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

[Brian Whatcott <betwys1@SBCGLOBAL.NET>]

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-----
> =46rom: 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
> =20
> 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 i=
> n
> revising our existing programmes!)
>
> Since I have just discovered that our 'weighing air using two balloon=
> s and
> a meter stick' experiment isn't accurate (apparently it only shows th=
> at
> 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
> 519-756-6220


Brian Whatcott    Altus OK    Eureka!