[Physics] Does a magnetic field do work on a moving rod

electromagnetismmagnetic fieldswork

enter image description here

In a case such as this one, my textbook says that I might be tempted to think the magnetic force is doing work (against the moving rod). It then says this isnt the case because "charges move horizontally, and similar to the Hall effect, the left side becomes positively charged and the right side becomes negatively charged; the electric field produced here is what really causes the negative work".

That's nonsense to me. If the magnetic field is not doing any work then the force vector could not be pointing in the direction shown in the figure. But maybe I'm wrong. How is it in this case that the magnetic field does no work?

Best Answer

The resolution to this problem is simple once you know how...

Remember work done is force times distance moved in the direction of the force. The electrons are moving upwards, the Lorentz force $-ev \times B$ is in the direction shown in the diagram. BUT, the force did not do any work, because the force is perpendicular to the direction of travel of the electron!

What has happened is that the electron has been merely deflected sideways (but its energy has not changed). This deflection means there is now a slight imbalance in the density of charge because the electrons have all been slightly shifted to one side. That means there is an electric field induced (via Gauss Law)...and it is this electric field, which is also in the direction shown by $F$ in the diagram, that ends up doing the work on the electrons.

Related Question