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An otherwise infinite, straight wire has two concentric loops of radii a and b carrying equal currents in opposite directions as shown in Fig. The magnetic field at the common centre is zero for A. `a/b=(pi-1)/pi`B. `a/b=pi/(pi+1)`C. `a/b=(pi-1)/(pi+1)`D. `a/b=(pi+1)/(pi-1)` |
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Answer» Correct Answer - B (b) `B_(centre)=0` `(mu_0I)/(4pib)odot+(mu_0I)/(4pib)odot +(mu_0I)/(2b)odot +(mu_0I)/(2a)ox=0` `implies (mu_0I)/(2pib)+(mu_0I)/(2b)-(mu_0I)/(2a)=0 implies 1/(2pib)+1/(2b)=1/(2a)` `implies a/b=pi/(pi+1)` |
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