A current-carrying, straight wire is kept along the axis of a circular loop carrying a current. This straight wire

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will not exert any force on the circular loop The magnetic force on a wire carrying an electric current i is given as​ \vec{F} = i . ( \vec{l} $\times$ \vec{B} ) , where l is the length of the wire and B is the magnetic field acting on it. If a current-carrying straight wire is kept along the axis of a circular loop carrying current, then according to the right-hand thumb rule, the magnetic field due to the wire on the current-carrying loop will be along its circumference, which contains a current element i d \vec{l} . So, the cross product will be​ ( \vec{l} $\times$ \vec{B} ) = 0 \Rightarrow \vec{F} = 0 Thus, the straight wire will not exert any force on the loop.

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