Re: [Phys-L] more about basic DC circuits

[John Denker <jsd@av8n.com>]

In the context of:

Q4:  True or false:  In a circuit, energy can be "used up".
  Knight, page 896.

On 12/16/2013 10:14 AM, Prof John Ertel wrote:

> Q4: If we ignore the "thermal energy" that can leave {or enter
> through thermocouple or Peltier effects)   a circuit, this one seems
> TRUE also. BTRW: In the context of this question, where should we
> account for the transport of thermal energy or "mechanical energy"
> being converted to "electrical energy" and coming into the "complete
> circuit" from inductive pickup or piezo-electric effect.

I wouldn't have said that.

In my universe, conservation of energy is of the utmost importance.
It is hard to imagine anything more fundamental, more emblematic of
the the unity and grandeur of physics.

The idea of redefining «energy» so as to not include «thermal energy»
(whatever that might be) seems like an enormous step in the wrong
direction.  Redefining «energy» so that it is OK to "use up" the
energy seems like an enormous step in the wrong direction.

We already have a problem because students are confused by the
ambiguity and contradiction between the physics energy and the
"DoE energy".  The latter is some kind of loosely-defined "useful 
energy" or "available energy".  If the physics teacher starts
throwing around the word "energy" in such a way that it does not
refer to the physics energy, things get real ugly real fast.

Both Knight and Chabay & Sherwood fumble this issue, in similar
albeit not-quite-identical ways.

Knight really takes the cake with the statement
  «It’s not current that the bulbs use up, it’s /energy/.»

which it is explained by saying that current is conserved.  I say 
that current is not conserved but energy is, so the quoted passage
embodies at least two misconceptions in a single sentence.

Even if we pretend the statement refers to conservation of charge,
it amazes me that anyone could make such a statement without
noticing that energy is conserved, just as strictly as charge.
So it seems off-the-wall to use conservation ideas to assert that 
energy is used up whereas charge (or current!?!) is not.

The thousand-pound moose on the table is /entropy/.  The only
thing in this scenario that is irreversibly used up is the
non-entropy.  This is not mentioned -- not even hinted at -- in
Knight or in Chabay & Sherwood.

When the DoE-energy gets used up, it is not for reasons having
anything to do with the physics energy.  The physics energy is
still there.  It may perhaps become less-available and less-useful,
but that's because of entropy, to the extent that it can be
quantified at all.

The true physics is very easily explained by saying that the
electrical resistor is creating a lot of entropy.  This is
incomparably better than replacing one wrong idea (current used
up) with another wrong idea (physics energy used up).