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Unformatted text preview: 21. (a) If the currents are parallel, the two ﬁelds are in opposite directions in the region between the wires.
Since the currents are the same, the total ﬁeld is zero along the line that runs halfway between the
wires. There is no possible current for which the ﬁeld does not vanish.
(b) If the currents are antiparallel, the ﬁelds are in the same direction in the region between the wires.
At a point halfway between they have the same magnitude, µ0 i/2πr. Thus the total ﬁeld at the
midpoint has magnitude B = µ0 i/πr and
π (0.040 m)(300 × 10−6 T)
= 30 A .
4π × 10−7 T · m/A ...
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