Consider a single charge moving only under the influence of Magnetic Field $\vec{B}$. The charged particle moves in a circle and the work done by $\vec{B}$ is 0. Now consider a current element in a uniform magnetic field ($\vec{B`}$). Now the derivation of the expression of force results in the the equation $\vec{F} = i \vec{l} \times \vec{B}$ (for a uniform magnetic field) where $\vec{l}$ is the vector joing the ends of the current element. For simplicity, I will consider a straight conductor. When the two vectors are perpendicular, the current element experiences a net non-zero force which does not cause a torque and causes translational motion. Therefore, in this case the magnetic field does positive work.
How is this contradiction possible? Magnetic field, which did no work on a single charge, now does positive work when the current carrying elements clumped together in a conductor. Is there any implicit assumption in the whole process which causes this? Can someone please give a good explanation?
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