Hey Crazies.
There’s this thing that teachers like to say in introductory physics courses:
Magnetic force cannot do work.
But this is the Science Asylum and we can’t ever forget Rule #8:
Question Everything!
Before we get into magnets though, let’s start with a more fundamental question:
What is work?
Well, you can’t really define work without also defining energy.
Yes, I’ve covered energy before… in several videos.
Here’s a summary of what we know so far: Energy isn’t anything by itself.
It’s just a number.
We assign this number to actual things like planets, people, or particles.
The specific value of that number doesn’t matter.
Whatever number that is, as long as there is symmetry across time, the total number
never changes.
That’s the conservation of energy.
In any practical sense, all that really matters is what the energy does.
That’s what work is!
So let’s put it this way: Energy is the amount of stuff you CAN do.
Work is the amount of stuff you HAVE done or WILL do.
That’s it.
But what do you mean by “stuff”?
I mean anything that could exist or happen.
Lifting, Swimming, Climbing, Dancing, Swinging, Waving, Running, Slithering
Uh, doing other things
Boiling, Lighting, Chemical Reactions, Nuclear Explosions
or the very existence of matter itself.
Well, that escalated quickly.
Ok, now back to magnets.
If we want to know if magnets can do work, we just need an example of them doing stuff.
Boom!
Magnets doing stuff!
Guess my work is done here.
Thanks for liking and sharing.
Hold on a second!
I was reading in this electrodynamics book that it’s not actually the magnet doing
the work.
Just because it’s in a book, it doesn’t make it true.
When textbooks say magnetic force can’t do work, they have to do this convoluted
dance around it.
Kind of like this: While it may appear that the magnets are doing work,
it is really just the electrons in the magnet doing the work.
No!
Regardless of what objects or particles are doing the work, they will be doing that work
through a force.
Work is always done by a force in the direction of a displacement.
That’s how we get units of newton meters or foot pounds.
There is always a force.
So, where do the textbooks get the idea?
The innately human desire for very simple rules.
The argument goes a little something like this:
Imagine there’s a proton moving to the right.
Suddenly, a magnetic field is turned on directed into your screen.
The right-hand rule tells us the magnetic force on that proton will be upward perpendicular
to the direction the proton is moving.
In fact, this cross-product means the magnetic force will always be perpendicular to that
motion.
Since work is only done along the direction of motion, magnetic force can’t do it.
It seems like a solid argument until you stop and think about it.
Yes, the magnetic force on this proton in this specific case isn’t doing any work.
All it did was change the path of that proton from straight to circular.
However, with attracting magnets, there is definitely a magnetic force and that force
is definitely along the direction the magnets move.
That magnetic force is doing work.
Maybe those are two different kinds of magnetic forces.
They’re not, but I’ll give you a couple more examples anyway.
Let’s say you replace that proton with a wire carrying an electric current to the right.
That current will have an upward magnetic force on it just like the proton did,
but the current can’t go in a circle.
It’s bound to the wire, so the wire will just move straight upward.
That’s a force in the direction the wire is moving, which means work is being done
on the wire.
Let’s go even deeper!
Remember that proton?!
The upward magnetic force we found earlier is not the whole story.
Protons do a lot more than just move.
They also have another property called Quantum Spin that can turn a particle like a proton
or electron into a tiny magnet.
That tiny magnet can also respond to the magnetic field
causing another magnetic force that can do work on that proton.
In fact, that’s all a permanent magnet is: A pile of lined-up electrons acting as tiny
magnets, held in place by a bunch of protons and other electrons.
That’s exactly how these magnets do work.
That precious simple rule is turning out to be more like this:
Magnetic force can’t do work on a single charged particle
assuming you ignore the particle’s spin.
It’s kind of like that English rule: I before E except after C
unless you’re a weird beige foreign neighbor running a feisty heist.
With that many exceptions, is it really a rule anymore?
So, are you satisfied with this answer?
Let me know down in the comments.
Thanks for liking and sharing this video.
A special thank you goes out to Patreon patrons like Rick Finn who are literally making my
life possible right now. Thank you so much!
And until next time, remember, it’s ok to be a little crazy.
The featured comment comes from Bobby Harper who asked:
If we didn’t have Einstein’s relativity to explain time discrepancies in GPS satellites,
would we call it Dark Time?
Yep, I’m pretty sure we would.