Question

Self inductance of a long solenoid is directly proportional to-
(Where $L$ is the length of solenoid)

• A. $L$
• B. $L^2$
• C. $1/L$
• D. $1/L^2$

Answer 1

The correct option is C $1/L$

$N$=total number of turns in solenoid

$l$=length of solenoid

$R$=radius of cross section of solenoid

Magnetic field inside solenoid =$B={\mu }_o\frac{N}{l}i$ where, $\frac{N}{l}$=number of turns per unit length

Now, emf induced= $E=NBA$

$⟹E=N\times {\mu }_o\frac{N}{l}i\times \pi R^2$

$⟹E=\frac{{\mu }_o{N}^2\pi R^2}{l}i=Li$

where $L$=self inductance

Thus, $L=\frac{{\mu }_o{N}^2\pi R^2}{l}$

Hence, $L\propto \frac{1}{l}$

Answer-(C)