Question

A current $I$ is passed through a Solenoid of $n$ turns. What is the magnetic filed outside the solenoid ?

• A. ${\mu }_{0}nI$
• B. ${\mu }_{0}I$
• C. ${\mu }_0{n}^{2}I$
• D. $0$

Answer 1

Answer: (D)

$0$

Magnetic field inside a solenoid has additive contributions from current elements both going into and coming out of the sectional plane. Thus the net magnetic field inside the solenoid becomes

$B_{net}={\mu }_{0}nI$

However for outer fields you can observe that the fields produced by both current elements oppose one another. So, the net field comes down outside a solenoid.