LAB ACTIVITIES
Chapter 7
INVESTIGATION 7.2.1
Inquiry Skills
Factors Affecting Electric Force
between Charges
The purpose of this investigation is to determine the relationship between the electric force and the distance between
charges, and between the electric force and the magnitudes of
charges. We start by investigating each relationship separately.
Later in the investigation, we combine the relationships to
yield a general equation for the electric force between small
spherical charged objects. Your teacher may require you to
perform this investigation with lab equipment other than
what is suggested here or with a simulation program in place
of lab equipment.
There are many ways to proceed. You could, for example,
try small, graphite-coated insulating spheres. One sphere is
suspended from a thread, and two identical spheres are
attached to insulating rods. Usually, no equipment is available for determining the magnitude of the charge on each
sphere. However, once you have placed a charge on a sphere,
you can reduce it in half by touching the charged sphere with
an identical neutral sphere. You can measure electrostatic
forces, at least to low precision, with the setup suggested in
Figure 1.
Questioning
Hypothesizing
Predicting
LEARNING
Planning
Conducting
Recording
Analyzing
Evaluating
Communicating
TIP
Hints: Keep the two spheres at the same height. Measure
distances and angles carefully. Redo the experiment if
necessary. Perform the experiment as quickly as you can
without sacrificing accuracy, since your charged spheres
will be leaking charge to the atmosphere as time passes.
Hypothesis/Prediction
(a) Communicate your hypothesis/prediction in words,
mathematical notation, or with a graph.
Materials
ebonite rod and fur
three small, graphite-coated spheres (one attached to a
long thread, the others to insulating rods)
protractor
ruler
balance
Procedure
Part A: Electric Force and Distance
protractor
1. Measure and record the mass of the sphere on the
thread.
v
2. Set up your equipment carefully and check that it will
function properly.
thread
charged sphere
attached to
insulating rod
charged
sphere
r
q1
q2
Figure 1
Setup for
Investigation 7.2.1
Questions
3. Charge the sphere on the thread and one sphere on
the rod. Bring the rod’s sphere close to the hanging
one at the same height. Measure the angle of deflection for the hanging sphere and the distance between
the two spheres.
4. Repeat step 3 several times, using the same charges at
different distances.
(i) How does the electric force between small, spherical
charged objects depend on the charges of the objects
and the distance between them?
(ii) How can this relationship be expressed in a single
equation?
372 Chapter 7
NEL
Unit 3
(d) Find a relationship between the magnitude of the
electric force and the distance between the charges.
Graph the relationship to ensure you are correct.
INVESTIGATION 7.2.1 continued
Part B: Electric Force and the Charges
5. Charge two spheres and bring them together,
ensuring they are at the same height. Measure the distance and angle carefully.
6. Ensure that the third sphere on the insulating rod is
neutral. Touch it to one of the charged spheres.
Repeat step 3, keeping the two spheres the same distance apart. (You will have to change the height
slightly.)
7. Ensure that the third sphere is neutral again. Touch it
to the other charged sphere. Repeat step 3, keeping
the two spheres the same distance apart.
8. Continue with this process using the neutral sphere
until you have enough data or the angle is too small
to measure accurately. If your final data set is too
small, try repeating the procedure by placing larger
charges on the spheres.
(e) Copy and complete Table 2.
Table 2 Constant Distance, Changing Charge
Fraction of q1
1,
1 1
, ,
2 4
etc.
?
Fraction of q2
1,
1 1
, ,
2 4
?
v
FE
?
?
etc.
(f) Find a relationship and a proportionality statement
between the magnitude of the electric force and the
two charges. Graph the relationship to ensure you are
correct.
(g) Combine the two relationships into one proportionality statement.
Evaluation
(h) Comment on the accuracy of your prediction.
Analysis
(b) Draw a free-body diagram for the charged sphere on
the thread. Find the electric force on the sphere in
terms of the force of gravity and the angle v between
the thread and the vertical.
(c) Copy and complete Table 1. Plot the electric force
versus the distance between the charges.
(j) Discuss any discrepancies between what you have
found and Coulomb’s law. Suggest explanations of
these discrepancies where possible.
Synthesis
Table 1 Constant Charges, Changing Distance
NEL
(i) List the most likely sources of random and systematic
error in this experiment. Discuss ways to minimize
these errors.
r
v
FE
?
?
?
(k) If you have used a simulation, is Coulomb’s law consistent with Newton’s third law? Explain and discuss
how it can be verified. Perform the experiment if you
have time.
Electric Charges and Electric Fields 373