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## Homework Statement

The problem is basically about tow infinite parallel wires separated by a distance ##d## with equally strong but opposite currents. You have to calculate the B-field outside the wires (not the field in between them).

## Homework Equations

Ampères law:

##\oint \mathbf B \cdot d\mathbf l = \mu_0 I_{enc}##

B-field outside one infinite wire with current I:

##B=\frac{\mu_0 I}{2\pi s}##

where ##s## is the distance from the wire.

## The Attempt at a Solution

Using the second formula on each wire and adding the resulting fields, we get the right answer, which obviously is bigger than zero. If we instead use Ampère's law, where we enclose both wires by an amperian circular loop, we get that the enclosed current is zero, since they run in opposite directions, which in turn makes the B-field equal to zero, which is obviously the wrong answer. In what way am I using Ampères law wrongly?