Re: First EM lab/Stop Press

[Brian Whatcott <inet@INTELLISYS.NET>]

Putting BC's dipole expression aside in favor of the old faithful point
charge model
to account for the shielded field from the charged conductive plane (?)
and using my 60 nanocoulomb estimated charge on a pen,
I see that f = q1.q2/(4.pi.d^2)
and assuming that the 60 nanocoulombs of charge reflects  4 electron charges
per carbon atom in the confetti at 3.29 cm.

I get f = q1 * q2 / ( 4 * pi * 3.29E-2^2)
           =  6E-8 * 6.4E-19  / (4 * pi * 3.29E-2 * 3.29E-2)
           = 2.8E-24 N
or 2.9E-25 kg but the chemists figure it's 12.01 / 6.024E25 = 2E-25 kg

Which even so is far too close.
What did I do wrong?
 Ah yes:  Herb's confetti lifting distance estimate was 17% too small.
Of course!
:-)
Brian W

At 02:05 AM 1/13/02, bc wrote:
> OK
> If the separation (dipole) is << distance (l) to the pen's charge (q) and
> it is
> concentrated (far field) the force is
>  ~ {2[Qd]*q / l^3} * 1 / 4pi*e   (Qd is the dipole moment) (SI units)
>
> bc
>
>
>
> Brian McInnes wrote:
>
> > on 13/1/02 4:36 PM, Herbert H Gottlieb at herbgottlieb@JUNO.COM wrote:
> >
> > > On Sat, 12 Jan 2002 21:59:12 -0500 Joe Heafner <heafnerj@VNET.NET>
> > > writes:
> > >>
> > >> Take a plastic ball point pen and rub it in your hair so that it
> > >> acquires a net charge. Hold it a couple of centimeters above a small
> > >> piece of paper (e.g. a tiny bit of paper from a notebook sheet).
> > >> Find the height at which the paper is just lifted by the electric
> > >> force. From this, estimate the polarizibility of carbon.
> > >
> > > I don't get is Joe. I tried what you said
> > > and found that the height distance is EXACTLY 3.28759
> > > centimeters. Using this data how do I estimate the
> > > polarizibility of carbon??
> > >
> > > Maybe it would be better to use an ordinary wood pencil
> > > that has some hardened CARBON inside?
> >
> > There are plenty of carbon atoms in the paper, Herb.
> >
> > The interesting situation that Joe has put forward comes from Chabey and
> > Sherwood's text he refers to.  It's a homework problem there.
> >
> > An estimate is needed of the charge on the pen; this estimate will be based
> > on hands-on homework done earlier by the student with charged (sticky)
> tape.
> >
> > Brian McInnes

Brian Whatcott
  Altus OK                      Eureka!