Question

Assume that each iron atom has a permanent magnetic moment equal to 'a' A-$m^2$. Density of atoms in iron is n atoms/$m^3$. Then maximum magnetic field B on the axis inside long cylinder of iron can be

• A. ${\mu }_{0}na$
• B. $\frac{{\mu }_{0}a}{n}$
• C. $\frac{{\mu }_{0}a}{n}$
• D. $\frac{{\mu }_{0}na}{2}$

Answer 1

The correct option is A ${\mu }_{0}na$

We get maximum Field when all ripple get aligned. At this point the solid cylinder acts as a solenoid. So the magnetic field inside a solenoid on its axis is given by,
$B={\mu }_{0}ni$, where $ni$ is the net current flowing through it per unit length.
Here, $ni=na$. Hence,
$B={\mu }_{0}na$