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Q. A long solenoid with $1000 \,turns / m$ has a core material with relative permeability $500$ and volume $10^{3} cm ^{3}$. If the core material is replaced by another material having relative permeability of $750$ with same volume maintaining same current of $0.75 \,A$ in the solenoid, the fractional change in the magnetic moment of the core would be approximately $\left(\frac{x}{499}\right)$. Find the value of $x$.

JEE MainJEE Main 2021Moving Charges and Magnetism

Solution:

$\frac{\Delta M }{ M }=\frac{\Delta \mu}{\mu}=\frac{250}{500}=\frac{1}{2}$
$\frac{1}{2}=\frac{ x }{499} \Rightarrow x \simeq 250$