Lab 1: Electrostatics In Your Home Introduction

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Physics 1bLab 1: Electrostatics in Your HomeSpring 2007IntroductionMost everyone has been “shocked” by the ability of electrons to transfer from one object to another particularly on dry winter days! In this lab, you will explore how electrons are transferred betweencommon house hold objects such as pieces of tape, StyrofoamTM, computer monitors, and even yourfingers. Along the way, you will show that the force between charged surfaces (those with an excess ordeficit of electrons) decreases with distance.This lab is divided into three parts. In the first section, you will be performing investigations similar tothose performed by Benjamin Franklin during the mid-1700's. Like Franklin, you strive to learn howcharges interact, and you will be asked to speculate on the how's and why's of electrostactics. Yourexperiments might not go exactly as your prejudiced mind may anticipate. Don't worry! As Franklinwrote in a letter to Cadwallader Colden on April 23, 1752, “Frequently in a variety of experiments tho'we miss what we expect to find, yet something valuable turns out, something surprising, and instructing,tho' unthought of.”1 Set your preconceptions aside and get ready for perceptive and accurate observation.In the second part of this lab, you will be quantifying the electrostatic properties of charged packingpeanuts. During the winter holiday you may have had one or more encounters with packing peanuts thatseemed determined to stick to you, the floor, and everything else, while at the same time, they seemedequally determined to stay away from one another. This type of behavior is caused by a build up ofelectrons on the Styrofoam that causes them to simultaneously repel one another (like repels like) andcling to objects with a deficit of electrons (opposites attract). Using just two charged packing peanutsyou can (and will!) measure how the amount of repulsion is related to the separation between the piecesof Styrofoam.In the final section of the lab, you will explore the difference between induction and conduction. Byusing these techniques to charge a standard aluminum pie plate, you will determine the sign of the“charge-transferred,” as well as the efficiency of each method.This lab offers you a chance to safely play with electrons using things that should be lurking about yourhome already. Even with the power off, electricity, in the form of static, is all around us.For your lab report please refer to the attached Post-lab questionnaire. The questionnaire must becompleted and handed in to your Lab TF’s mailbox before 6 PM on February 20.Equipment (bold items will be supplied) Scotch Magic TapeTMStyrofoamTM cupTV or CRT computer monitor2 Styrofoam packing peanuts (puffs)Needle, thread and scissorsRuler Lamp2 Rods /chopsticks / mixing spoons1 sheet of graph paperAluminum pie-plateStyrofoam plateReadingsIf you are interested in learning more about Benjamin Franklin and his scientific activities, you mayenjoy reading Benjamin Franklin's Science by I. Bernard Cohen (Harvard University Press, 1990).Online information on Franklin can be found at http://sln.fi.edu/franklin/1John Bigelow, The Works of Benjamin Franklin, (Knickerbocker Press, NY, 1904) Vol II, p. 370Page 1 / 8

Lab 1: Electrostatics in Your HomeI. Sticky ElectrostaticsPhysics 1bSpring 2007The likelihood of one atom latching onto another atom's electrons issomething you deal with more often than you may think. You may havenoticed the generation of static charge (and cling) when you take off yourwinter fleece (which is polyester). What's going on?Referring to the Triboelectric table at the right, you can predict whichcombinations of clothing are going to produce the greatest zap whenseparated. Neutral atoms will bond together to create complete shells of electrons. For a detaileddescription, see: l/bond.htmlPage 2 / 8(readily lose electrons)(readily steal electrons)So why does scuffing your feet cause charge to buildup? Neither yoursocks nor the carpet are perfectly smooth surfaces. When you scuff yourfeet across the floor you increase the number of atoms in your socks thatwill come in contact with the carpet. The more atoms “touch” and pullapart, the more electrons will get transferred. Similarly, when you rub aballoon through your hair, you increase the number of atoms in your hairthat touch atoms on the balloon. The friction between your socks and thecarpet or between your hair and the balloon has no effect on chargetransfer. What matters is the amount of surface area that comes intocontact. If you want to test this idea, try rubbing two balloons together.You'll find they have more friction between them than between one balloonand your hair, but because they are identical materials the atoms in eachballoon have an equal hold on their electrons and no charge is transferred.Most PositiveWhen two surfaces touch (like your socks on a carpet) chemical bonds cantemporarily form between surfaces, as neighboring atoms share electrons. When the surfaces are made of two different materials, the atoms in onesurface often exert a stronger pull on the electrons than does the othersurface. As a result, when the surfaces pull apart, electrons are stripped outof the weaker atoms by the stronger. These stolen electrons create anegative charge on one material, leaving positive “charge” (actually, a lackof charge/electrons) on the other surface. It is strictly the act of one surfacetouching and then not touching another surface that causes the chargetransfer.Most NegativeRubbing materials together can generate static electricity. You can test this by scuffing your feet acrosscarpeting on a dry day and then touching a doorknob. ZAP! But is it the friction of scuffing your feetacross the floor that causes the charge buildup, or is something else going on?Rabbit's furLuciteBakeliteAcetateGlassQuartzMicaHuman hairNylonRayonWoolCat's furSilkPaperCottonWoodSealing waxAmberResinsHard rubberMetalsPolyesterPolystyrene (Styrofoam)OrlonSaran luloidVinyl (PVC)TeflonTriboelectric Series: Experimentershave established lists, called triboelectric series, of the relative affinitiesmaterials have for gaining and losingelectrons. By studying these lists, youcan learn that rubbing wool onStyrofoam leads to negatively chargedStyrofoam (and positively chargedwool). Materials with similar properties(e.g. hair, wool, fur) clump together onthe list and don't interact strongly. Ingeneral, objects listed near one other,like cotton and amber, interact poorly.This list's author notes, the series isreproducible only in rare circumstances. Cleanliness, humidity, andmanufacturingdifferencesaffectordering. Adapted from Electrostaticsand its Applications, edited by A.D.Moore, (Wiley & Sons, NY, 1973).

Physics 1bLab 1: Electrostatics in Your HomeSpring 2007I. Procedure1. Stick a piece of plastic adhesive tape (Scotch Magic tape works well) about 40 cm long onto atable top. This is your base tape.2. Cut two 12-20 cm long pieces of tape. Create a non-sticky handle on the end of each piece byfolding over a couple cm section. These are your working strips.3. Stick your working strips firmly to your base tape. Make sure they are in full contact with thebase tape by pressing them down firmly with your fingers.4. Grasping their handles, briskly pull your working stripes off of the base tape (imagine you areremoving a band-aid). Letting the strips dangle freely, slowly bring the strips together.Experiment with bringing the tape together with the like sides facing each other (non-sticky tonon-sticky) and the opposite sides facing each (non-sticky to sticky). What happens? How doesthe orientation of the tape affect what you see? What do you think is causing this effect?5. One at a time, pass each of the working strips lightly between your fingers. Try bringing the tapeback together again. Is the behavior of the tape different?6. Carefully stick the two strips of tape together (sticky to non-sticky) so that you have a doublethick piece of tape, and run your fingers down the length of the working strips.7. Grasping one tape tab in each hand, quickly pull the strips of tape apart, repeating step 4 fromthis new starting configuration. Do the strips behave differently this time? Is the behavior thesame or different from step 4?8. Create four new working strips that are all about 10-cm long.9. Create two double thick pieces of tape using your 4 new working strips. Use a pen to mark thetabs of the top and bottom stripes in each pair so you can track which strips started on the top andbottom. (The piece with the non-sticky side exposed is the top.)10. Quickly pull the two pairs of tape apart and test all possible combinations of bottom and topstrips as you tested the strips in step 4. What do you discover?11. At this point you do not know which strips are positive and which are negative. Using twoobjects from the list on page 2 (like hair and Styrofoam), create a negatively charged object.12. Test a top and bottom piece of tape with the negatively charged object. How are the top andbottom pieces of tape charged?13. TV and CRT computer screens tend to charge up. What type of charge (positive or negative) doyou think collects on the screen?14. Test your hypothesis with your positively and negatively charged pieces of tape. What type ofcharge do you find on the screen?Page 3 / 8

Physics 1bLab 1: Electrostatics in Your HomeSpring 2007II. Electrostatic ForcesThe electrostatic or Coulomb force between electrically charged objects is one of the four fundamentalinteractions of matter. Like the gravitational interaction, it has an infinite range, but unlike thegravitational interaction (where there is only one kind of mass, and the interaction is always attractive),there are two kinds of electrical charge and the force can be either attractive or repulsive. Theelectrostatic force between point charges is proportional to the product of the charges and falls offinversely with the square of the separation between the charges. This is true for spherically symmetricdistributions of charge when the separation between their centers is much larger than the radius of thecharge distribution. This relationship was determined quantitatively by Charles Augustine de Coulombin 1785 and is known as Coulomb's law:!QQ F k# 12 2 &" r %where k, the Coulomb constant, has a value of about 9x109 N m2/C2. It takes 6.2 ! 1018 electrons tocreate a 1 Coulomb of charge! (1 electron has a charge of 1.6 ! 10 "19 C .)In the next activity you will test the Coulomb inverse square law for two charged Styrofoam “puffs”(sometimes called packing peanuts) and calculate the amount of charge on these puffs.In your experimental setup, you will suspend a charged packing peanut from thread. Before you start theprocedure, consider what should happen. Initially, the charged packing peanut dangles straight down dueto gravity. If an object with the same charge is brought near the puff, it will swing around from thecharged source until the Coulomb force and gravity are balanced. In this new configuration, the puff isbalanced between tension (T) from the string holding it up, gravity (mg)pulling it down, and the electrostatic force (FCoulomb) pushing it sideways.The vertical, y-component forces balance to:[Eq. 1]mg Tcos!and the horizontal, x-component forces balance to:[Eq. 2]FCoulomb Tsin !You can solve for the Coulomb force by dividing Eq. 2 by Eq. 1:FCoulomb Tsin ! !!"!!FCoulomb mg tan ![Eq. 3]mgTcos!xFor small angles tan ! " sin ! , (since for small angles y ! L )LxEquation 3 becomes[Eq. 4]FCoulomb mgL! Q1Q2 & , where r is the distance" r2 %Now the Coulomb force is given by Fc k#to a second puff off to the right; thuskQ1Q 2x mg2rL[Eq. 5]2Equation 5 shows that the displacement of the puff, x, is inversely proportional to r .Page 4 / 8

Physics 1bLab 1: Electrostatics in Your HomeSpring 2007II. Procedure1. Cut two 80-cm lengths of thread and one 20-cm length of thread.2. Using the needle, string an 80-cm piece of thread through each puff asshown. The puffs should be centered on the thread.3. Select one puff to be your test puff, and the other to be your charge source.Pull the thread tight at the base of the test puff, and stab the needle through the puff so that itcreates a straight pointer at the puff's bottom (Step 3a). Use the 20-cm piece of thread to tiescissors to the second puff (Step 3b).4. Attach both puffs to long rods (chopsticks, kabob sticks, ormixing spoons are fine) to form bi-fiber suspensions as shown(Final Setup). The suspensions should be as identical as possibleso that the puffs hang at the same height.5. Tape your test puff rod to the edge of a table or counter severalinches from one edge. Place a bright light straight in front of thepuff so that the puff casts a sharp shadow on the surface behind it. Tape aruler to that surface such that the shadow from the needle touches the 0 onthe ruler.6. Tape a second ruler to the surface behind the puff so that its 0 end is linedup with the rod on the test puff rod. (see Final Setup).7. Use a heavy book to hold the source rod in place. Initially, separate thetwo rods by about 20 cm.8. Charge the puffs by rubbing them with fur, hair, wool, or some other electron source.Page 5 / 8

Physics 1bLab 1: Electrostatics in Your HomeSpring 20079. After you have charged both puffs, you should notice that the test puff's needle's shadow nolonger lines up with 0 on the ruler. Record the shadow's new position as well as the separationbetween the two rods.10. Move the charge source puff rod progressively closer to the test puff rod, and repeat step 9 aftereach move. Your two rods should be about 1-cm apart for your last measurement. Charge candischarge (“evaporate”) from your puffs (especially on humid days), so you will need to workquickly. Working with a partner is encouraged!11. When you are done with the initial measurements, you may want to verify that your puffs haven'tdischarged too much. How can you do this?12. During the previous several steps you qualitatively measured how one charged puff moves inresponse to a charged source. You can use the numbers your recorded to glean a quantitativeunderstanding of how the two puffs repelled one another. From the earlier discussion, we knowthe square of the separation is related to the displacement of the test puff:QQx1[Eq. 5]k 1 2 2 mg !!!!!!! 2 " xrLr13. Plot x versus 1/r2. Include error bars. Note, plotting x versus 1/r2 means that x is plotted on thevertical axsis and 1/r2 is plotted on the horizontal axsis. Try to draw a straight line that passesthrough all of your data points (the line should pass through the error bar associated with eachdata point not the point itself).14. Equation 5 can be rewritten into the standard form for a straight line, y mx b with b 0,L1Lx kQ1Q 2 2 therefore the slope of the line, m, equalskQ1Q 2 .mgrmgSince you charged the two puffs the same way, it is reasonable to assume they have equalamounts of charge on them, or Q1 Q2 so Q1Q2 Q2,Lslope kQ 2 [Eq. 5]mg15. What is the value of the slope of your line? Be sure to include units. The slope of the line isrelated to the amount of charge on the puffs. You can measure L and look up g and k. We havemeasured a handful of puffs and needles and determined puffs have an average weight of0.05 0.01 grams and sewing needles typically weigh 0.12 .02 grams. Filling these values into theequation above, solve for Q2. What is the charge on each puff? How many electrons did each pufftake from the fur or hair you used as an electron source? Does this number seem surprisingly large orsmall to you, considering the effects you observed due to the electrostatic charge?Do not disassemble your test puff rod until completing the next section!Page 6 / 8

Physics 1bLab 1: Electrostatics in Your HomeSpring 2007III. Charging by Induction - The ElectrophorusAlessandro Count Volta is credited with inventing the electrophorus perpetuum in 1775. This practicalmachine allowed the (apparent) perpetual generation of charge. The principle behind it is simple. Likecharge repels like charge. When a neutral object is brought near a negatively charged dielectric, the freeelectrons in the neutral object flow as far from the charged dielectric as they can get. If the neutral objectis than touched with a conductive object connected to ground, those electrons will actually flee theneutral object, leaving it positively charged. If the neutral object is actually touched to the chargedsource, the electrons on the charged object will flow onto the neutral object, making it negativelycharged.In this final part of the lab, you will create your own electrophorus perpetuum in a manner similar tothat used by Volta. A regular Styrofoam pie plate becomes a charged dielectric when it is rubbed againstyour hair or a wool sweater. Combine this with an aluminum pie plate with Styrofoam-cup handle, andyou're ready to “create” charge!Page 7 / 8

Physics 1bLab 1: Electrostatics in Your HomeSpring 2007III Procedure1. Tape an upside-down Styrofoam plate to a table or counter top. The tape should only touch theedges of the plate. This is your dielectric.2. Tape a Styrofoam cup to the inside of an aluminum pie plate. The cup will serve as an insulatinghandle for moving the charged pie plate.3. Charge the Styrofoam plate by rubbing it with fur, or wool.4. Untape your test puff rod from the table and negatively charge it as you did earlier. Bring itclose to the Styrofoam plate. Is it attracted or repelled? What type of charge is on the Styrofoamplate? When you are done, hang your puff back up on the side of the table.5. Make sure the aluminum pie plate is neutral (uncharged). Touching it with your hands shouldwork. However, you can verify its neutrality by touching it to a water faucet, which serves as anexcellent “ground”.6. Holding on to its Styrofoam handle, move the neutral aluminum plate as close to the Styrofoamdielectric as possible without letting them touch! While keeping the plates as close together aspossible, momentarily touch a finger to the top surface of the aluminum pie plate, and then raisethe aluminun plate.7. Now while touching only the handle bring the aluminum pie plate near the test puff. Is the puffattracted or repelled by the aluminum plate? What sign is the charge on the aluminum plate? Isthis the same or opposite of the charge on the Styrofoam plate? Was the process used to chargethe aluminum plate induction or conduction?8. You can recharge the aluminum plate as many times as you want, as long as you don't allow thetwo plates to touch. The process of charging the plate requires energy, which is introduced by thework done when the aluminum plate is separated from the charged Styrofoam. Try untaping theStyrofoam plate from the table and repeating steps 5 and 6. What happens? When you are done,retape the Styrofoam plate to the table.9. Repeat steps 3, 5-7, but this time firmly touch the plates together but don't touch the pie plateyourself. How does the test puff react? What do you notice about the amount of chargetransferred this time? Was the process used to charge the aluminum plate induction orconduction?10. Which process do you think is more efficient at transferring charge, induction or conduction?11. Draw figures to show the motion of charges throughout this section of the experiment. Where didthe charge start? Where did it move when you held the aluminum plate near the Styrofoam?What happened when you momentarily touched the aluminum plate with your finger? Whathappened when you raised the pie plate away from the dielectric?Page 8 / 8

Physics 1b Lab 1: Electrostatics in Your Home Spring 2007 Page 3 / 8 I. Procedure 1. Stick a piece of plastic adhesive tape (Scotch Magic tape works well) about 40 cm long onto a table top. This is your base tape. 2. Cut two 12-20 cm long pieces of tape. Create a non-sticky handle on t

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