Magnetic force does no work. OR does it?

[fisico30]

Hello Forum,

it is well known that the magnetic force |F_mag|= q v B sin(theta) does no work on a moving charge particles, i.e. it does not increase or decrease its kinetic energy. It only changes its trajectory.

However, in the examples of a U shaped wire loop with a movable metal bar that is free to slide, if we move the bar with velocity v while the loop is inside a magnetic field B, an induced emf= vBL is generated. It is said that the induced current (charges in motion) is due to the magnetic force...that seems an inconsistency: the charges are now set into motion by F_mag...So F_mag does work on them...or doesn't?

thanks,
fisico30

 

[Matterwave]

The metal bar will slow down, the magnetic field is simply transferring the kinetic energy of the moving bar into the kinetic energy of the charges.

 

[fisico30]

Hi Matterwave

so your point is that the magnetic force is not actually doing the work...it is just transferring it? Is the magnetic field kind of acting as a proxy you mean?

 

[Matterwave]

Sort of I guess. It's just transferring one kinetic energy into another.

 

[sardar]

Hello fisico30,

Thank you for bringing up this interesting topic. It is true that the magnetic force does no work on a moving charged particle in the traditional sense of increasing or decreasing its kinetic energy. This is because the magnetic force is always perpendicular to the velocity of the particle, so it cannot change the magnitude of its velocity.

However, in the case of the U shaped wire loop with a movable metal bar, the situation is a bit different. Here, the magnetic force is not acting on a single charged particle, but rather on a collection of charged particles in the metal bar. The movement of the bar through the magnetic field creates a changing magnetic flux, which induces an electric field and subsequently an electric current in the loop. This induced current is what we observe as the movement of charges in the bar.

So while the magnetic force itself does no work on individual charged particles, it does contribute to the overall work done in the system through the generation of the induced current. This is because the magnetic force is indirectly causing the movement of the charges in the bar.

I hope this explanation helps to clarify the role of the magnetic force in this scenario. Thank you for your question and for your interest in physics.

Sincerely,