PASCO ES-9070 COULOMB BALANCE - Manual (Page 15)

012-03760E Coulomb Balance
11
Therefore C = C
test
(V
test
- V)/V. Once you’ve measured the capacitance C, measure the charge of
the charged sphere is follows:
➀ Discharge the conducting sphere on the insulating thread, by touching it to a grounded probe.
➁ Holding the sphere by the insulating thread, touch it to the charged sphere, then to the inner
conductor of the ice pail.
➂ The charge on the original charged sphere, q, can now be determined using the equation:
q = 2CV;
where C is the capacitance of the electrometer and ice pail and V is the reading on the electrom-
eter. (The factor of two arises because, in using the test sphere to sample the charge on the
original sphere, only half the original charge was transferred.)
Calculations for the Coulomb Constant
The Coulomb constant can now be determined by using any data pair from your force versus
distance data.
➀ Convert your torsion angle measurement (q
corrected
) to a force measurement, using your measured
torsion constant for the torsion wire: F = K
tor
q
corrected
.
➁ Determine the charge that was on the sphere using Method I or Method II above. If you are using
Method II, you will need to recharge the sphere to the voltage previously used while taking data,
so that you can determine the charge using the electrometer and the Faraday ice pail.
➁ Plug your collected data into the Coulomb equation, F = k q
1
q
2
/R 2 , to determine the value of k.
Do this for several sets of data. Average your results to determine a value for k.