Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-L] [**External**] Lenz's law

Would we say this is similar to friction? We say the work done by sliding friction is negative, and we justify this by saying either that delta K is negative (object slows down) or the force is pointed opposite to the displacement, but it still depends on our *belief* that the friction force is pointed opposite to the displacement. In the annals of observation, this one appears to be ironclad but it does sometimes seem lacking to just say "well if it weren't true then energy wouldn't be conserved." If that's the best we can do then I'm good with it, but I do prefer to level with a student that maybe "that's the best we can do." 

Stefan Jeglinski

On 4/25/24 1:24 PM, Carl Mungan via Phys-l wrote:

A nice example of that is the motional emf problem with the usual
arrangement of a slide bar on top of a stationary U-shaped wire with
the whole arrangement in a uniform vertical magnetic field. If you give the
slide bar a push, Lenz's law must give an induced current in such a
direction that the magnetic braking force on the bar is opposite the
direction of the push. Otherwise the bar will subsequently speed up instead
of slowing down!

On Thu, Apr 25, 2024 at 1:19 PM Philip Keller via Phys-l <
phys-l@mail.phys-l.org> wrote:


I don't know that this is an answer that has been approved here in the
past...but it seems to me that if you could change the minus sign (and
nothing else!) then your various experiments would all violate conservation
of energy.  Motors would have helping EMF rather than back emf.  Generators
would become easier to turn as the current increased -- you could even let
go and let the force on the electromagnetically induced current take over
for you!

So I would say that Lenz's law is consistent with the law of conservation
of energy.  But as I said, I think others will disagree...  :)

On Thu, Apr 25, 2024 at 1:09 PM pschoch via Phys-l <phys-l@mail.phys-l.org
wrote:


So, here's where I get myself into trouble.   I encourage my students to
understand why things happen, what causes them, and not just memorize the
formulas.  And now we've gotten to Induction -- Lenz's law.

The textbook shows them that the induced EMF opposes the change causing
it, thus the negative sign -- experimentally.  Now I've had a student ask
me what the underlying physical principle is that results in the negative
sign.   Honestly, I don't think I understand this well enough to give an
explanation.

Any help would be appreciated,
Peter Schoch

_______________________________________________
Forum for Physics Educators
Phys-l@mail.phys-l.org
https://www.phys-l.org/mailman/listinfo/phys-l


_______________________________________________
Forum for Physics Educators
Phys-l@mail.phys-l.org
https://www.phys-l.org/mailman/listinfo/phys-l