Re: Lenz's Law

[Barlow Newbolt <NewboltW@WLU.EDU>]

I apologize.  In order to be correct I should have said in the
"blue-eyed" world or the "brown-eyed" world, or in the "hazel-eyed"
world.  It didn't intend to leave out any worlds!

> > > NewboltW@WLU.EDU 03/22/00 11:32AM >>>
.....a minor point.  It helps to know that it is mutual inductance
because every electricity book in the "blue-eyed" world tells how
currents influence each other through mutual induction.
      WBN

> > > jlu@HEP.ANL.GOV 03/22/00 11:05AM >>>
        You seem to be asking two questions. How does the induced
field
impede the increase of current in the straight wire, and does the
straight
wire "see" resistance in the loop?
        Draw the induced field lines coming out of the loop.  Each
line
is a circle (DivB=0) and some of those circles cut the straight wire,
so
that the stright wire is in a region of increasing B opposing the B
from
the straight wire current.  That changing B generates an EMF in the
straight wire that opposes the original increase in the current (via
dB/dt = -curlE, the minus sign being, as you stated, Lenz' law).
        Resistance in current loop does not affect the derivative of
the current in the loop, so if you trace through all the linkings you
will see that it affects the voltage change on the straight wire that
is required to effect a current change in that wire.
        I don't see how it helps to know that the name of this effect
is "mutual inductance".
                                Regards,
                                        Jack

Adam was by constitution and proclivity a scientist; I was the same,
and
we loved to call ourselves by that great name...Our first memorable
scientific discovery was the law that water and like fluids run
downhill,
not up.
                          Mark Twain, <Extract from Eve's
Autobiography>

On Wed, 22 Mar 2000, Stuart Leinoff wrote:

> Can someone help me out and check my reasoning.
>
> Given: a long straight wire lying on a horizontal table carrying
conventional
> current north.  Next to the wire, on the right, is a metal ring
lying
flat on
> the table.
>
> If the current in the wire is increased, there will be a growing
B-field inside
> the metal ring directed down into the table.  This should (I'm
pretty
sure)
> induce a counter-clockwise current in the ring which will cause a
B-field up out
> of the table in opposition to the change in field that induced the
current in
> the first place (Lenz's Law?)
>
> What bothers me is that (by my understanding is that by Lenz's law)
this induced
> current in the ring should somehow oppose the increasing current in
the wire,
> and I don't see the mechanism for that.  If the ring was part of a
circuit with
> resistance, the induce current would do work, which would mean that
it would
> take more work to increase the current in the straight wire when the
loop was
> present than without the loop.
>
> What mechanism would make increasing the straight wire current more
difficult
> with the wire loop there?
>
> Stu Leinoff
> Adirondack Community College