012-03760E
Coulomb Balance
7
Experiment: (Part A) Force Versus Distance
suspended
sphere
sliding
sphere
slide assembly
pendulum
assembly
Procedure
➀
Set up the Coulomb Balance as described in the
previous section.
➁
Be sure the spheres are fully discharged (touch
them with a grounded probe) and move the sliding
sphere as far as possible from the suspended sphere.
Set the torsion dial to 0
×
C. Zero the torsion balance
by appropriately rotating the bottom torsion wire
retainer until the pendulum assembly is at its zero
displacement position as indicated by the index
marks.
➂
With the spheres still at maximum separation, charge both the spheres to a potential of
6-7 kV, using the charging probe. (One terminal of the power supply should be grounded.)
Immediately after charging the spheres, turn the power supply off to avoid high voltage leakage
effects.
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IMPORTANT:
Read the section
Tips for Accurate Results
. It has some helpful hints about
charging the spheres.
➃
Position the sliding sphere at a position of 20 cm. Adjust the torsion knob as necessary to balance
the forces and bring the pendulum back to the zero position. Record the distance (R) and the angle
(
q
) in Table 1.
➄
Separate the spheres to their maximum separation, recharge them to the same voltage, then
reposition the sliding sphere at a separation of 20 cm. Measure the torsion angle and record your
results again. Repeat this measurement several times, until your result is repeatable to within ± 1
degree. Record all your results.
➅
Repeat steps 3-5 for 14, 10, 9, 8, 7, 6 and 5 cm.
Analysis
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NOTE:
In this part of the experiment, we are assuming that force is proportional to the
torsion angle. If you perform Part C of the experiment, you will test this assumption when you
calibrate the torsion balance.
Determine the functional relationship between the force, which is proportional to the torsion angle
(
q
); and the distance (R). This can be done in the following ways:
➀
Plot log
q
versus log R.
Explanation:
If
q
= bR
n
, where b and n are unknown constants, then log
q
= n log R + log b.
The slope of the graph of log
q
versus log R will therefore be a straight line. Its slope will be equal
to n and its y intercept will be equal to log b. Therefore, if the graph is a straight line, the
function is determined.
Figure 6. Experimental Setup
