this post was submitted on 16 Mar 2025
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Tried to read about this but it all goes over my head. If anyone wants to ELI5 why magnetic forces do no work, that would be great :)
IIRC it depends on the frame of reference. Relative to a magnetic field, the only thing a magnetic force does is changing the velocity direction of affected objects. All work regarding the absolute magnitude of the velocity is zero (no lateral acceleration).
How fucking smart do you think 5 year olds are?
Well, you have a point here.
"So the squirrel went "neep! neep! neep!" all the way home."
When you hold two strong magnets an inch apart, then let them go it looks like they move themselves (add velocity)?
What makes the magnets start moving towards each other from a complete stop?
It's the magnets, doing work!
It's usually said about a charge under a magnetic feild. The magnetic force goes perpendicular to the direction of motion of the charge(F=qv×B*). Work is done only if the force is applied along the direction of motion. So on a moving charge, magnetic force does no work.
Not sure how that plays on magnets though. Magnets are magnetic because electrons go in circles producing the feild, and it might be because electric feild comes in and do the work but it's not clear for me either
I just did a quick read on it on it. “Work” is the application of force over time in the direction the object is moving. Pushing a shopping cart for example is work, because you have to constantly apply force to it.
From what I’ve read, it seems that magnetic force doesn’t do work because it doesn’t apply force in the direction the object moves. Magnetic force only “deflects” or changes the direction of an object with an existing velocity. It’s only a deflection because the force applied is always perpendicular to the direction of the velocity.
To use the previous shopping cart example, picture a shopping cart that already has a forward velocity that passes a magnet. The magnet only applies a force to the side of the cart towards the magnet. This doesn’t push the cart itself, but changes the direction of its velocity towards the magnet.
But that's true of gravitational forces too, otherwise satellites wouldn't have constant speed. It's silly and misleading to say that magnetic forces do no work. If I turn an electromagnet on next to a spoon, the spoon will move and the magnetic force did some work.
Gravity does no work on satellites or objects that go in circular orbits. The force is there but it does no work and hence no energy change/transfer. Work is defined based on energy change by work-energy theorem
My point is that whether a force does work doesn't depend on what the force is. It makes no more sense to say that magnetic forces do no work than it does to say that gravitational forces do no work.
But magnetic force does no work to a charged particle in any way. While gravitational force CAN do work and it does work on most cases(every non circular orbits or just a mass falling down). That's why magnetic force case is emphasised.
But on your take about magnets, its not the magnetic force that do the work but the associated electric force
I suspect I'm using a naive macro model and inaccurate terminology whereas you're using a micro model and accurate terminology.
Is there even such a thing as magnetic force? Reading up a bit, it only talks about magnetic fields.
There is magnetic force. A moving charge across a magnetic feild experiences it and it is always perpendicular to motion of the charge. So it changes the direction of motion. Since magnets are basically objects with electrons spinning in an oriented fashion, making a current loop(like an electromagnet), it is also appling to the macroscopic case. But the work done is probably done by electric feild in some manner as the title implies. I don't know how exactly it plays out though.
The title is the punchline. Magnets move things with their fields, the magnet itself doesn't need to apply force on the things it moves. 'work' is a physics term for applying a force over a distance.