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RE: Coulomb's force lab.



We have the same apparatus. Most students get better results than Ludwick
reported, but some fail miserably.

(1) The second ball will not accept as much charge if it is near the first
ball charged.

(2) I think 2 minutes is too long to wait. All you do by waiting is have
charge leak away.

Here is the procedure we use for charging and measuring (assuming instrument
is already zeroed, etc.)

(a) Move the balls as far apart as possible.
(b) Touch each ball with the charging wand while they are far apart. Adjust
the power supply between charging the first and second balls if you are
giving the balls different charge, but do it quickly.
(c) Turn off the power supply.
(d) Move the sliding ball to the separation desired.
(e) Adjust the torsion dial to get the pivoting ball to the zero mark
(f) Practice this so you can do all these steps within 30 seconds.

Also...
For each data point (e.g. 6000 volts ball one, 4000 volts ball two, 8 cm
separation) do the procedure three times and average the torsion readings.
This means turning the power supply on and off, moving the balls apart
during charging then bringing them back together, working quickly.

Also...
Stand in the same position and hold the charging wand with the same
orientation for all charging. Charges on clothes (especially synthetics
like rayon, dacron, nylon) and the position of the wand and wire, will
affect how much charge the balls take. We need to make these variables more
constant.

I have a high-voltage probe and found the Pasco HV power supply has a pretty
accurate meter. They assume most people can't confirm the voltage, so they
did a pretty nice job of calibrating it for you. This seems to be an
exception to the old rule of "don't trust meters on power supplies."

Ludwick did not mention having the balls far apart during charging. If this
was not done, that is the bigget problem. The next biggest problem is
speed.


Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817