A solenoid of length $L$ has $N$ turns and carries a current $I$. What is the magnetic field inside the solenoid?

Solution

1. The magnetic field inside a solenoid is $B={\mu }_{0}nI$, where $n$ is the number of turns per unit length.
2. The number of turns per unit length is $n=N/L$.
3. Substituting $n$ into the formula gives $B={\mu }_{0}NI/L$.
4. Hence the answer is (A).