I: d \"We\" 1: INTERACTIONS OF ELECTRIC CHARGES CHAPTER 1 THE INTERACTIONS OF ELECTRIC CHARGES In this chapter we will examine experimentally the basic aspects of electric interactions. Using three fundamental physical principles. conservation of charge. Coulomb's law, and the superposition principle. we will attempt to explain the results of our experiments. We will find that certain observations cannot easily be' explained. 1.1_ The basic properties of electric interactibns Whether or not you have previously Studied electricity. you have probably heard somewhere that electric interactions have the following basic properties: - There are two kinds of charge. called \"+\" and \"\".~ ' Like charges repel. unlike charges attract. The electric force: - is proportional to the amountsot' both charges. 0 acts along a line between the charges. and 0 decreases rapidly as the distance between the charges increases. Note that the description above is an informal one: later on we will make it more precise. We need a simple. inexpensive. reproducible method of producing electrically charged objects so that we can further explore the properties of electric interactions. A possible material is \"invisible tape" such as Scotch brand Magic\" Tape. (Note: cheaper generic brands of invisible tape often work better than Scotch Magic Tape for our peculiar purposesl You may have noticed that invisible tape frequently sticks to your hand when you pull a strip off the roll. it this behavior is due to electric interactions. then this tape may be a suitable experimental material. ' we will be using very simple apparatus. yet our experiments will raise fundamental questions about the nature of the electric interactions of atoms and molecules. 1.2 Is invisible tape electrically charged? tn the following sections we will see whether invisible tape exhibits all the properties of electric interactions in the list above. If it does. we can conclude that the tape becomes electrically charged when we strip it off or another piece of tape. and we can study the behavior of pieces or\" tapelas a concrete example of electric interactions. ' 1.2.1 Preparing a \"U" tape Use a strip of tape aboutzt) crn-or 8 inches long (about as long as this paper is widet. Shorter pieces are not exible enough. and longer pieces are difcult to handle. Fold over one end of the stnpto make a non-sticky handle: 1: INTERACTIONS OF ELECTRIC CHARGES Cl Prepare a U tape as follows: ' Stick a strip oftape with a handle down onto a smooth flat surface such as a desk. Smooth this base tape down with your thumb or fingertips. This base tape tarot-tides a standard surface to work from. (Without this base tape. you get different effects on different kinds of surfaces.) Stick another tape with a handle down on top of the base tape. Smooth the upper tape down well with your thumb or ngertips. Write U (for Upper) on the handle of the upper tape. With a very quick motion. pull the U tape up and off the base tape. leaving the base tape stuck to the desk. Your hand and a U tape Cl Hang the U tape vertically from the edge of the desk. and bring your hand near the hanging tape. In the box provided. write a brief description of what happens; Does it matter which side of the tape you approach? - .7..la If the tape is.in good condition and me mom IS not too humid. you should find that there is an attraction between the hanging strip and your hand when you get close to either side of the tape. If you don 't .te'e an attraction. re-make the tape. You can reuse the same piece of tape by again sticking it down on the base tape and then pulling it off. as follows: Standard procedure for making a" _U tape STANDARD PROCEDURE FOR MAKING OR REMAKING A U TAPE 0 Run your nger along the base tape that is on the desk. - Stick the U tape down on the base tape. smoothing the tape down well. - Jerk the U tape very quickly off the base tape. - Test to make sure that the tape is attracted to your hand. Upper tape am==u Base tape Pull rapidly \\ 7 a 1: INTERACTIONS OF ELECTRIC CHARGES 1.2.2 Two U tapes: Repulsion or attraction? Il'U tapes are electrically charged. how would you expect two U tapes to interact with each other? Would you expect them to repel each other, attract each other. or not to interact at all? Circle your prediction. and briefly state your reason. in the box below: Prediction: ' -.- attraction repulsion: no interaction I I. The \"at\" symbp' nit-nits that 'th't! on .t'ld should check this a- -. . ll. the back of the chapter. In order to really learn the material, you must always make a serious attempt before checking your work. But do check and correct your work periodically. or as you go along. CI Prepare a second U tape in exactly the same way (remember to write U on the handle). Bring it near the hanging U tape. Since the hanging tape is attracted to your hands. try to keep your hands out of the way. For example. you might approach the vertically hanging tape with the other tape oriented horizontally. held by two hands at its ends. Describe what happens: You should nd that the two U tapes repel each other rather strongly. If the air is very humid. you may have to do the experiment very fast. C] After observing the repulsion. hang the second U tape beside the rst. If you don't observe repulsion. re-prepare the tapes as explained above. Sometimes when a piece of tape rst comes off the roll it is initially in a state that interferes with the experiment. but repeating the standard procedure usually produces reproducible results. 1.2.3 Direction of force We are told that electric forces between two objects act along a line drawn from one object to the other. This may sound a little obvious: perhaps you can't think of a force that acts at an angle to the line between two objects. However such forces do exist. as we'll see later (in Chapter 12}. Let's see what happens with our tapes. Force along line between objects Force at an angle to line between objects 1: INTERACTIONS OF ELECTRIC CHARGES Suspend a U tape from a thread or a hair. Hold the thread or hair in your hand, or use a short piece of tape to stick the upper end of the thread or hair to the desk. Approach the suspended tape from various directions with another U tape. Do you find that the force does indeed act along a line drawn from one object to the other? Hanging from thread or hair Force along line connecting the two charged objects? F ? 1.2.3a * 1.2.4 Two U tapes: Effect of distance Move a U tape very slowly toward a hanging U tape. Observe the deflection of the hanging tape from its original position, at several distances (for example, the distance at which you first see repulsion, half that distance, etc.) Make a very rough graph of the strength of the repulsive interaction as a function of the distance between the two tapes (you can't really go all the way to 0 distance!). The deflection of the tape away from its original vertical position is a measure of the strength of the interaction. Deflection from original position (measure of U-U repulsion) Distance between the two tapes 1.2.4a Note: The real world is messy You may have noted several difficulties in making accurate measurements of displacement versus distance. For example, the tapes are both attracted to your hand, as well as repelling each other. If you tried to use a ruler, you may have found that the tapes are attracted to the ruler, too. For now, we'll settle for rough observations, but keep these difficulties in mind.6 1: INTERACTIONS OF ELECTRIC CHARGES 1.2.5 Effect of amount of charge You may have already discovered that if you handle a U tape too much, it no longer repels another U tape. Next we will learn a systematic way for getting this to happen. Making a tape not interact O Make sure you have an active U tape. Hold onto the bottom of the U tape and slowly rub your fingers or thumb back and forth along the slick side of the tape. Describe the changes in how this U tape interacts with your hand and with another U tape: with hand with another U tape 1.2.5a So we have a way of making a fape nui interact with other ordinary objects. which will be useful in the future. If the U tape was electrically charged, as we suppose, then by running a finger along the slick side we have apparently "neutralized" it-it now appears electrically neutral (uncharged). It will be a while before we can explain this effect. Decreasing the "amount of charge" Let's partially neutralize a U tape and see what happens: Prepare two charged U tapes. Hang one of them from the desk, and note how strongly the other tape repels it. Partially neutralize one of the tapes by running your finger along the length of the slick side of the tape, being careful that your finger touches only a portion of the width of the tape. Again observe how strongly the two tapes repel. What is the effect of this partial neutralization? Circle your observation: weaker repulsion stronger repulsion no effect 1.2.5b You should observe that the two tapes repel less strongly when you partially neutralize one of them. We'll be more quantitative about this later, but for now you see that to a certain extent we can control the amount of "charge" on a tape, and that the force between two tapes seems to depend on the amount of charge on each tape (since there has to be some charge on both tapes in order to get a repulsion). 1.2.6 Unlike charges So far we have observed that two U tapes repel each other, that the force acts along a line between the tapes, that the strength of the repulsion decreases as the tapes get farther away from each other, and that the strength of the interaction depends on the amount of charge on the tape. These observations are consistent with the hypothesis that the U tapes are electrically charged, and that all U tapes have like electric charge.1: INTERACTIONS OF ELECTRIC CHARGES 7 How could you prepare a tape that might have an electric charge UNLIKE the charge of a U tape? Think of a plan, and outline it below, before going any farther in the book. (Don't cheat yourself by looking ahead-the point is to think for yourself!) : : Perhaps you reasoned along these lines: We don't know how the U tape became charged, but if the tapes started out neutral, maybe the U tape pulled some charged particles off of the bottom tape (or vice versa). So now the bottom tape might have an equal amount of charge, of the opposite sign. There are a number of assumptions implicit in this line of reasoning, for example the idea that neutral matter is made up of both positively and negatively charged particles, and the notion that it might be possible to remove charged particles from a surface. We will discuss many of these ideas in greater detail later on. However, we note a particularly important one here: the charges on the U tape were not "created" when we pulled the tapes apart; they already existed somewhere. Conservation of charge . . We will have more to say about this later, but here is a simple statement: Charge cannot be created or destroyed: In an extremely wide variety of experiments, no one has ever observed electric charge to be created or destroyed. These results are summarized by the important principle called "conservation of charge": the net charge of the universe, or of any closed system, cannot change. By a closed system we mean any: region through whose boundaries charged particles neither enter nor leave. In this book, we will sometimes need to refer to results from other areas of science. . Such statements will be marked by a double vertical bar on the left of a paragraph, to indicate clearly that we are not fully justifying the results. A full justification would require describing a wide range of mutually supporting evidence coming from a large number of experiments and theoretical analyses. If you continue your studies you may have the opportunity to study the full justification for the results that we describe