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- cross-posted to:
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This is all based, most likely, on Griffiths’ textbook. Quoting here from this post https://www.reddit.com/r/Physics/comments/1b97gt/magnetic_fields_do_no_work_but_magnetic_cranes/ :
The statement “magnetic fields do no work” is incorrect. Griffiths has mislead a generation of physics students on this. A correct version of the statement is that “magnetic fields do no work on objects with no magnetic moments” which is rather trivial. One could also correctly make the same statement about electric fields. However, electric monopoles are very common, so a situation in which there are no electric moments never occurs in normal circumstances.
tl;dr: use Jackson ;)
Thanks.
Somehow an entire generation of teachers just decided that magnets don’t attract or repel each other.
Those blatantly wrong things that somehow people still insist on teaching are ridiculous.
You are correct about which book the quote is from. Idk about Jackson’s books so can’t say anything.
I tried to include the explaination (tbh I dont get it fully still) in the title that technically its true bc ‘something else does the work’ (hence the title)
An attempt at a generalized joke:
– We are recuting you for the GOP. It says here you are a conservative force?
– Yeah. I go around in circles and do no work.
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!
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.
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=q*v×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.
