Holding a charged rod close to the electroscope plate

Transcription

1 Lab 1. Electrostatics Goals To understand and verify behavior of two kinds of charge, denoted positive and negative, respectively. To understand response of electroscope when a charged rod is brought near, so that electrical charges on rod interact with charges already present in electroscope. To visualize charge transfer between charged rods, electroscope, and or objects, and to understand how electroscope is used to compare net charges on two objects. Introduction Electroscopes are used to detect presence or absence of electric charge. They come in various forms, but a picture of a typical electroscope is shown in Figure 1.1. Inside electroscope a metal needle pivots on a wire support shaped something like a paper clip. This structure inside electroscope is connected to outside by a metal rod passing through a plastic insulator. The metal disk on top simply allows charge to be detected more efficiently; orwise its geometry is not too important. The term electrostatics refers to charges that are basically stationary, rar than continuously moving as in a wire carrying an electric current. An analogy may be made to water in a bathtub as opposed to a flowing stream of water. Some important things to remember are: Electric charges come in two varieties that are designated positive and negative. Charges of same variety repel one anor while charges of opposite variety attract one anor. Charges exert greater forces on one anor when closer toger (Coulomb s law). All materials are composed of positive and negative charges. In metal objects, a small fraction of negative charge is relatively free to move from one place to anor within object. (This is why metals are called conductors.) Electric charges in insulators such as rubber and glass are essentially fixed in place. The positive charges in solid materials are in atomic nuclei and are not free to move. Electric charges in static equilibrium have no net force acting on m. When rubbed with silk, a glass rod acquires a net positive charge on its surface by giving up electrons to silk, which has a stronger affinity for electrons. 14

2 CHAPTER 1. ELECTROSTATICS 15 The plastic (polyvinyl-chloride, or PVC) acquires a net negative surface charge when rubbed with wool by stealing electrons from wool. Caution: The glass rod is brittle. Return it to tray when not in use. If placed on table, rod can roll off and break. Avoid handling glass rod, plastic tube, and wool and silk fabrics any more than necessary. Their electrostatic properties are degraded by moisture and oil from your hands. Figure Figure Grounding Grounding electroscope. electroscope. 2. Holding a charged rod close to electroscope plate Holding a charged rod close to electroscope plate Ground electroscope as illustrated in Figure 1. This works because your body can absorb or Ground give up small electroscope amounts of as charge illustrated without in Figure suffering 1.1. any This ill works effects. because You could your use body a wire can absorb connected or give to up earth small(or amounts ground, ofhence charge without term ground ), suffering any but ill your effects. body You is handier could use in this a wire case. connected Rub toglass earth rod to (or charge ground, it, and hence n move term ground ), it close to butcircular your body disk ison handier top of in this electroscope case. To charge without glass touching rod, hold disk silkwith cloth byrod. What edgedo soyou thatobserve? it hangsas below rod your is hand moved and away stroke from hanging disk, silk what withappens? glasshyposize rod. This procedure what is happening keeps moisture to from charges. yourif hand at any from time damping you suspect silk. that Then needle position is stuck, part gently of tap glass rod case that of touched electroscope silk just with above your finger. circular The case diskis onnot top ofconnected electroscope to top without plate. touching Tapping case disk will withnot affect rod. 1 What charge do you on observe? plate or Asneedle. rod is moved away from disk, what happens? Hyposize what is happening to charges. If at any Repeat same sequence with plastic tube after rubbing it with wool. Again record your time observations. you suspect (Note: that rubbing needle is plastic stuck, pipe gently with tap wool case can of produce electroscope enough electrostatic with your charge finger. The to produce case is not tiny connected sparks when to top pipe plate. is brought Tappingnear case plate willof not affect electroscope. charge Sparks on transfer plate orcharge needle. to electroscope. The effect of charge transfer is subject of next experiment. If Repeat you suspect same a spark, sequence ground with electroscope plastic tube after and repeat rubbing it experiment with wool. The without wool bringing cloth is thicker rod so close. The needle can deflect considerably before a spark occurs.) than silk, and is less susceptible to moisture. The best procedure is to put to wool in palm of Now 1 Theexplain effectiveness your ofhyposis chargingwith procedure aid depends of some strongly simple on cartoons a ambient humidityseries and of cleanliness pictures with of words of explanation; your TA will have some helpful suggestions for making simple drawings. Show what electric charges on electroscope are doing as charged rods are brought close and n moved away. You will need a sequence of several cartoon pictures to show locations of charges on electroscope for different positions of each rod. If you can t support your hyposis by your observations and pictures, you may need to make anor hyposis. glass rod. On a humid day, it may take some time to properly charge rod. Cleaning glass with a glass cleaner helps considerably. On a dry day, charging procedure can produce much more charge. As you move rod toward electroscope, stop when you see needle move. Sparks between rod and electroscope will invalidate this part of experiment. If needle moves more than half distance up scale, you have probably produced a spark. Sparks transfer charge to electroscope. The effect of transferred charge will be studied below. 3. Charging electroscope by direct contact Ground electroscope again. This time touch charged glass rod to disk, and n move

3 CHAPTER 1. ELECTROSTATICS 16 your hand and rub it against rod. Take care to avoid sparks, as described in footnote above. Again record your observations. Now explain your hyposis with aid of some simple cartoons a series of pictures with words of explanation; your TA will have some helpful suggestions for making simple drawings. Show what electric charges on electroscope are doing as charged rods are brought close and n moved away. You will need a sequence of several cartoon pictures to show locations of charges on electroscope for different positions of each rod. If you can t support your hyposis by your observations and pictures, you may need to make anor hyposis. Charging electroscope by direct contact Ground electroscope again. This time touch charged glass rod to disk, and n move rod away. What happens to needle of electroscope? Make a hyposis about what happened when you touched disk with rod using some cartoons as visual aids. Without grounding electroscope, test your hyposis by bringing charged glass rod near disk at top of electroscope but without touching it. What happens to electroscope needle? Explain wher this observation supports your hyposis or not. If observation doesn t support your hyposis, redo whole procedure and make sure that observed behavior is repeatable an important aspect of scientific process. Record all your hyposes, wher y turn out to be correct or incorrect. By using scientific method we hope to reach correct explanation in end. If behavior is repeatable, n make anor hyposis to explain your observations and test it again. To double check your understanding, bring plastic tube close to (but not touching) electroscope that was touched at outset with charged glass rod and observe what happens. Is your hyposis consistent with se additional observations? Explain with aid of anor cartoon sequence. Repeat entire process outlined in previous paragraph, but start this time by touching initially uncharged electroscope with charged plastic tube. Can a net electric charge be left on electroscope by touching it with rods? How does sign of charge on electroscope compare to charge on rod that touches it? Summarize your findings for this exercise. Charging electroscope by induction Ground electroscope again, as in Figure 1.1. With your finger still touching edge of disk and your thumb still touching body of electroscope, bring charged glass rod up close to or side of disk (away from your finger) without touching disk. Now remove your finger from disk first and n move glass rod away. What do you observe on electroscope? Make sure that it is repeatable. Make a hyposis about what happened to charge in electroscope and record it. Then test your hyposis using what you learned so far. Modify your hyposis as necessary. Explain your reasoning with anor cartoon sequence. Now beginning with charged plastic tube repeat process described in previous paragraph. Summarize your results for this section.

4 CHAPTER 1. ELECTROSTATICS 17 Effect of lit match on electroscope charge Using your knowledge of behavior of electroscope and charged rods, determine what variety of charge is released when a match is burning. Hold burning match about 2 cm above disk. (Hold match at least 1 cm from plate.) Try it with electroscope initially uncharged, positively charged, and negatively charged. Explain in detail your procedure, results, reasoning, and conclusions. More cartoons are needed here. Summary Summarize your findings concisely. Provide a brief explanation of your most important observation in each experiment. AA extract information from representation correctly Labs: 1-12 Grading Rubric No Effort Progressing Expectation Exemplary No visible attempt is made to extract information from experimental setup. Information that is extracted contains errors such as labeling quantities incorrectly, mixing up initial and final states, choosing a wrong system, etc. Physical quantities have no subscripts (when those are needed). Most of information is extracted correctly, but not all of information. For example physical quantities are represented with numbers and re are no units. Or directions are missing. Subscripts for physical quantities are eir missing or inconsistent. All necessary information has been extracted correctly, and written in a comprehensible way. Objects, systems, physical quantities, initial and final states, etc. are identified correctly and units are correct. Physical quantities have consistent and informative subscripts. AB construct new representations from previous representations construct a different representation. Representations are attempted, but omits or uses incorrect information (i.e. labels, variables) or representation does not agree with information used. Representations are constructed with all given (or understood) information and contain no major flaws. Representations are constructed with all given (or understood) information and offer deeper insight due to choices made in how to represent information. Labs: 1-12 AF Sketch Labs: 1-3, 5, 7-9 No representation is constructed. Sketch is drawn but it is incomplete with no physical quantities labeled, or important information is missing, or it contains wrong information, or coordinate axes are missing. Sketch has no incorrect information but has eir a few missing labels of given quantities. Subscripts are missing or inconsistent. Majority of key items are drawn. Sketch contains all key items with correct labeling of all physical quantities have consistent subscripts; axes are drawn and labeled correctly.

5 CHAPTER 1. ELECTROSTATICS 18 BA identify phenomenon to be investigated Labs: 1, 2, 4, 6, 7, 9, 11, 12 BE describe what is observed concisely, both in words and by means of a picture of experimental setup. Labs: 1, 2, 4, 6, 7, 9, 11, 12 BF identify shortcomings in an experiment and suggest improvements Labs: 1, 2, 4, 6, 7, 9, 11, 12 No Effort Progressing Expectation Exemplary No phenomenon is mentioned. No description is mentioned. identify any shortcomings of experimental. The description of phenomenon to be investigated is confusing, or it is not phenomena of interest. A description is incomplete. No labeled sketch is present. Or, observations are adjusted to fit expectations. The shortcomings are described vaguely and no suggestions for improvements are made. The description of phenomenon is vague or incomplete but can be understood in broader context. A description is complete, but mixed up with explanations or pattern. OR The sketch is present but relies upon description to understand. Not all aspects of design are considered in terms of shortcomings or improvements, but some have been identified and discussed. The phenomenon to be investigated is clearly stated. Clearly describes what happens in experiments both verbally and with a sketch. Provides or representations when necessary (tables and graphs). All major shortcomings of experiment are identified and reasonable suggestions for improvement are made. Justification is provided for certainty of no shortcomings in rare case re are none. BG identify a pattern in data search for a pattern The pattern described is irrelevant or inconsistent with data. The pattern has minor errors or omissions. OR Terms labelled as proportional lack clarityis proportionality linear, quadratic, etc. The patterns represents relevant trend in data. When possible, trend is described in words. Labs: 1, 2, 4, 6, 7, 9, 11 BI devise an explanation for an observed pattern explain observed pattern. An explanation is vague, not testable, or contradicts pattern. An explanation contradicts previous knowledge or reasoning is flawed. A reasonable explanation is made. It is testable and it explains observed pattern. Labs: 1, 2, 4, 6, 7, 9, 11

6 CHAPTER 1. ELECTROSTATICS 19 GB evaluate specifically how identified experimental uncertainties may affect data Labs: 1, 2, 7, 8 GD record and represent data in a meaningful way Labs: 1, 2, 4-12 GE analyze data appropriately Labs: 1, 2, 4-12 IA conduct a unit analysis to test self-consistency of an equation Labs: 1-9, 10 No Effort Progressing Expectation Exemplary evaluate experimental uncertainties. Data are eir absent or incomprehensible. analyze data. No meaningful attempt is made to identify units of each quantity in an equation. evaluate experimental uncertainties, but most are missing, described vaguely, or incorrect. Or only absolute uncertainties are mentioned. Or final result does not take uncertainty into account. Some important data are absent or incomprehensible. They are not organized in tables or tables are not labeled properly. analyze data, but it is eir seriously flawed or inappropriate. identify units of each quantity, but student does not compare units of each term to test for self-consistency of equation. The final result does take identified uncertainties into account but is not correctly evaluated. The weakest link rule is not used or is used incorrectly. All important data are present, but recorded in a way that requires some effort to comprehend. The tables are labeled but labels are confusing. The analysis is appropriate but it contains minor errors or omissions. check units of each term in equation, but student eir mis-remembered a quantity s unit, and/or made an algebraic error in analysis. The experimental uncertainty of final result is correctly evaluated. The weakest link rule is used appropriately and choice of biggest source of uncertainty is justified. All important data are present, organized, and recorded clearly. The tables are labeled and placed in a logical order. The analysis is appropriate, complete, and correct. The student correctly conducts a unit analysis to test selfconsistency of equation. EXIT TICKET: Quit any software you have been using. Straighten up your lab station. Put all equipment where it was at start of lab. Report any problems or suggest improvements to your TA. Have TA validate Exit Ticket Complete.