Submitting assignments Schedules Course Info Ch. Reviews Guides Problems Labs Videos AP Info Announcements Contact Shortcuts WebAssign Canvas Equations Lab FAQ Software IWP

 L153.  Investigations in Electrostatics For Parts A and B, advance reading of the textbook is actually discouraged. Part C, on the other hand, will require some textbook reading.

As you work, write your observations and conclusions in a Word document. Number and letter your responses the same as given in the instructions.

Goals

To investigate how objects acquire electric charge and what principles govern the interactions between charged objects

Prelab

Introduction

You start this lab without having done any reading about electrostatics in the text. We realize, however, that you have probably acquired some of the basic ideas about electricity just from experience (e.g., receiving mild shocks after shuffling your feet over a rug or having seen television commercials about "static cling"). You also probably know the number of kinds of electric charge, the laws of attraction and repulsion, and some of the basic ideas of atomic theory.  In doing this lab, however, we ask you to put aside your preconceived ideas about electricity and electric charge and see what you can discover just from the activities contained within. The activities are simple but can provide insights into the phenomena of electrostatics beyond what is presented in the textbook.

Experimental Considerations

As you carry out your observations, keep in mind the following points; they may keep you from making incorrect conclusions and wasting time.

• When you rub objects from the list below, you have to rub vigorously in order to produce the effects that are needed.
• Some objects may exhibit different effects in different areas. When you test for these effects, be sure to test the area you actually rubbed. You can also rub the entire object uniformly if that's feasible.
• At several points in the lab, you'll be using two strips of sticky tape, called top and bottom tape. It's possible for the effects produced by these two kinds of tape to become reversed by inadvertently touching them to other objects. It's wise to check the tape pieces occasionally to be sure that a reversal has not occurred.
• Try each activity at least twice to verify results. The activities are short and take very little time to repeat.

Theoretical Considerations

We need to make a distinction between observations and inferences. Strictly speaking, an observation is something detected with one or more of the five senses. In this lab, your observations will be made primarily by sight although sound and touch can also play a role. An inference is a conclusion based on observations.  Suppose for example that a student walks into the classroom soaking wet. That's an observation. You may conclude that it's raining outside. That's an inference (assuming you can't see outside). While the observation can't be refuted, there are other possible inferences. Perhaps the principal shot the student with a super soaker in the hallway.

A principle (not the same as principal) is something that explains a category of observations. Scientific principles are examples. From observing the fact that objects fall to the ground when we release them, we might formulate the principle that all objects are attracted to the Earth. This is a form of reasoning called induction in which a number of specific observations are used to formulate a general principle. The principle is, of course, subject to additional testing. When we observe a helium balloon floating upward, we either have to abandon our principle or revise it. We always try to revise it first. In the case of the balloon, we might postulate that there were an upward force on the balloon. (This would be correct; the upward force is buoyancy.) We realize there are other forces that have to be considered together with gravity in determining how an object moves.  So we would revise the principle to include the effects of other forces. Eventually, we might end up with something like Newton's 2nd law.

A principle can be used to make predictions of new observations. For example, if we saw five students come into the classroom wet and inferred that it was raining outside, we could use that principle to predict that the next student to come into the classroom would be wet. This is a form of reasoning called deduction. We start with a general principle and predict a specific observation. Of course, deductions can be wrong. The next student might be dry, because she used an umbrella.

Throughout this lab, you'll be making observations and inferences, formulating principles using induction, and making predictions using deduction. Try to be as objective an experimenter as you can. Don't let your preconceived ideas about what should happen influence what you observe. In writing your observations and principles, avoid use of words like electrons, positive, and negative for which you don't have direct experience. Of course, we'll eventually be using these words, but for Part A, they're off limits. Also, go beyond the obvious observations of "this thing attracted or repelled that thing".  Of course, those are interesting and important observations. However, also note the strengths of these effects and what you can do to make them larger or smaller. You can also include observations that you make with senses other than sight.

Equipment

You'll need to gather the following items from your lab kit.

• Orange or brown nylon thread (This is in the ziplock bag.)
• Roll of sticky tape
• 2 Styrofoam cups
• Balloon
• Square of aluminum foil
• Saran wrap
• Aluminum pie plate
• Sheet of Plexiglas
• Animal fur
• Hard plastic ruler

Part A

Investigation I

1. Suspend a Styrofoam cup by taping a piece of thread to it. Then rub the outside of the cup vigorously with the fur. Rub another cup with fur and bring it near the suspended cup. What do you observe?

2. Tear a 10-15 cm strip of sticky tape from a roll and stick it down firmly to the table top, leaving a short overhang to pick it up again. Run a finger along the entire length of the tape, except for the overhang, to make sure the tape sticks completely to the table. Strip it off the table top, being careful to hold only the end with your fingertips. Without touching the tape anywhere else, hang the strip of tape vertically over the table’s edge such that the non-sticky side faces outward. If your tape touches your hand – or anything else – you'll need to start the procedure over. Repeat the above procedure with another strip of tape and then bring this strip of tape near the first strip (the non-sticky sides should face each other). What do you observe when the strips are far apart? when brought close together?

To reactivate the sticky tape, repeat the procedure above. Tape can be reused. Reactivate the tape strip whenever something inadvertently comes in contact with it.

1. Formulate principles from your previous observations. Base them on your observations rather than on previous knowledge. Remember that words like negative and positive are not allowed at this point. In order to illustrate this process, here's an example of a principle you may have formulated based on your observations above:

Principle A: Similar objects rubbed in the same manner repel.

Now write a principle having to do with the strength of the forces that you observed. Name it Principle B. Name successive principles with a continuation of the lettering sequence. This will make it easy to refer to them.

Investigation II

1. Rub Saran wrap vigorously against the sheet of Plexiglas. After rubbing, let the plastic wrap lie on top of the Plexiglas, and then turn the Plexiglas square over. What happens?

2. Again rub the Plexiglas square with the Saran wrap. Now bring the Saran wrap near a tape strip prepared as in step 2 of Investigation I. What happens? Bring the rubbed surface of the Plexiglas near the tape strip. What happens?

3. Take one of the tape strips and stick it back down to the table. Mentally label this tape strip B. Tape another strip directly on top of the one already on the table. Mentally label the top piece with a T. Now unstick both strips from the table at the same time without unsticking the strips from each other. Run them over your lips or a piece of metal.  (In making this unusual request to 'kiss' the tape, we're using our own knowledge of electrostatic effects in order to produce a particular, unseen result. You may accept it for now, or you may ignore it. The experiment will probably work even if you don't kiss the tape.) Pull the two tape strips apart quickly. Then bring the pieces of tape toward each other, non-sticky sides facing. What do you observe as you bring the strips closer together?

4. Hang both the B and T tape strips vertically over the table’s edge such that the non-sticky sides face outward. We'll refer to these as the test strips. Prepare another pair of top and bottom strips by repeating the procedure in step 3. What do you observe as you bring the new top piece near each of the original B and T test strips? as you bring the new bottom piece near the test strips?

We're going to introduce some new words in order to make it easy to talk about what you did by rubbing the cups and Plexiglas and by sticking and unsticking the tape. One word is charge (used as a verb).  When you charge an object by rubbing, unsticking, or other actions to be determined, you alter it in such a way that it exhibits effects that we'll group together under the term, electrostatic effects. Right now, these are just words. They don't explain the underlying mechanisms that cause the effects. We will also say that an object is charged when it exhibits such effects as a result of rubbing, unsticking, etc. Finally, we'll use the term charge (as a noun) to mean whatever an object has that causes it to exhibit electrostatic effects.

1. With the above definitions in mind, here's a question to answer by experiment:  Is the tape charged as it comes off the roll? Design a method for deciding and then try it. Describe your method and your findings. Does the off-the-roll tape have the same charge as the B or the T strip? Note that answering this question is a deduction based on a prior principle or a generalization of observations. Therefore, use the principle or generalization in explaining how you come to your conclusion about the charge of the off-the-roll tape.

2. Rub the exterior of a Styrofoam cup with fur. Bring the rubbed cup close to the B test strip. What do you observe? Predict what would happen if the cup were brought near the T test strip. Then try it and describe your observations.

3. Which of the observations in Investigation II can be explained with the principles formulated in Investigation I? What new principles can you write? Remember to continue the lettering sequence for principles that you've already started.

Part B

Investigation III

1. Rub a Styrofoam cup with fur. Pass the cup over one of the following:  a) the back of your hand, b) another person’s hair. What do you observe?

2. Rub an inflated balloon with fur and bring the balloon near various objects in order to observe its effects. How are these effects similar or different if you rub the balloon against your hair instead? Bring the rubbed balloon near but not touching a thin stream of water. Describe what you observe.

3. Tear some scrap paper into very tiny bits and place them on the table top. Rub a plastic ruler or the Plexiglas with fur and bring the rubbed strip near but not quite touching the bits. Careful observation is required here in order to see everything that happens. Describe what you observe.

4. Which of your observations in steps 1-3 of this investigation can be explained with principles that you formulated previously?

5. Formulate any additional principles that you need to explain your observations in this investigation.

6. Tear off about a 6" square of aluminum foil. Bring it near both top and bottom test strips. How are the strips affected?

7. Rub the Plexiglas with fur. Place the piece of foil on the Plexiglas and then turn it over. What does the foil do? Now remove the foil and bring it near the test strips. Is the result different from the previous step?

8. How can you explain your observations with the foil using the principles you've devised?

Part C

Investigation IV

Now it's time to bring in atomic theory to help explain at a fundamental level what is happening to produce the effects that you've observed. For that purpose, read sections 19-1, 2, and 6 of your textbook. From this point on, you will need to use terms such as positive, negative, neutral, electron, ion, polarization, conductor, insulator.

1. Here’s a check on what you learned in the previous investigations. In those investigations, you tested the effect of a number of charged objects on top and bottom tape strips. If an object repelled the bottom strip, you could be certain that the object had the same charge as the bottom strip. However, if an object attracted a bottom strip, that test alone was not enough to determine the charge of the object. Explain why not.

2. Rabbit fur is the most electropositive material of the materials you've been using. This means that, when rubbed against other materials, rabbit fur acquires a positive charge. That is, it loses electrons to the material it is rubbed against. If the rabbit fur is rubbed against the balloon, the fur acquires a positive charge, while the balloon acquires negative charge. Knowing this fact, devise a way to determine which of the test strips has positive charge and which has negative charge. (i) Describe your method, (ii) carry it out, and (iii) give your results.

3. Which of the following objects is neutral:  balloon, aluminum foil, Styrofoam, Plexiglas? (That is, it affects top and bottom strips the same way no matter what you rub it with.) Give an explanation for your results. It's both appropriate and necessary at this point to use some of the concepts from the textbook reading.
1. Many of the materials that you've been using can be arranged in an electronegativity series. (See Table 19-1 in the text for an example.) Rabbit fur would be the least electronegative material (or most electropositive) material in such a series. It's important to understand that electronegative and negative (or electropositive and positive) do not describe the same property of a material. Electronegative describes a materials ability to acquire electrons. Negative describes the excess of electrons that a sample of a material actually has. Thus, material X may be more electronegative than material Y, but material Y can have a greater negative charge than material X. Suppose, for example, that you have three initially neutral samples of materials A, B, and C and want to arrange them in an electronegativity series. After rubbing A against C, you test both materials with test strips. You find that A is positive and C is negative. That means C is more electronegative than A, because C took electrons from A. Next, you rub B against C and test both materials with test strips. You find that B is negative and C is positive. The charge of C changed after being rubbed with B. That means B is more electronegative than C, because B took electrons from C. The electronegativity series from most to least electronegative would then be B, C, and A. If you rubbed materials A and B together, what would their charges be? Explain.

2. Why wouldn't the method just described work to determine the position in an electronegativity series of the aluminum foil or pie plate?

3. Carry out the experiment described in step 4 to order these materials in an electronegativity series:  balloon, Plexiglas, Styrofoam. Much rubbing may be required to make a material switch from one charge to another. This seems to be particularly true of the balloon and Styrofoam.