We have purchased a Pasco torsion balance to measure
Coulomb repulsive forces, F. The apparatus is designed
to conduct three activities:
a) Keep Q1=Q2 constant and show that F*d^2 remains
constant when the distance d changes. Arbitrary units
for F and Q.
b) Keep d constant and show that F/(Q1*Q2) is constant
when the product of two charges changes. Arbitrary units
for F and Q.
c) Measure epsilon_o. In this part F and Q must be
expressed in SI units.
I did not try (c) because my experience with (a) and (b)
is not great. Results are not reproducible; differences as
large as 10% are common. What is the remedy? The air
is dry and I always allow the same time (2 minutes)
between the moment of charging the spheres and the
moment of reading of F.
The charging is performed by touching each sphere
(a ping pong ball coated by carbon) with a wire
connected to a 6000 power supply. That power supply
was recommended by Pasco. The procedure followed
is also that recommended by Pasco. Distances to walls,
table, my body, etc. do not change significantly from
one measurement to another. I am assuming that Q,
in arbitrary units, is the same as the reading on the
large voltmeter (built into the power supply).
I suspect that the amounts of Q received by the balls
is not always the same. Ideally one expects Q=C*V,
where C is the capacitance of the sphere. No knowledge
of C is required in parts (a) and (b); the proportionality
between Q and V is sufficient. But I strongly suspect that
something else, not under my control, has the effect on
how much Q is deposited when a sphere is touched by
the probe connected to the power supply. Any advice?
Ludwik Kowalski