There are two long co-axial solenoids of same length l.

Question:

There are two long co-axial solenoids of same length $l$. The inner and outer coils have radii $r_{1}$ and $r_{2}$ and number of turns per unit length $\mathrm{n}_{1}$ and $\mathrm{n}_{2}$, respectively. The ratio of mutual inductance to the self-inductance of the inner-coil is:

  1. (1) $\frac{n_{1}}{n_{2}}$

  2. (2) $\frac{n_{2}}{n_{1}} \cdot \frac{r_{1}}{r_{2}}$

  3. (3) $\frac{n_{2}}{n_{1}} \cdot \frac{r_{2}^{2}}{r_{1}^{2}}$

  4. (4) $\frac{n_{2}}{n_{1}}$


Correct Option: , 4

Solution:

(4) The rate of mutual inductance is given by

$\mathrm{M}=\mu_{0} \mathrm{n}_{1} \mathrm{n}_{2} \pi \mathrm{r}_{1}^{2}$             ...(1)

The rate of self inductance is given by

$\mathrm{L}=\mu_{0} \mathrm{n}_{1}^{2} \pi \mathrm{r}_{1}^{2} \ldots$ (ii)

Dividing (i) by (ii)

$\Rightarrow \frac{\mathrm{M}}{\mathrm{L}}=\frac{\mathrm{n}_{2}}{\mathrm{n}_{1}}$

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