Ampere's Law allows us to easily calculate magnetic fields in highly symmetric situations, much as Gauss' Law allowed us to determine electric fields.

Ampere's Law states that the line integral of B • ds around a closed (i.e., complete) loop is proportional to the current passing through the loop:

Around a closed loop ∫ B • ds = μ_o I_enc

Question: Rank the three closed loops above according to the magnitude of the net current enclosed, from largest to smallest. Four current-carrying wires are present. The three in red carry currents of I, 2I, and 3I out of the page; the one in blue carries a current of 3I into the page.

1. Loop 3 > Loop 1 > Loop 2
2. Loop 3 = Loop 1 > Loop 2
3. Loop 3 > Loop 2 > Loop 1
4. Loop 2 > Loop 3 > Loop 1
5. none of the above

The answer is that Loop 3 has 6I passing through it while Loop 1 and Loop 2 each have 3I (the 3I into the page cancels the 3I out of the page for Loop 2). So:
Loop 3 > Loop 1 = Loop 2