A long solenoid carries a steady current. An Amperian loop is chosen completely outside the solenoid. What is the magnetic field along this loop according to the ideal solenoid model?
Zero
In an ideal (infinitely long, tightly wound) solenoid, the magnetic field inside is uniform, \(B = \mu_0 n I\), directed along the axis.
For an Amperian loop lying entirely outside the solenoid, no current passes through the loop, so \(\oint \vec{B} \cdot d\vec{l} = \mu_0 I_{enc} = 0\). The field lines that leave one end return over an effectively infinite region, so the external field is taken as zero.
The other options fail: the field is maximum and uniform only inside the solenoid; outside it is neither equal to the inside value nor a uniform non-zero field in the ideal model.
Hence, the answer is Zero.
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