Re: Lenz's Law

[Maurice Barnhill <mvb@UDEL.EDU>]

It is unphysical to have a current in a wire, however long, whose ends
are not connected to a voltage source of some kind.  The wire plus
connections must form a loop, and that loop will allow the application
of Lenz's Law.

It is true that the ends of the wire are far away, and that the
connections can be placed far away, but you can't arrange for all of the
area of the resulting loop to be far away. Some of that area is
immediately adjacent to the original wire.  So the approximation that
all important parts of the problem except the straight wire are too far
away to affect the analysis is incorrect.

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

--
Maurice Barnhill, mvb@udel.edu
http://www.physics.udel.edu/~barnhill/
Physics Dept., University of Delaware, Newark, DE 19716