36. If the number of turns per unit length of a coil of solenoid is doubled, the self-inductance of the solenoid will: (1991)
Solution:
Self-inductance of a solenoid is $L = \mu_0 n^2 A l$, where $n$ is the number of turns per unit length.
Thus, self-inductance is directly proportional to the square of turns per unit length ($L \propto n^2$).
When $n$ is doubled, $L' = (2n)^2 = 4L$, so it becomes four times.
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