A long straight wire carries current out of the page. Three loops are shown. If the integral in each case is carried out by going counter-clockwise around the loop, rank the loops by the value of
ò B · dl , from largest to smallest.

1. 1>2>3
2. 1=2=3
3. 3>2>1
4. 1>2 but we can't say for sure where 3 fits in
5. 1=2 but we can't say for sure where 3 fits in
6. 2>1 but we can't say for sure where 3 fits in

Ampere's Law says the ò B · dl is proportional to the current passing through the loop. Each loop encloses the same current, so 1=2=3.