specializes in making a physics lab

Lab 3: Electrostatics with Aluminum Balls (2)

By the end of this activity, you should be able to do:

1. Hands-on opportunity to optimize an electrostatics setup

2. Make observations of Coulomb’s Law by removing charge from one Aluminum ball

3.1 Goals

1. Transfer the maximum charge, so that each Al sphere has QQ. Then ground one Aluminum ball, such that after redistribution of the charges, you have Q/2Q/2 on each Al sphere.

2. Determine the Coulomb force FF as a function of distance rr due to different charges

3.2 Prediction

1. Given the approximate charge QQ from the previous lab and the mass mm, and distance rr, find the Coulomb force FF for that point. Compute the Coulomb force FF for the different charges Q/2Q/2 and Q/4Q/4.

2. Make a graph of the computed values FF versus rr. Label the x- and y-axes.

3.3 Materials

1. Al foil, scissors, tape

2. Fine insulating string (about 2 m)

3. PVC rod and fur, sweater, wool, or socks

4. Metric ruler

3.4 Procedure

1. Create the same setup as from Electrostatics with Aluminum Lab (1) and transfer the maximum charge to the Al balls;

2. Measure the separation distance rr (from where to where?);

3. Ground one Al ball, after the collision, each Al ball will have Q/2Q/2 charge (granted they are the same size);

4. Measure the reduced separation distance r2r2;

5. Repeat the last two steps to measure Q/4Q/4 until the Al spheres touch each other;

6. Based on the charge and the separation distance, compute the force FF and graph as a function of rr.

Milestone: Once you have created the setup with the Al balls at the end of the string and are able to transfer charge, call the instructor.

3.5 Measurement

Create a table with columns similar to this table. Here QQ refers to the transfer charge and we will call the first charge QQ, then next charge Q/2Q/2, etc. The qq values is the calculated charge based on your model. Note that in the first row, QQ=qq, but in the next row, Q/2Q/2 is the expected value, whereas q2q2 is the experimental value that you measured.

3.6 Graph

Make a graph of the charge qq versus the separation distance rr. The qq values are experimental and therefore data points. Secondly, add the computed values of QQ, Q/2Q/2, Q/4Q/4, etc., the first value will be the same. Then, add a curve with the predicted function Q(r)Q(r) or simply compute additional values of 0.7QQ, etc.

Pay attention that the x-axis and y-axis of your graph uses a linear scale.

3.7 Discussion

Share the highest measured value of QQ with units. Compare it with the textbook value. What was the separation distance rr? How did you measure the separation distance rr, can you state precisely from which to which point? How large were the Al balls? What are some of the challenges in this experiment? How could you find the power law exponent of Coulomb’s law with this technique? What are some suggestions to improve the uncertainty of the measurement? Which observable contributes the most to the uncertainty of the charge qq?

3.8 Summary

Report the charge qq that you can remove. Describe your model of the atom. Comment on the behavior of charge and separation distance. How valid is your model? What is the connection of these results with Coulomb’s Law?

3.9 Additional Reading

· Triboelectric Series: https://www.alphalabinc.com/triboelectric-series/