Re: [Phys-l] Cheap Classroom projec to sense water temperature

[John Denker <jsd@av8n.com>]

On 05/05/2010 11:30 AM, CARABAJAL PEREZ, MARCIAL ROBERTO wrote:
> The point of the project is to experience firsthand the copper
> temperature coefficient value, the difficulties to use it for such
> purpose, and the suitability of more specific devices such as
> thermistors. 

OK.

> The idea is to use cheap operational amplifiers, or 2-3
> transistors, diodes and resistors, and battery. The indicator can be
> any analog or digital tester, or even a few leds. One team should
> make this with the original copper coil, another team with carbon
> resistors, and another team should make similar but with a
> thermistor. After that, they can develop conclusions.

You're making it seem waaaay more complicated than it
really is.

Just clip the digital voltmeter to the copper wire.
The effect is plenty big enough to be seen this way,
as you can easily verify by thinking about the theory
and/or doing the experiment.

You don't need resistors (other than the device under 
test), op amps, transistors, diodes, batteries, or LEDs.

You also don't need the water.  The freezing point and
boiling point of water provide a couple of nice fixed
points for reference, but the range of interesting
resistance/temperature physics greatly exceeds the
range of liquid water.

  Selling the project as "applied" physics on the
  basis of the application to water temperature is
  not necessary.  Students are clever enough to realize 
  that any transducer with a good coefficient can be 
  applied to water _and to innumerable other things_.

As CB hinted, the temperature coefficient of modern
thin-film resistors is not interesting.  Antique
carbon-composition resistors are much more interesting
... probably too interesting, because there is no
simple theory to explain what happens.

The comparison with the most direct physical interest
is to compare copper wire with brass, bronze, or steel
wire.  Go to the hardware store and get some "picture
hanger" wire if you don't have a better source.