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Mutual Inductance Between Concentric Tilted Square and Circular Loops

A circular wire loop having radius RR is located in the xx-yy plane with its center at origin OO. A small square loop of side aa (a≪Ra \ll R) consisting of two turns is centered along the axis of the circular loop at z=3Rz = \sqrt{3}R, as shown in the diagram. The plane containing the square loop forms an angle of 45∘45^\circ relative to the zz-axis. If the mutual inductance between these two loops is expressed as μ0a22p/2R\frac{\mu_0 a^2}{2^{p/2} R}, find the value of pp.

Question Diagram 1
Official Numerical Answer7

Step-by-Step Solution

To find the mutual inductance MM between the circular wire loop and the small square loop, we can pass a current II through the circular loop and calculate the magnetic flux linked with the square loop.

The magnetic field at a distance zz along the axis of a circular loop of radius RR carrying current II is given by: B=μ0IR22(R2+z2)3/2B = \frac{\mu_0 I R^2}{2(R^2 + z^2)^{3/2}}

Given z=3Rz = \sqrt{3}R, substituting this into the equation gives: B=μ0IR22(R2+3R2)3/2=μ0IR22(4R2)3/2=μ0IR22×8R3=μ0I16RB = \frac{\mu_0 I R^2}{2(R^2 + 3R^2)^{3/2}} = \frac{\mu_0 I R^2}{2(4R^2)^{3/2}} = \frac{\mu_0 I R^2}{2 \times 8R^3} = \frac{\mu_0 I}{16 R}

Since a≪Ra \ll R, the magnetic field B⃗\vec{B} can be considered uniform over the area of the small square loop.

The area of the square loop is A=a2A = a^2. The normal to the plane of the square loop makes an angle θ=45∘\theta = 45^\circ with the zz-axis (the direction of the magnetic field B⃗\vec{B}).

The magnetic flux Φ1\Phi_1 through a single turn of the square loop is: Φ1=B⃗⋅A⃗=BAcos⁡(45∘)\Phi_1 = \vec{B} \cdot \vec{A} = B A \cos(45^\circ) Φ1=(μ0I16R)a2(12)=μ0Ia2162R\Phi_1 = \left(\frac{\mu_0 I}{16 R}\right) a^2 \left(\frac{1}{\sqrt{2}}\right) = \frac{\mu_0 I a^2}{16 \sqrt{2} R}

Since the square loop consists of N=2N = 2 turns, the total magnetic flux Φtotal\Phi_{\text{total}} linked with the square loop is: Φtotal=NΦ1=2×μ0Ia2162R=μ0Ia282R\Phi_{\text{total}} = N \Phi_1 = 2 \times \frac{\mu_0 I a^2}{16 \sqrt{2} R} = \frac{\mu_0 I a^2}{8 \sqrt{2} R}

The mutual inductance MM is given by: M=ΦtotalI=μ0a282RM = \frac{\Phi_{\text{total}}}{I} = \frac{\mu_0 a^2}{8 \sqrt{2} R}

Rewriting the denominator in terms of powers of 2: 82=23×21/2=27/28 \sqrt{2} = 2^3 \times 2^{1/2} = 2^{7/2}

Therefore: M=μ0a227/2RM = \frac{\mu_0 a^2}{2^{7/2} R}

Comparing this expression with the given formula M=μ0a22p/2RM = \frac{\mu_0 a^2}{2^{p/2} R}, we get: p2=72  ⟹  p=7\frac{p}{2} = \frac{7}{2} \implies p = 7