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# Re: [Phys-l] T dS versus dQ

Whats the time constant of their thermometer? -- I think there are similar devices (saw? one at the advanced labs. conference.) that include a pressure gauge in addition to the thermometer. A very small thermistor (head of pin size) might do, or, for which I have a reference, a ribbon copper / constantan TC "eroding" type that has a tc < 10* 1 E-6 s. specifically designed to measure shocks in gasses.

Unfortunately, the Vernier has insufficient range (only about 2 atmospheres, even the Pasco is inadequate (about 7 atm.)

The make and take (tissue paper igniter) has a compression ratio closer to 100;1 than 10:1 !! Digikey has jillions of gauges -- didn't find time constant specs tho. A bunch of 0=> 500 psi 1/8" NPT about \$100.

This is with the only insulation
being the plastic tube. You can then demonstrate cooling during
expansion by turning the screw the other way rapidly

Is this because it's not adiabatic (isothermal -- uninsulated), or, partially shows the difference between non- and iso- entropic compression?

NASA thinks the power and compression strokes of a gasoline engine are isentropic.

http://www.grc.nasa.gov/WWW/K-12/airplane/compexp.html

Wiki. is a bit better:

Gas turbines are described thermodynamically by the Brayton cycle, in which air is compressed isentropically, combustion occurs at constant pressure, and expansion over the turbine occurs isentropically back to the starting pressure.

In practice, friction and turbulence cause:
• non-isentropic compression: for a given overall pressure ratio, the compressor delivery temperature is higher than ideal.
• non-isentropic expansion: although the turbine temperature drop necessary to drive the compressor is unaffected, the associated pressure ratio is greater, which decreases the expansion available to provide useful work.

etc.

http://en.wikipedia.org/wiki/Gas_turbine#Theory_of_operation

bc thinks slow enuf is an analytic statement, and should trust Wiki. more than NASA?

On 2010, Jan 14, , at 13:28, WC Maddox wrote:

The experimentalists among those receiving these Tds vs dQ messages may
want to show the temperature difference between slow pressure increase
and rapid pressure increase by using the device shown here:
http://www.arborsci.com/detail.aspx?ID=1157. It has a digital
temperature display. If you turn the screw slowly you will not see a
temperature change. If you turn the screw rapidly you get ~ 1 degree (C)
increase for full travel of the piston. This is with the only insulation
being the plastic tube. You can then demonstrate cooling during
expansion by turning the screw the other way rapidly.

End Message
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