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Asked in GUJCET 2015 · Force between parallel currents
Given: two long straight parallel wires carrying equal currents in the same direction, a distance d apart.
Idea: each wire sits in the magnetic field of the other, and the force on it is ⃗F=Iℓ⃗×⃗B.
Wire 1's field at wire 2 is perpendicular to the plane containing the wires, with the sense given by the right-hand rule.
Taking Iℓ⃗×⃗B for a current in the same direction gives a force on wire 2 pointing towards wire 1, and by symmetry the same for wire 1.
So wires carrying currents in the same direction attract each other. (This is how the ampere itself was defined.)
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