength and magnetic dipole moment. Magnetic dipole moment for a bar magnet is the product of the magnetic pole strength of the bar magnet and its length.