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



A question for you:

What is more likely to fall on Friday the 13th, Christmas or Easter?

I mention that because on 03/30/2014 01:10 AM, Savinainen Antti wrote:

I saw an exam question (actually a specimen paper question) which
asked:

Lenz's law is a consequence of the following concervation principle:

a) momentum
b) charge
c) current
d) energy.

It seems to me the only acceptable answer is "none of the above".

It is super-easy to fool people with this sort of question.
This has been a problem for thousands of years, but it has
gotten worse lately.

I blame the multiple-guess tests. One of the standard tricks
for gaming the test uses the "principle" stated by Sherlock
Holmes: when you have eliminated the impossible, whatever
remains, however improbable, must be the truth.

Alas, that trick only works if you have an exhaustive list of
possibilities.

Multiple-guess tests are exceedingly bad preparation for life
in the real world. I mean seriously, when was the last time
a student came to you and said "I'm confused. Here are four
possibilities for what the problem is; pick one."

Why do we allow people to make life-changing decisions about
students, teachers, and entire schools based on tests that
are obviously invalid? I say it's time to get out the torches
and pitchforks:
http://www.av8n.com/physics/img48/vanhelsing-villagers.jpg

It is a standard rhetorical device to argue that something must
be black or white, and it's obviously not completely black, so
therefore it must be completely white. It is super-important
to be able to recognize and reject this sort of nonsense.

I also blame the science-fair tradition that says the student
should test "the" hypothesis as if there were only one. Real
scientists, and indeed anybody with an ounce of sense know that
the real-world rule is:
Consider /all/ the plausible hypotheses.
When the Boy Scouts schedule a camping trip, they plan for
the possibility of good weather or bad weather. Do the
science-fair people think that scientists are less thoughtful
than Boy Scouts?

More generally, we need to do a much better job of teaching
people to deal with ill-posed questions. The first rule is,
be sure you can recognize ill-posed questions as such. Whenever
you get a question, start by asking how badly ill-posed it is.

This is a teachable skill. Start with something simple such
as the Friday the 13th question, /explain/ the principles
involved, and then gradually work up to more elaborate examples.

==============================

Tangentially unrelated: There is no such thing as conservation
of current [as mentioned in item (c) above]. Conservation of
charge corresponds to /continuity/ of current, but current per
se is not conserved. If you have current flowing in a resistive
loop, the current dies away. It doesn't move to a new place,
it just dies away in place. This is the perfect example of
what happens when something is *NOT* conserved.

I don't know whether to laugh or cry when I see textbooks that
repeatedly and emphatically talk about "conservation of current".

Why do we put up with stuff like this?