Two long ideal solenoids (with radii 20 mm and 30 mm respectively)have the same number of turns of wire per unit length.The smaller solenoid is mounted inside the larger, along a common axis.It is observed that there is zero magnetic field within the inner solenoid.The current in the inner solenoid must be:
A)two-thirds the current in the outer solenoid
B)one-third the current in the outer solenoid
C)twice the current in the outer solenoid
D)half the current in the outer solenoid
E)the same as the current in the outer solenoid

Question

Quizplus · Accepted Answer

The magnetic field inside an ideal solenoid is proportional to the current and the number of turns per unit length. For the net magnetic field inside the inner solenoid to be zero, the magnetic field due to the inner solenoid must cancel out the magnetic field due to the outer solenoid. Since they have the same number of turns per unit length, the currents must be equal in magnitude but opposite in direction.

Two Long Ideal Solenoids (With Radii 20 Mm and 30