[Phys-L] Re: Weighing air (Was: electricity)

[Brian Whatcott <betwys1@SBCGLOBAL.NET>]

I picked up a  Camel brand "universal clamp-in valve" at 2 for $5
and Norma provided the 2 liter soda bottle.
A pen knife made a suitable hole in the plastic cap.
Here are preliminary feasibility data.

Weight empty: 79.2 gm
wt with ?? 20 psig air   82.8 gm
Difference  3.6 gm
wt filled with water ?? 2.2 kg
  ( A little too heavy for my Brainweigh scale rated at 1.5 kg,
so a kitchen scale served its turn.)
(34.7 psia/14.7 psia ) * 2.12 /3.6  =
~1.39 gm/liter ???   given as 1.29 gm/liter at 0degC

Brian Whatcott

At 12:28 PM 2/4/2005, you wrote:
> Building on Vickie's suggestion. I recall that thin plastic soda bottles
> can handle about 50 psi.
>
> If one drills a hole in a cap, and screws in a threaded tire valve
> (available at many autoparts stores for go-faster alloy wheels)
> Then one could compare the weight of the bottle at ambient
>   pressure, and one inflated to 15 psi, even 30 psi, using a digital
>   postal balance.
>
> This would give a reasonably plausible result, quite cheaply.
>
> Brian Whatcott
> p.s. Care obviously needed with inflation)
>
> At 10:58 AM 2/4/2005, you wrote:
> > Actually, the two balloons on a meter stick doesn't even show that, b=
> > ecause of the phenomenon of the buoyant force. =20
> >
> > One "right" way to weigh air (in principle) is to take a vacuum bell =
> > jar, weigh it on a sensitive balance, pump it out, seal it off, and w=
> > eigh it again.  The density of air is about 1 kg per cubic meter, whi=
> > ch works out to about 1 g per liter.  You might be able to "weigh" ai=
> > r directly if you put a lightweight  Nalgene (plastic) bell jar on to=
> > p of a digital scale capable of measuring milligrams.    I haven't ac=
> > tually tried this, but with a lightweight bell jar it might have a fa=
> > ir chance of working.  (Heavy bell jars would overwhelm the scale.)  =
> > I really like the Nalgene bell jars for lecture demos.  They're light=
> > weight, easy to handle, easy to see through, and the top is flat, whi=
> > ch lets one use the bell jar on a document camera in the larger class=
> > rooms.=20
> >
> > By the way, Easter is coming, and so are marshmallow peeps.  Students=
> >  just  love peeps in a bell jar.=20
> >
> > Vickie Frohne
> >
> >
> >
> > ----Original Message-----
> > from: Forum for Physics Educators on behalf of Alison Innes
> > Sent: Fri 2/4/2005 10:13 AM
> > To: PHYS-L@LISTS.NAU.EDU
> > Subject: Re: electricity
> >
> > Thank you to everyone who responded to my request about
> > electricity.  It's great to know that I'm on the right track
> > and finally 'getting' stuff  that was lost to me in grade
> > school. (I also have my work cut out for me in
> > revising our existing programmes!)
> >
> > Since I have just discovered that our 'weighing air using
> > two balloons and a meter stick' experiment isn't accurate
> > (apparently it only shows that air under pressure weighs
> > more than air that isn't), does anyone know
> >  of any simple experiments/demonstrations that can show
> > kids that air has weight? Or is it something we have to
> > take on faith?
> >
> > Thanks for your help-- I know my requests aren't for
> > the usual 'deep'  topics discussed on the list, but I
> > really do appreciate it.
> >
> > Alison Innes
> > Education Coordinator
> > Bell Homestead National Historic Site


Brian Whatcott    Altus OK    Eureka!