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AIIMS 2017
Exam
AIIMS
Year
2017
Subject
Physics
Chapter
Current Electricity

AIIMS 2017 · Physics · Current Electricity

A and B are two points on a uniform ring of resistance R . The A C...

AA and BB are two points on a uniform ring of resistance RR. The ∠ACB=θ\angle A C B=\theta, where CC is the centre of the ring. The equivalent resistance between AA and BB is

Options

  1. ARθ(2π−θ)4π2\frac{R \theta(2 \pi-\theta)}{4 \pi^2}
  2. BR(1−θ2π)R\left(1-\frac{\theta}{2 \pi}\right)
  3. CRθ2π\frac{R \theta}{2 \pi}
  4. DR(2π−θ)4π\frac{R(2 \pi-\theta)}{4 \pi}

Correct answer

A. Rθ(2π−θ)4π2\frac{R \theta(2 \pi-\theta)}{4 \pi^2}

Step-by-step solution

Resistance per unit length =ρ=R2πr=\rho=\frac{R}{2 \pi r} Lengths of sections A P BandAQBand A Q B are rθr \theta and r(2π−θ)r(2 \pi-\theta) Resistances of sections A P BandAQBand A Q B are R1=ρrθ=R2πrrθ=Rθ2π and R2=R2πrr(2π−θ)=R(2π−θ)2π\begin{aligned} & R_1=\rho r \theta=\frac{R}{2 \pi r} r \theta=\frac{R \theta}{2 \pi} \\ & \text{ and } R_2=\frac{R}{2 \pi r} r(2 \pi-\theta)=\frac{R(2 \pi-\theta)}{2 \pi}\end{aligned} As R1andR2R_1 and R_2 are in parallel between A and B,theirequivalentresistanceis, their equivalent resistance isReq=R1R2R1+R2=Rθ2π⋅R(2π−θ)2πRθ2π+R(2π−θ)2π=R2θ(2π−θ)4π2R2π[θ+2π−θ]=R(2π−θ)θ4π2\begin{aligned} & R_{\mathrm{eq}}=\frac{R_1 R_2}{R_1+R_2} \\ & =\frac{\frac{R \theta}{2 \pi} \cdot \frac{R(2 \pi-\theta)}{2 \pi}}{\frac{R \theta}{2 \pi}+\frac{R(2 \pi-\theta)}{2 \pi}}=\frac{\frac{R^2 \theta(2 \pi-\theta)}{4 \pi^2}}{\frac{R}{2 \pi}[\theta+2 \pi-\theta]}=\frac{R(2 \pi-\theta) \theta}{4 \pi^2} \\ & \end{aligned}$$
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