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On the axis of a circular current loop of radius R, at a distance x ≫ R, the magnetic field can be written in terms of the moment m as:
A(μ₀/4π)(2m / x²)
B(μ₀/4π)(m / x³)
C(μ₀/4π)(2m / x³)
D(μ₀/2π)(m / x³)
Answer & Solution
Correct answer: C. (μ₀/4π)(2m / x³)
1. Axial field: $B = \dfrac{\mu_0 I R^2}{2(x^2+R^2)^{3/2}}$.
2. For $x \gg R$, drop $R^2$: $B \approx \dfrac{\mu_0 I R^2}{2x^3}$.
3. Using $m = I\pi R^2$, write $B = \dfrac{\mu_0 m}{2\pi x^3}$.
4. Rearranged: $B = \dfrac{\mu_0}{4\pi}\dfrac{2m}{x^3}$, the axial dipole form.
_Source: NCERT Class 12 Physics Ch 4 "Moving Charges and Magnetism", p.155_
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