Re: [Phys-L] Lenz's law and conservation of energy

[John Denker <jsd@av8n.com>]

On 04/03/2014 04:41 PM, Rauber, Joel wrote:
> You find the energy argument to be unnecessary and non-constructive;
> but do you find it invalid?  Your executive summary seems to imply
> "no" to the question of validity to the energy alone argument.
>
> However further down, where you refer to "covering all the bases" as
> necessary for a "contrafactual" argument; you suggest that the energy
> argument alone does not suffice; from which I infer a response of
> "yes" to the validity of the energy argument (since all the bases
> aren't covered).

Interesting question.  It's a moving target.  Let me explain:

1) The energy argument /as it originally appeared/ stood alone
 and purported to prove the result.  I consider this to be invalid
 (as well as unnecessary and non-constructive).  It is invalid
 because it is incomplete, and does not prove what it purports
 to prove.  It does not cover all the bases.

2) Conservation of energy could be used as a lemma within a much 
 more elaborate argument.  The overall argument could be arranged
 to be valid, to cover all the bases.  However, the overall argument
 would still be non-constructive, and all the more unnecessary.


The constructive derivation, based on V = flux_dot and V = IR,
provides so much more bang for the buck that I am having a hard
time figuring out why anybody would bother with a complicated
argument that /at best/ only gives you the sign of the effect.
What's the attraction?  I just don't get it.

I know there are some students who have a phobia about learning
the Maxwell equations, but c'mon folks, how hard is it to learn
V = flux_dot?  Given the choice between learning Lenz's law and
learning V = flux_dot, I'm gonna go with the latter, because it
is about 1000 times more useful.  It gives you Lenz's law and
999 other things for free.