[Phys-L] circular definition of "success" .... was: standard DC circuits

[John Denker <jsd@av8n.com>]

Several people have said that this discussion hinges on matters
of opinion.  Well, some of it does and some of it doesn't.

 -- If somebody says "my favorite thing is Japanese tea ceremony"
  that's an opinion.
 -- If somebody says "my favorite thing is American football"
  that's an opinion.

Everybody is free to choose their own opinions, provided 
(a) they respect the right of others to choose differently, 
and (b) they keep track of what they are doing.  By that I
mean you shouldn't conduct the tea ceremony by football
rules, or vice versa.

In contrast: Physics facts are not matters of opinion.

On the third hand, even within physics, certain questions 
of emphasis and questions of pedagogical tactics can be
matters of opinion ... within limits.

  Once upon a time, Professor A decided that electrical field
  lines were super-important.  He focused on that to the near-
  exclusion of all else.  He designed an end-of-year interview
  that emphasized field lines.  Needless to say, students who
  took his course did far better on that interview than those
  who didn't.  He declared that his method was "successful"
  by his definition.

  Meanwhile, Professor B decided that steering charges were
  were super-important.  He focused on that to the near-
  exclusion of all else.  He designed an end-of-year interview 
  that emphasized steering charges.  Needless to say, students 
  who took his course did far better on that interview than 
  those who didn't.  He declared that his method was "successful" 
  by his own definition.

  Meanwhile, Professor C decided that macroscopic engineering
  methods (starting with Kirchhoff's laws) were super-important.
  He focused on that to the near-exclusion of all else.  He 
  designed an end-of-year interview that emphasized black-box
  methods.  Needless to say, students who took his course did
  far better on that interview than those who didn't.  He 
  declared that his method was "successful" by his own 
  definition.

Each of these guys thought the other two guys were nuts, and 
each could "prove" it by giving his exam to the other guys'
students.  Each guy thought that the flaws in his model were
merely minor nonidealities, while the nonidealities in the
other guys' models were fatal flaws.

This is the reason why the word "academic" is used as a curse-
word by people outside of academia.  When teachers focus on
their own private notion of "success" the arguments become
circular.  This is scholasticism verging on narcissism.
 
This sort of circularity is deeply ingrained in the education
business.  By definition, a test is "valid" if it measures
what it is supposed to measure.  Folks in the college of
education are OK with that ... but folks in the real world
find it maddeningly circular.

I would like to see more objective metrics, something rooted 
in real-world notions of success.

If you gave me the interview that defines "success" for the
_Matter and Interactions_ course, I would not be "successful".
If the question is based on figure 19.14 and the required 
answer is given by figure 19.17, my answer would have less 
than 40% similarity to the "textbook" answer.

I do not think this is because I'm stupid.  There is strong
objective evidence that my answer agrees with the Maxwell
equations and with experiment, and that figure 19.17 does 
not. 

I can already see the reply to this message.  Somebody is
going to say that figure 19.17 suffers from nothing worse
than minor nonidealities -- not fatal flaws.

I'm not buying it.  I say that folks who cannot tell the
difference between /charge/ and /voltage/ should not be
telling me how to do electrostatics.  When it comes to
conceptual physics, the distinction between charge and
voltage is a core bedrock concept.  I cannot consider
this a matter of opinion.

I mention this because AFAICT virtually all the charge 
distribution diagrams in chapter 19 are predicated on
not being able to tell the difference between charge 
and voltage.  The book says that the "+" signs represent
charge and that clusters of "+" signs represent high
charge density.  The problem is, the diagrams put the
"+" signs in regions of high voltage, which are *NOT* 
the same as the regions of high charge density.

The circuit in question consists of an ideal battery
discharging through a serpentine nichrome resistance
wire. Here is a color-coded diagram showing the voltage:
  http://www.av8n.com/physics/img48/rwire_material.png

The question is, what is the corresponding charge
density?  If you are "successful" based on the _M&I_
definition of "success", you should have no trouble 
writing down the charge density, approximately.  You
might start with something simple and qualitative,
such as identifying the location with the highest
positive surface charge density.  Do it now, if you 
dare.

By way of contrast, here is my diagram showing the 
charge density:
  http://www.av8n.com/physics/img48/rwire_charge.png

I don't think this is a matter of opinion.  There is
a well-defined objective notion of charge.  My charge
diagram could be /mistaken/ ... but I don't think so.
No diagram is perfect, but I reckon this one does a
decent job of capturing the bedrock principles.  More 
importantly, if/when I make mistakes, I want people 
to tell me.  I do *not* want people accepting wrong 
diagrams from me or anybody else and pretending that 
the discrepancies are mere matters of opinion.

There are plenty of real-world situations that require 
knowing where the charge density is ... the real charge, 
not some phony-baloney pseudo-charge.  If you find
yourself in such a situation, you will discover that
"success" according to the _M&I_ definition is several 
sandwiches short of a picnic.

To come at this from a different angle:  Anybody with
good intuition about electric field lines could look at
figure 19.17 and know it isn't right.  Ditto for figure 
19.32.  Alas the M&I students can't do any of that, 
because every vestige of field lines has been scrubbed 
out of the book.

The claim that steering charges are super-important
while field lines are unimportant just cracks me up.
You need the field lines, if nothing else to help
figure out where the charges must be!  The other half 
of the claim says that students cannot draw the field 
lines very accurately, so we shouldn't bother.  That 
cracks me up again, because evidently neither the
students -- nor the professors -- have been drawing
the charge distributions very accurately.

Last but not least, a few words about critical thinking:

On 11/25/2013 10:06 AM, Bruce Sherwood wrote:
> Clearly a lot of class discussion needs to be devoted to talking
> about how to learn.

Agreed!

One of the key elements of "how to learn" involves
looking at something like figure 19.17 and asking
whether it is consistent with everything else we
have learned.

It seems remarkable that over the years, not a single 
student -- or teacher -- has said "Hey wait a minnit,
these charge distributions cannot possibly be right."

It seems that all too often, "success" in school has 
been defined in terms of the triumph of conformity 
over creativity, the triumph of regimentation over 
reasoning.

This is a fixable problem.

One of the things I really like about phys-l is the
high concentration of "Hey wait a minnit" type of
questions.