Suppose you want to produce a strong magnetic field that is constant in magnitude and direction in a region of space. Which one of the following would you choose to produce this field?

a. The region surrounding a long, straight, current-carrying wire.
b. The region surrounding a loop of a current-carrying wire.
c. The exterior region of a long current-carrying solenoid.
d. The interior region of a long current-carrying solenoid.

d. The interior region of a long current-carrying solenoid.

Explanation:

The magnetic field produced by a straight wire is uniform but the magnetic field lines are not dense to strengthen it.

In case of a current carrying loop of wire the magnetic field always remains in a plane perpendicular to the tangential direction of current so the direction of magnetic field changes with space and hence it is not uniform.

Similar is the case for the magnetic field lines to the exterior of a solenoid for which the magnetic field lines vary with position in space and become rare as we move away from the pole ends.

Explanation:

to create a strong magnetic field in a region, we use a solenoid as the magnetic field inside the interior of the solenoid is given by

B = μo ni

where, n be the number of turns per unit length and i be the current.