Q.
The figure shows a circular loop made of a wire of radius R. The resistivity of the material varies as a function of θ such that ρ=ρ0sin2θ. The positions of the Jockey such that the magnetic field at the centre (O) due to the current in the loop is zero, will be
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a
θ=π2
b
θ=π
c
θ=3π2
d
θ=π4
answer is A.
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Detailed Solution
Consider two sectors one of α and other of (2π−α)B at O will be zero if I1α=I2(2π−α) ⇒I1=ER1 ,⇒ I2=ER2 So αR1=2π−αR2 ⇒R1=∫0αρ0sin2θA Rd θ =ρ0RAα-sin2α2∵∫sin2θdθ=12x-sinθcosθ+CSimilarly we can get R2=∫α2πρ0sin2θA Rd θ =ρ0RA2π-α+sin2α2 ∴αR2=2π-αR1⇒αρ0RA2π-α+sin2α2=2π-αρ0RAα-sin2α2 ⇒sin2α=0Onsolvingweget α=π2,π,3π2
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