[Phys-L] Re: Simple heat engine labs

[John Denker <jsd@AV8N.COM>]

On 05/17/05 22:38, Paul D. Price wrote:
> I am looking at adjusting aspects of our curriculum and am looking for a
> "better" heat engine lab.  Better by my definition would be
> A) Simple equipment (basically what I can cull together from syringes etc)

How simple is too simple?  What about ye olde steam turbine
made from teakettle + pinwheel?

> B) The ability to take quantitative data via labpro or direct measurements
> of volume, temp etc....
> C) The inclusion of a simple thermoelectric converter in some way

You can throw a thermocouple almost anywhere and use
it "in some way" i.e. as a temperature gauge.  OTOH
if you want to make it an _essential_ part of the
engine, (C) seems inconsistent with (B).  If volume
is "the" independent variable, then charge isn't,
and vice versa.

Also "quantitative data" and cobbled-together equipment
are not usually compatible.  Unless you have a sophisticated
design and precise tolerances, there are going to be friction,
heat leaks, fluid leaks, etc. that seriously degrade the
accuracy of the desired measurements.

The essential problem is that thermo theory, especially
at the intro level, focuses on ideal engines, but real
engines (especially small ones, and very especially
cobbled-together ones) are grossly nonideal, so there's
not much connection between the theory and the demo ...
which mostly defeats the purpose of quantitatively
measuring the demo.

For on the order of $100 you can buy a working-model
Stirling-cycle engine, e.g.
   http://www.pmresearchinc.com/solar001.htm

For the same sort of price you can buy a working-model
single-cylinder steam engine (from e.g. Jensen or
PM Research or ...).  Again you're making a tradeoff:
compared to a Stirling engine, it's easier to
explain how a single-cylinder engine works ... but it
doesn't work nearly as well (i.e. it is more grossly
nonideal).

To summarize: I've mentioned three points on the tradeoff
curve.  In order of increasing sophistication:
  a) teakettle + pinwheel
  b) single-cylinder steam engine
  c) Stirling engine.

In roughly the same category as (a), there's always the
wheel with rubber-band spokes as illustrated and discussed
in Feynman.  Also dippy-birds.  Also radiometers.
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