Verifying Coulomb's Law

Source: Adapted from PSSC Physics Laboratory Guide


Theory: Back in the late 1700's, Charles Coulomb developed a mathematical expression to describe the attraction and repulsion of electric charges, namely:

F = k q1 q2 / r^2

That is to say, the force between two charged objects is inversely proportional to the square of the distance between them.


Objective: In this experiment, you are going to demonstrate the veracity of Coulomb's Law. To do this you will show that the Electrostatic Force between two charged bodies is inversely proportional to the square of the distance between them.


Apparatus: In this experiment, you will use the following materials:


1 Suspended Pith Ball (A)

1 Chamber for the Suspended PithBall

1 Mounted Pith Ball (B)

1 Wood Blocker (C)

1 Plastic Comb (not shown)



Experiment Background: Without expensive and/or time consuming apparatus, it is difficult to directly measure electrostatic forces. We can, however, do so indirectly. Force diagrams for the Suspended Pith Ball, before and after the introduction of the other charge, are seen below.


Since the angle theta is relatively small, the Electrostatic force is almost parallel to the separation distance, d. Using this approximation, we see that the force diagram and objects diagram are similar in the geometric sense (AAA similarity). This implies to us that:

Electrostatic Force is proportional to Distance Ball is Displaced

The distance the ball is displaced is easily measured, as is the separation between charges (r).


Procedure: Place a charge on a comb by rubbing it through the hair on the back of your head. Transfer this charge to the Mounted Pith Ball. Use the Mounted Pith Ball to transfer its charge to the Suspended Pith Ball. Now recharge the Mounted Pith Ball again.

Slide the Mounted Pith Ball into the chamber and take down all data in the data table. Repeat this step, changing the separation (r) slightly each time, until you have completed a total of 10 trials.

Now, recharge the Mounted Pith Ball. This time, transfer half of the charge on the Mounted Pith Ball to another pith ball. Repeat the above procedure using the half charged Mounted Pith Ball, until you have completed 10 more trials, taking down all data.


Report: Graph your results using a computer program such as Graphical Analysis or ClarisWorks (or on a piece of graph paper). At least one plot should be of Electrostatic Force vs. Inverse Square of Separation Distance.

Write up your lab report and turn in one week from now. The report should address the question of whether or not your experiment showed the validity of Coulomb's Law. Include the following sections at a minimum: Title/Name, Introduction, Procedure/Apparatus, Data/Results, Discussion. Make sure to include all sources of error in the report.





Coulomb's Law

Electrostatic Force Experiment Data Table