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ExamsNEETPhysics

Two coaxial solenoids are made by winding thin insulated wire over a pipe of cross-sectional area $A=10\,\text{cm}^2$ and length $20\,\text{cm}$. If one solenoid has 300 turns and the other has 400 turns, their mutual inductance is $(\mu_0=4\pi\times10^{-7}\,\text{T m A}^{-1})$ :

  1. $4.8\pi\times10^{-4}\,\text{H}$
  2. $4.8\pi\times10^{-5}\,\text{H}$
  3. $2.4\pi\times10^{-4}\,\text{H}$
  4. $2.4\pi\times10^{-5}\,\text{H}$

Correct answer: $2.4\pi\times10^{-4}\,\text{H}$

Solution

For two long coaxial solenoids, $M=\mu_0\dfrac{N_1N_2A}{l}$. Here $A=10\,\text{cm}^2=10^{-3}\,\text{m}^2$ and $l=20\,\text{cm}=0.2\,\text{m}$. Substituting $N_1=300$, $N_2=400$ gives $M=4\pi\times10^{-7}\times\dfrac{300\times400\times10^{-3}}{0.2}=2.4\pi\times10^{-4}\,\text{H}$.

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