MCQ
A copper wire is wound on a wooden frame, whose shape is that of an equilateral triangle. If the linear dimension of each side of the frame is increased by a factor of $3$, keeping the number of turns of the coil per unit length of the frame the same, then self inductance of the coil
  • A
    decreases by a factor of $9$
  • B
    increases by a factor of $27$
  • increases by a factor of $3$
  • D
    decreases by a factor of $9\sqrt 3 $

Answer

Correct option: C.
increases by a factor of $3$
c
Total length $L$ will remain constant

$\mathrm{L}=(3 \mathrm{a}) \mathrm{N} \quad(\mathrm{N}=\text { total tums })$

And length of winding $=(d) N=\ell$

$(d=\text { diameter of wire })$

Self inductance $=\mu_{0} \mathrm{n}^{2} \mathrm{A} \ell$

$=\mu_{0} n^{2}\left(\frac{\sqrt{3} a^{2}}{4}\right) d N$

$\alpha a^{2} N \propto a$

So self inductance will become $3$ times.

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