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Q.
Two free parallel wires carrying currents in the opposite directions
J & K CETJ & K CET 2007
Solution:
When two infinitely long parallel conductors carrying current $I_{1}$ and $I_{2}$ are placed a distance $r$ apart, then force on the unit length of a conductor due to the other conductor is given by
$F=\frac{\mu_{0}}{4 \pi} \cdot \frac{2 I_{1} I_{2}}{r}$
The force is attractive, if current in two conductors is in same direction; and repulsive, if currents are in opposite directions.