Electrostatics Lab Exercise
April - May 2021

Shortly before beginning my graduate program, I decided to use the knowledge from my stockroom and lab instructor positions to improve the Electrostatics lab for PHYS 112 and PHYS 212. My initial goal was to make a few improvements to the lab procedure, but in the end I rewrote the lab entirely. Making changes to labs the department offers requires coordination between the stockroom and faculty, so the new exercise was designed to use equipment already in stock and cover the same concepts at the original.

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Lab Exercise Highlights

Remaining Work

Unfortunately, I left the department before I could see the lab implemented. There are some parts of the exercise which could use more testing for consistency, and the included triboelectric series is in need of additional research or empirical verification.

Learning Goals

Part 1

  • Observe how all neutral objects can be polarized.
  • See the impact of surface area and volume on polarization.
  • Compare polarization between insulators and conductors.

Part 2

  • See localization of charge on an insulator.
  • See the difficulty in discharging an insulator.
  • Use a standard and reasoning to determine the charge on an object.
  • Use and verify a triboelectric series.
  • Observe conductivity of human body.

Part 3

  • Use reasoning to verify whether charge by induction succeeded.
  • Distinguish between polarization and net charge.
  • Understand an object with a net charge can be polarized.

Question Explanations
1) Were there any objects the tape didn't interact with? Was it repelled by any objects? The tape should be attracted to all the objects because all neutral objects can be polarized.
2) What impact does surface area seem to have on the attractive force due to polarization? The sheets of paper and aluminum should show much greater changes in weight than the crumpled pieces. This is because there are more atoms in close proximity to the charged PVC pipe.
3) Was polarization stronger in the paper or the aluminum? Did it matter whether the paper and aluminum were flat or crumpled? There is a lot of nuance to this question. Generally, polarization in insulators is stronger; this should be seen between the crumpled objects, though the difference will be quite small. However, the large surface area of the sheets means the aluminum sheet will likely exhibit stronger polarization than the paper sheet.
4) What implications does this finding have for your previous observations? The scale will give a negative reading due to the attraction from the pipe. This doesn't matter very much for the crumpled pieces because the pipe is further from the scale, but it is significant for the sheets.
5) Why is the tape more attracted to one end of the rod than the other? Because the glass rod is an insulator, charging one side of it won't charge the other side.
6) How did you check whether the tape was discharged? An instruction isn't given for how to discharge the tape to test the students' ingenuity and understanding.
7) Which method(s) was most effective for discharging the tape? Why? It should be very difficult to discharge the tape quickly using any method other than rinsing it under water. Even brushing the tape against a grounded conductor will have little effect.
8) Is the sign of the charge for each object consistent with what the triboelectric series predicts? Explain any exceptions. The pipe will be negatively charged, and the glass rod positively charged. The silk will take on a negative charge from the glass rod but will lose the charge quickly through the student's hand. This will leave the silk neutral, so it will attract the tape due to polarization!
9) Without the triboelectric series, would it be possible to determine if the tape were positively or negatively charged? This is meant as a philosophical question. The answer is no: without a standard of positive or negative charge on hand, the tape could just as well be positive or negative.
10) Why are we using two conductors instead of two insulators? This is a concept check. Charge by induction only works for conductors.
11) Should the PVC pipe be held perpendicular or parallel to the rectangle? Holding the PVC pipe parallel to the long face of one of the rectangles will maximize the charge induced. This will make later observations easier but is not strictly necessary.
12) Do your observations show the metal rectangles were charged? Are the observations consistent with each other? The observations will likely be inconsistent with each other if they heard a spark (which will be very quiet). The spark indicates the rectangles were properly charged. However, the tape will likely be attracted to both rectangles. Why? Because the tape is also polarizing the rectangles!
13) Did the tape interact differently from before? Why or why not? The new instructions are meant to beat the polarization caused by the tape stick. By holding the tip of the tape stick closest to the rectangle, the like charge is moved to the other end of the rectangle, thereby repelling the rest of the tape!
14) Were there any objects in the exercise which couldn't be polarized? No, not even the charged objects as Part 3 demonstrates. It might be reasonable based purely on observation evidence to conclude charged insulators can't be polarized.
15) Was it important for the pieces of paper and aluminum to be similar in mass? What about volume? Explain. The similarity in mass is not directly important but makes measurements and comparisons convenient. It is important they have similar volume, though, so the amount of atoms available to polarize is similar. Technically, the relative molar densities and valence electrons are probably what really matters, but volume analysis is good enough for us.
16) Is it possible for a charged object to repel an uncharged object? No, the uncharged object will become polarized and attract the charged object.
17) If two objects electrostatically attract, does that mean both are charged? What if they repel? If two objects electrostatically attract, it's possible one is neutral and just polarized. Repulsion is only possible, though, if both objects are charged.
18) Is it possible to distinguish between whether an object has been polarized or charged? How? Building off the previous question, polarization can be distinguished from charge by observing repulsion.
19) Why is it important to hold the rod as close as possible to the metal objects when charging by induction? Is this proximity important when polarizing objects? The proximity is important in both cases. Electrostatic forces are dependent on the inverse square of the distance, meaning they diminish quickly as distance increases.
20) Illustrate the charge by induction process using a three-step diagram. The important steps are 1) the two conductors are touching 2) the inducer is brought near one conductor 3) the two conductors are separated.