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Re: [Phys-l] T dS versus dQ

From: WC Maddox

I have never had any problem getting a small but measurable ( ~ 1dec C) change in temperature due to "rapid" compression. Perhaps the unit I have has a more loosely fitting piston. Rapid is not all that fast. It takes ~30 sec for the pressure to increase from 0 to highest value marked on scale (30 N/sq cm). If you do not see a temperature increase you may want to risk raising the pressure above the highest marked value. Judging by the spacing of the pressure scale markings I can get the pressure up to ~40 without the apparatus exploding. Another reason for not noticing a temperature rise is that since the column is not heavily insulated the temperature starts dropping quickly once you stop the compression ( 0.5 deg in 7 sec).

End Message

On 1/14/2010 11:03 PM, LaMontagne, Bob wrote:

We use that apparatus in lab for some of our general physics sections. The screw is hard to turn and we have never been able to turn it fast enough to see a temperature change. For all intents, we give it to our students as a way of graphing P versus V when T is constant.

Can you share how you manage to turn the screw that fast? I would love to add temperture to the lab as well.

Maybe ours are just stiffer than yours to turn.

Bob at PC
From: [] On Behalf Of WC Maddox [maddox@physics.Auburn.EDU]
Sent: Thursday, January 14, 2010 4:28 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] T dS versus dQ

From: WC Maddox

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: 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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