Download Coulomb`s Law

6/29/17
PHYS 1402 – General Physics II
Coulomb’s Law
Leader: _________________________
Skeptic: _________________________
Recorder: __________________________
Encourager: ________________________
Equipment Needed
Coulomb Balance Apparatus (Pasco)
Kilovolt Power Supply (Pasco)
Typical Setup
Block Diagram View
Objective
The objective of this lab is to investigate the dependence of the electrostatic force
on the distance between two charged spherical conductors.
Introduction
In this experiment we will investigate the factors that affect the electrostatic force
between two charged spherical conductors. In this lab, we will place the same
charge on two spherical conductors and vary the distance between them. We will
measure a quantity proportional to the force, namely the angle through which a
torsional pendulum turns as the distance is varied. From the measured angle, we
will be able to determine the force. Our final product will be a graph of the force
1
Coulomb’s Law
493736061
Page 1 of 6
6/29/17
versus the distance, and we will investigate the relationship between the force and
the distance.
The apparatus for this experiment will be a torsional pendulum. The two
spheres will have equal charges of like sign. When the spheres are brought near
to each other the pendulum to rotate about its axis to a new equilibrium position.
The angle through which the pendulum rotates can be measured by measuring the
angle required to return the torsional pendulum to its original position. The
relationship between the force and the angle in degrees through which the
torsional pendulum turns is given by the equation F  K tor where
6
K tor  1.45  10 N


(1). The value of Ktor for each Coulomb Balance
Apparatus depends on the tension of the wire but has been found experimentally
to be only slightly different from one apparatus to another. It is also time
consuming and tedious to measure the value of Ktor, so to find the force that
produces a given deflection we will use equation (1).
Procedure
1. Setup
The Coulomb balance apparatus should already be set-up. Gently touch both of
the spheres to remove any charge. If not already done, move the angle indicator
to 0˚. Zero the torsional pendulum by rotating the plastic piece holding the wire
on the bottom of the apparatus until the line on the counter balance pan aligns
with the index mark. Move the sliding sphere to its maximum separation from the
sphere attached to the torsional pendulum.
Note: Check the slide calibration. This should be done before any data is taken.
Move the slide to 3.8 cm. The balls should just touch at this point. (This is the
center to center distance.) If this calibration is off have you instructor or the
laboratory technician reset it for you. It has been set previously but can be
disturbed.
Setup of the Balance
1
Coulomb’s Law
493736061
Page 2 of 6
6/29/17
2. Set up the high voltage power supply
Make sure that the high voltage (HV) power supply switch is in the off position,
and that the adjustment knob is turned fully counter clockwise. The charging
probe should be plugged into the plug labeled 6 kV on the front of the power
supply. Make sure that the HV power supply is plugged in.
.
3. Qualitative Observations
Make sure that the sphere on the slide is at the maximum distance from the other sphere.
Turn on the power supply and rotate and adjust the voltage so that it reads 6 kV. Touch
each of the spheres successively for several seconds with the probe. Remove the tip, turn
down the voltage and turn off the power supply. Rapidly but gently move the sliding
1
Coulomb’s Law
493736061
Page 3 of 6
6/29/17
sphere so that it is close to the other sphere but not touching. Continue pushing the two
spheres closer together.
Q1) Describe what you see.
Q2) Is the force attractive or repulsive in this case? What does that mean about the
charges on the spheres?
Separate the spheres again. Gently touch them to remove any charge. Turn on the power
supply and rotate and adjust the voltage so that it reads 6 kV. This time touch only the
sphere on the slide for several seconds with the probe. Remove the tip, turn down the
voltage and turn off the power supply. Gently but quickly slide the spheres together so
that they are close but not touching.
Q3) Describe what you see?
Q4) You should have observed at first the spheres attracted. Use the fact that the spheres
are coated with graphite which is a conductor to explain why at first the spheres attracted.
Draw a picture illustrating your explanation.
Q5) Once the spheres touched you should have observed that they then repelled. Explain
why.
Gently touch each of the spheres to remove the charge from them. And separate them to
maximum separation.
1
Coulomb’s Law
493736061
Page 4 of 6
6/29/17
4. Quantitative Measurements
In this part of the lab we will now investigate some of the features of the electrostatic
force quantitatively. You need to perform each trial (in italics) rapidly but carefully so
that you obtain good data.
Turn on the power supply and rotate and adjust the voltage so that it reads 6 kV.
Touch each of the spheres successively for several seconds with the probe. Remove the
tip, turn down the voltage and turn off the power supply. Rapidly but gently move the
sliding sphere so that it is at the 20 cm mark. Measure the torsion angle by turning the
knob so that the pan returns to the equilibrium position. Record the distance and the
torsion angle in the data table below.
Distance (cm)
Torsion Angle
Force
Discharge the spheres, and return them to maximum separation. Repeat the procedure for
separations of 14, 10, 9, 8, 7, 6, and 5 cm respectively.
Q6) Did the force become stronger or weaker as you moved the spheres together?
Q7) Does this suggest that there is a direct or inverse relationship between the force and
the distance? Explain.
Use equation (1) to calculate the force for each trial and fill it in the data table.
Open LoggerPro 3.1 . Click OK to proceed without the interface. You should have a
data table in which you can enter your force and distance data to construct a graph of
Force vs. Distance. (Remember when we say Force vs. Distance, Force is the vertical
axis and Distance is on the horizontal axis.)
1
Coulomb’s Law
493736061
Page 5 of 6
6/29/17
Q8) Does your graph appear to agree with your answer to Q7)? Explain.
We expect some sort of inverse relationship. To find the power, click on the Analysis
menu and then Automatic Curve Fit. Choose a Power fit, click on Try fit, and then
accept the fit.
Q9) Record the power given by the fit.
Q10) Is the exponent you obtained close to -2? (It will very unlikely be exactly -2)
An exponent of -2 would mean that there is an inverse square law between the force and
the distance.
Q11) Identify any likely sources of error in this experiment. Do not use meaningless
catchalls like “experimental error, “measurement error”, or “human error”. Be specific.
Think carefully about how the experiment was conducted and what factors and
assumptions might not have worked perfectly.
Q12) Complete the following relationship between the Force and the distance
F ~ ____________
1
Coulomb’s Law
493736061
Page 6 of 6