Coulomb?s inverse square law :
Coulomb?s inverse square law states that the force of attraction or repulsion between the two magnetic poles is directly proportional to the product of their pole strengths and inversely proportional to the square of the distance between them.

**Force between two magnetic poles**

In 1785, Coulomb made use of his
torsion balance and discovered the law governing the force between the two
magnetic poles.

*Coulomb?s inverse square law*

Coulomb?s inverse square law
states that the *force of attraction or*
*repulsion between the two magnetic poles
is directly proportional to the product of their pole strengths and inversely
proportional to the square of the distance between them.*

If *m _{1}* and

F α m_{1}m_{2} and F α 1/d^{2}

F α m_{1}m_{2}/ d^{2}

F = k m_{1}m_{2}/ d^{2}

where k is the constant of
proportionality and k = ?/ 4 π where ? is the permeability of the medium.

But ? = ?_{o} ? ?_{r}

?_{r} = ? / ?_{0}

where ?* _{r}* - relative permeability of the
medium

?_{o} - permeability of free space or vacuum.

Let *m*_{1} = *m*_{2}
= 1

and d = 1 m

k= ?_{0} / 4 π

In free space, ?^{o} = 4π ?
10^{-7} H m^{-1}

F = 10^{-}^{7} ? *m* ? *m /d ^{2}*

F = 10^{-7} N

Therefore, *unit pole is defined as that pole which when
placed at a* *distance of 1 metre in
free space or air from an equal and similar pole, repels it with a force of 10 ^{-7}
N.*

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