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Re: Didactic "white lies"

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Malcolm McLean <malcolm.arthur.mclean@gmail.com> writes:

> On Monday, 4 February 2019 20:33:35 UTC, James Kuyper  wrote:
>> On Monday, February 4, 2019 at 6:35:48 AM UTC-5, Malcolm McLean wrote:
>> > On Sunday, 3 February 2019 20:46:58 UTC, Keith Thompson  wrote:
>> ...
>> > If you never simplify, it's often too hard for most students to understand.
>> > You wouldn't try to teach general relativity on a primary school time
>> > and motion exercise, after all.
>> > > 
>> > > At what point in the course can your students safely assume that
>> > > you're no longer lying to them?
>> > > 
>> > You can't. An example is the experiment that shows that air is about 20%
>> > oxygen. A candle is lit on a boat, over which is place a bell jar,
>> > resting on a tray of water. The oxygen in the sir is used up, and the candle
>> > goes out, and the water level in the bell jar rises.
>> > Except that most of the effect is from the heat of the candle forcing 
>> > expanded air out of the bell jar. The air inside the bell jar then
>> > cools.
>> 
>> This experiment?
>> <http://www.math.harvard.edu/~knill/pedagogy/waterexperiment/>? Read the
>> explanation carefully - when done properly (and it's quite possible to
>> do it improperly), the initial water level is high enough that expansion
>> of the air does not result in any air leaving the bell jar.
>> 
>
> From the website:
>
> Argument: The experiment can be explained by physics alone. During the
> heating stage, air escapes. Afterwards, the air volume decreases and
> pulls the water up.
>
> Source of the Error: the argument could work, if indeed the heating of
> the air would produce enough pressure that some air could leave. In
> that case, some air would be lost through the water. But one can
> observe that the water level stays up even if everything has gone back
> to normal temperature (say 10 minutes). No bubbles can be seen.
>
> Knill has got it wrong. If the air escapes due to thermal exapansion, it 
> won't be replaced when temperature returns to equilibrium. No bubbles can
> be seen - OK, but attention is on the burning candle.

It's pretty clear that when he says "no bubbles can be seen" he means
"there are no bubbles", not that the students are just distracted by the
candle.

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Re: Didactic "white lies" Keith Thompson <kst-u@mib.org> - 2019-02-03 12:46 -0800
  Re: Didactic "white lies" "Pascal J. Bourguignon" <pjb@informatimago.com> - 2019-02-03 23:40 +0100
  Re: Didactic "white lies" Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-02-04 03:35 -0800
    Re: Didactic "white lies" Keith Thompson <kst-u@mib.org> - 2019-02-04 12:01 -0800
    Re: Didactic "white lies" James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-02-04 12:33 -0800
      Re: Didactic "white lies" Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-02-05 01:37 -0800
        Re: Didactic "white lies" Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2019-02-05 08:56 -0700
          Re: Didactic "white lies" Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-02-05 08:03 -0800
            Re: Didactic "white lies" scott@slp53.sl.home (Scott Lurndal) - 2019-02-05 16:39 +0000
            Re: Didactic "white lies" James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-02-05 10:35 -0800
              Re: Didactic "white lies" Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-02-05 12:34 -0800
        Re: Didactic "white lies" James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-02-05 10:31 -0800
    Re: Didactic "white lies" John Bode <jfbode1029@gmail.com> - 2019-02-05 08:55 -0800

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