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