Physics 360 Neumann Virtual Lab 1: Electrostatics
Name(s): ____________________________________________________________________________
PURPOSE To explore attractive and repulsive electric forces, and to practice explaining how objects can be charged by the means of direct charge transfer (friction or conduction) and induction. The following link will be useful: https://phet.colorado.edu/en/simulation/balloons-and-static-electricity Part I: Exploring Electrical Interactions 1. For this part of lab, you will need a plastic hair comb (not a brush, not a metal comb),
a piece of a PVC pipe, or an inflated rubber balloon (not the crinkly shiny kind), which you will have to rub on your hair. You can substitute a woolen blanket or sweater, or a fluffy carpet or rug instead of using your hair. You will also need at least 3 materials from the following list: small paper pieces (about the size of the circles made by a 3-hole puncher), small pieces of aluminum foil, a paper clip (plastic or metal), small pieces of Styrofoam or a Styrofoam peanut, rubber band cut into small pieces, running water (use a steady stream that does not splash or spew around), your arm or leg (not covered by cloth), your hair.
2. Rub the comb (or PVC pipe or balloon) vigorously on your hair. Avoid touching the
comb to anything that is not part of the experiment. Is the comb (PVC, balloon) positively or negatively charged? How do you know? Use the triboelectric series to the right (adapted from Keyence.com), to help you determine the direction of electron transfer.
3. Describe exactly the mechanism behind charging the comb. Is it conduction, friction, or induction? Which
ways do the electrons travel? 4. Lay the small pieces of materials/objects on a level surface, and bring the comb close, without touching. In
the case with running water or your arm/leg, make the comb almost (but not quite) touch them. Describe the interaction in the first three columns of the table below. You are welcome to add extra rows if desired.
5. In the last column of the table, describe in as much detail as possible what happens with the charges in the
objects/materials you picked. Hint: think of polarization, charge transfer, induction, etc.
Material Affected? (Yes or No)
Comparative observations about the strength of the reactions
What happens with charges?
6. Examine the picture below. The model’s hair is attracted to the plastic comb that was previously charged by
it. Describe in detail, what happened to the electrons during charging, to cause this reaction.
Part II: Charging a Peanut by Induction Method Read this bit of theory before going on: If a charged object is brought near, but not touching, a neutral object, the protons and electrons inside the neutral object experience electrical forces that push them in opposite directions. The result is a migration of charges inside the neutral object, as shown in this diagram:
This separation of charges, where an object has one side more positive than the other, is called polarization. We can describe the pith ball in this example by saying “it has been polarized.” Notice that the polarized pith ball is still neutral. By contrast, when electrons are actually transferred by physical contact, when electrons actually move from one object to another, the process is called conduction. Conduction is the charging process you have used up to now in this experiment when “charging by contact.”
(Charging without contact: polarizing while grounding.) 7. A neutral Styrofoam peanut is suspended from a string. A PVC pipe that has extra electrons placed on it (using
direct contact via friction) is brought close. The pipe is held not touching, directly under the peanut attached to string, as shown in this diagram.
8. Draw “+” and “-” signs in this diagram to illustrate the polarization that is happening inside the peanut.
Is the peanut charged? _______ Describe the electrical state of the peanut: The peanut is ________________.
Charged pipe under the
neutral peanut
9. With the PVC pipe still under the peanut, the top side of the peanut is now touched with one finger, and then the finger is removed, while the PVC pipe is still under the peanut.
10. Finally, move the PVC pipe away from the peanut. 11. The peanut is now charged. What type of charge (positive or negative) is
on the peanut? _______________
Grounding one side.
12. So how did this happen? How did the peanut obtain a charge if the negatively charged PVC pipe never physically touched it? Use the diagrams below to illustrate how it happened.
• Illustrate electrons moving from one place to another. • Draw “+” and “-” signs to illustrate charges, as they move or as they are left behind.
Pipe under, but not touching the peanut
You touch and ground one side of the peanut
Finger and PVC pipe gone
13. When a neutral object is charged by contact with an already charged object, how does the polarity of the
charge acquired by the neutral object compare to that of the charged object that touched it? 14. When a neutral object is charged by induction by first polarizing it with an already charged object, how does
the polarity of the charge acquired by the neutral object compare to that of the charged object that polarized it?
Part III: The mini-experiments 1-3 In this part, you will summarize the mini-experiments that you see in the video made by Professor V. Sergan – posted on Canvas: https://www.youtube.com/watch?v=JOnYVEV0csk&feature=youtu.be Use the preliminary results/questions that you discussed in Parts I-Ii, as well as the video to answer the following questions. Answer each question with one complete sentence. Experiment 1 – Force between charged objects 15. What kind of charge does the rubber rod acquire when you rub it with wool? Why? 16. What happens when you bring the second rubber rod in? 17. What kind of charge does the glass rod acquire when you rub it with silk? 18. What happens when a charged rubber rod and a charged glass rod are in the near vicinity of each other? 19. What happens when two charged glass rods are in the near vicinity of each other? 20. What does this experiment tell you about the existence of two different types of charge and of the interactions
between like charges and unlike charges? Experiment 2 – Force on an uncharged dielectric 21. Does a charged rod have any effect on a neutral rod suspended from a stirrup? 22. Explain why or why not. Experiment 3 – Force on an uncharged metallic object 23. Does a charged rod have any effect on a piece of aluminum foil suspended from a stirrup? 24. Explain why or why not. 25. How could this have happened if the foil was neutral in the beginning and not touched with the charged
object?
Part IV: The Electroscope Read this bit of theory before going on (copyright by Department of Physics and Astronomy, Sacramento State University). All reproductions and redistribution is prohibited without permission by the Department of Physics and Astronomy, Sacramento State University. The electroscope is a simple device used to detect charge. The main part of the electroscope is a very thin (about 10−7 m) piece of gold leaf hinged to a metal plate. When there is an excess of charge (a net charge) on the system, the force of repulsion of like charges causes the leaf to diverge from the plate. The tangent of the angle of divergence is roughly proportional to the amount of excess charge, with a 45 degree angle corresponding to a charge of roughly 10−9 C, (which corresponds to a voltage relative to the case of about 1000V). Thus we have a semi- quantitative way to measure charge.
The extremely fragile gold leaf has to be protected from drafts and dirt, so it is enclosed by a case with glass windows for viewing. The case and windows also must shield the leaves from undesired external electrical fields, which might be caused by nearby charged objects. For this reason, the case is made of metal, and the glass windows have been given an electrically-conductive coating. In order to prevent uncontrolled charge buildup on the case, one can ground it once in a while by touching it with a finger. The leaf and plate are electrically connected to a spherically shaped external terminal by means of a short metal support rod that passes through a plastic insulating bushing in the case. This external terminal provides the experimenter with a means of transferring charge to and from the leaf/plate, without opening the case.
There is also a metal cap, which can be placed over the terminal ball without touching it, but by resting on the case, makes electrical contact with it. With the cap in place, the entire leaf, plate, and terminal system is completely surrounded by a conducting surface that shields it from external electrical fields.
These are the links to three short YouTube videos that illustrate the usage of the electroscope (same links are available on Canvas): Boseman Science, Electrostatic induction: https://www.youtube.com/watch?v=dwJ-MM7yu4E Rimstar, Inductive Charging: https://www.youtube.com/watch?v=-JsVZwc1dOo GPB Education, Friction, Conduction, Induction: https://www.youtube.com/watch?v=7-tW9AOcXT4 Video by Professor Sergan, posted on Canvas: https://www.youtube.com/watch?v=JOnYVEV0csk&feature=youtu.be 26. Using the information in the passage above, your knowledge of charge transfer, and the four videos, explain
in a few sentences, how to charge the electroscope to a positive net charge, via the two methods. Make sure to talk about the direction of electron movement. You should also draw some simple diagrams to illustrate charge transfer, polarity, etc.
By direct charge transfer: By induction:
27. Describe a method that can be used to determine the polarity of charge on the electroscope. Some diagrams again might be helpful.
28. In Experiment 4 (Charge detection by an electroscope), the rubber rod was charged and slowly brought near the ball of the electroscope, but without making contact. Describe what you observed. Explain what happened from individual charges perspective, and draw a diagram.
29. In Experiment 5 (Charging the electroscope by contact), the rubber rod was charged, and the charge was
transferred to the electroscope. Describe what you observed. Explain what happened from individual charges perspective, and draw a diagram.
30. In Experiment 6 (Determining the polarity of charge on an electroscope), the electroscope was once again
charged using the rubber rod. Then a glass rod was slowly brought nearby. Describe what you observed. Explain what happened from individual charges perspective, and draw a diagram.
31. Where does the charge go when you touch the ball and the casing of the electroscope in Experiment 7
(Discharging the electroscope)? Answer with one sentence. 32. What does the aluminum cap do in Experiment 8 (Electrostatic shielding)? Answer in one or two sentences. 33. In Experiment 9 (Charging by induction), record your observations for each step (as described below), and
write a one-sentence explanation for each, from the perspective of charges. “With one hand ground the scope terminal to the case (ie. Have one finger touch both the case and the terminal.) Keep it there while you bring a charged rubber rod towards the electroscope terminal, but not touching the rod to the terminal, with you other had. Observe the leaf. Now, while the rod is held near the terminal, remove your grounding hand from the electroscope completely (without changing the position of the charged rod). Observe the leaf. Finally, slowly withdraw the charged rubber rod. Observe the leaf. What happens? Is there a final charge on the scope? Test to determine the sign of the charge on the scope. What is it? Is it the same as the charge you would get b the direct contact method? Explain and draw diagrams showing what has happened. How did the charge get on the electroscope?”
