When a piece of a magnetic substance is placed in a uniform magnetic field,the flux density inside it is four times the flux density away from the piece. The magnetic permeability of the material is

$A$ solenoid has a core of material with relative permeability $501$. The windings of the solenoid are insulated from the core and carry a current of $2.5 \ A$. If the number of turns is $900$ per metre,the magnetization in $A \ m^{-1}$ is:

$A$ domain in ferromagnetic iron is in the form of a cube of side length $1\; \mu m$. Estimate the number of iron atoms in the domain and the maximum possible dipole moment and magnetisation of the domain. The molecular mass of iron is $55\; g/mol$ and its density is $7.9\; g/cm^3$. Assume that each iron atom has a dipole moment of $9.27 \times 10^{-24}\; A m^2$.

Susceptibility of $Mg$ at $300\, K$ is $1.2 \times 10^{-5}$. The temperature at which susceptibility will be $1.8 \times 10^{-5}$ is......$K$.

The magnetic susceptibility of a paramagnetic material is $99$. The permeability of the material in $Wb / A-m$ is: [Permeability of free space $\mu_{0} = 4 \pi \times 10^{-7} \ Wb / A-m$]

$A$ domain in a ferromagnetic substance is in the form of a cube of side length $2 \mu m$. It contains $4 \times 10^{14}$ atoms and each atomic dipole has a moment of $16 \times 10^{-24} \text{ A m}^2$. The magnetization of the domain is . . . . . . .