[Physics] Positive work by a magnetic field

magnetic fieldswork

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?

Best Answer

The claim, "the magnetic force does no work," while technically true, is misleading in so many situations and actually helpful in so few that we probably shouldn't put as much emphasis on it as many textbooks do. The magnetic field stores energy, and a change of the configuration of a system (like moving a current) can convert that stored energy into other forms, so the system will certainly behave as if the magnetic field is doing work, but careful analysis will always show that it's not actually the magnetic force that's doing the work associated with the change of magnetic energy. Usually it's an electric force.

In your example, I think the subtlety lies in assuming that $I\vec{\ell}$ for the current $ = q \vec{v}$ for individual charges. The charges will tend to change trajectory, i.e. $\vec{v}$ changes, due to the magnetic field, but $\vec{\ell}$ maintains its direction. Why? Because the wire (magically? We never really explain it very well in class) constrains the charges to move along its length in spite of their lateral movement produced by the magnetic field. As others have said above, the wire exerts a force on the charges to keep them in a straight line, and so the charges exert a reaction on the wire which moves it.

So technically, the magnetic force isn't doing any work on the current; it's just redirecting the force that would otherwise be pushing the current down the wire so that that force has a component pointing perpendicular to the wire instead.

Now for the bonus question: a magnetic dipole, e.g. a current loop or a small refrigerator magnet, will experience a force in a nonuniform magnetic field (though not in a uniform one). The dipole will then accelerate if this is the only force acting on it, so work is being done on it. If it's not the magnetic force doing the work, then what is it? (Really, as in this case, it gets silly sometimes to insist the magnetic force isn't doing work. For many intents and purposes, you might as well treat it as though it is.)

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