Date: Mon, 25 Feb 2002 09:02:52 -0500 (EST)

Author: Steve Wonnell

Subject: Re: new demo: He balloons in liquid Nitrogen, quantitative

Post:


Dear Jerry,

Answers below:

On Fri, 22 Feb 2002, Jerry DiMarco wrote:

> Date: Fri, 22 Feb 2002 11:38:09 -0700
> From: Jerry DiMarco
> Reply-To: tap-l@listproc.appstate.edu
> To: tap-l@listproc.appstate.edu
> Subject: Re: new demo: He balloons in liquid Nitrogen, quantitative
>
> We don't do this demo but it sounds interesting. A couple questions
> come to mind. Are you sure the helium is at the stated temperatures when
> you measure its volume?

The balloon's volume responds quite quickly (a few seconds) to the change
in temperature, so to within experimental uncertainty, yes.

>Also, is it possible you are trapping air under
> the wrinkled balloon as you immerse it in LN?

When the balloon is first inserted, it's round in shape, so there's no
room for the air to trap. The balloon is still buoyed upward; if attached
to a stick, it will be most narrow at bottom, so that no air can hide.

> Jerry
>
>
> At 2/20/2002 01:45 PM, you wrote:
>
> >Idea: Fill a balloon with helium, and insert it in a large pyrex beaker
> >filled with liquid nitrogen. Measure the liquid N2 level before and after
> >inserting the balloon to determine the displacement; this gives you the
> >approximate volume of the deflated balloon. Insert balloon in another
> >pyrex beaker of water at room temperature to determine the volume of the
> >balloon. If PV=nRT applies, if He is a fairly good ideal gas, and if the
> >pressure exerted by the nitrogen and water on the balloon is still
> >approximately 1 atmosphere, the volume should decrease to 77/300 (26%) of
> >its original volume; we measured 420/1170 (36%) the first time
> >around. But our measurements are uncertain to within 100 ml, so that's
> >420 plus/minus 100 ml, or about a 25% error. Now I need to work on a more
> >precise measurement of the liquid nitrogen volume--maybe by weighing
> >the beaker instead.
> >
> >Anybody else do this demo?
> >
> >Steve Wonnell
>
k

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