Problem solving
If currents flow in the same direction in parallel straight conductors, they attract; if they flow in opposite directions, they repel: each is in the magnetic field of the other. This interaction was the basis for defining the unit of current: formerly 1 A was defined as the current that, flowing in two infinitely long parallel conductors 1 m apart in vacuum, gives a force of 2·10⁻⁷ N on each metre of length (in today’s SI the ampere is defined through the elementary charge, but its value did not change). In problems first find the needed quantity from F = BIl: F = BIl, I = F/(Bl), l = F/(BI). Check the force direction with the left-hand rule. The formula is written for a current perpendicular to the field lines; at other angles the force is smaller.
Make your own problem: choose B, I and l and ask for F; then solve an inverse problem with another unknown. Work in your notebook only.