Question

The S.I. unit of pole strength is?
A.)Am2
B.)Am
C.)Am-1
D.)Am-2

Answer 1

Hint: Consider pole quantity as two poles. Here pole strength is defined by considering two poles. Strength is nothing but force only which is acting between two poles. Rearrange the formula of magnetic dipole moment in terms of pole strength.

Complete step by step answer:
Units used to measure different quantities vary place to place. So to have a uniformity in units a system called ‘The International System of Units’ (abbreviated as SI) is used.
There are basic seven units defines in SI system:
Length - meter (m)
Time - second (s)
Amount of substance - mole (mole)
Electric current - ampere (A)
Temperature - kelvin (K)
Luminous intensity - candela (cd)
Mass - kilogram (kg)

The amount of strength by which a magnetic pole attracts other magnetic materials is called pole strength. It describes how strong our magnet is. It is denoted by m. The pole strength of North Pole is represented by +m and that of South Pole is represented by –m’
The magnetic dipole moment is defined in terms of product of pole strength and distance between two poles.

$\vec{M}=m\times2\vec{l}$
Therefore,

$m=\dfrac{M}{2l}$
Expressing all quantities in SI system
$unit\:of\:m=\dfrac{Am^2}{m}$
$unit\:of\:m=Am$
Answer- (b)

Additional Information:

We can find the pole strength of the magnet by moving the magnet around the coil. It is given by
$m=\dfrac{W}{I}=\dfrac{\int F.dl}{I}$
Where W is work done in moving magnet around the coil
     I is current set up in the a loop
     F is force experienced by magnet
If all quantities are measured in SI units i.e. wok done in Joule and current in Ampere then
$unit\:of\:m=\dfrac{Joule}{Ampere}=Weber$

Note: Pole strength is just a quantity. It may vary as fields get varied. Ex. Pole strength of magnet, pole strength in electric field. It is not necessary to memorize units. It is sufficient to know formula or definition from that you can deduce units and dimensions. But you must memorize the basic seven units which have been listed above.