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Re: Coulomb's force lab.



Michael Edmiston wrote:

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

Good to know.

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

I slide the second ball as far as possible before charging.

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

True, the "half-life" of the force is 9 minutes. But 30 seconds to too
short time for the torsion balance. One minute should be sufficient.

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.

I decided to wait about one minute after turning the power supply on,
just to be sure. I was not careful in holding the charging wire (red probe)
at the same end each time. Perhaps this was my problem. I will tape the
probe to a plastic ruler (so that my hand is farther away) and will try
again. I will share the results. It seems to me that charging would be
more reproducible is each ping pong ball was coated continuously
both inside and outside, and had hole to access the inside. In other
words, if it was a Faraday cup. Charging the inner surface (by the
probe) could then always deposit the maximum possible Q. Right?

Did you try to measure epsilon-o, Michael? How successful was it?
Ludwik Kowalski