Lab 1: Electrostatics in Your Home

Transcription

1 Introduction Most everyone has been shocked by the ability of electrons to transfer from one object to another - particularly on dry winters' days! In this lab, you will explore how electrons are transferred between common house hold objects such as pieces of tape, Styrofoam TM, computer monitors, and even your fingers. Along the way, you will show that the force between charged surfaces (those with an excess or deficit of electrons) decreases with distance. This lab is divided into three parts. In the first section, you will be performing investigations similar to those performed by Benjamin Franklin during the mid-1700's. Like Franklin, you strive to learn how charge interacts, and you will be asked to speculate on the how's and why's of electrostactics. Your experiments might not go exactly as your intuition might predict. Don't worry! As Franklin wrote 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 quantify the electrostatic properties of charged packing peanuts. During the winter holiday you may have had one or more encounters with packing peanuts that seemed determined to stick to you, the floor, and everything else, while at the same time, they seemed equally determined to stay away from one another. This type of behavior is do to the a build up of electrons on the peanuts that causes them to repel one another (like charges repel like charges) or cling to objects with a deficit of electrons (opposites attract). Using just two charged packing peanuts you can measure how the amount of repulsion relates to the separation between the pieces of Styrofoam. In the final section of the lab, you will explore the difference between induction and conduction. By using 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 play safely with electrons using things that can be found in your home. Even with the power off, electricity, in the form of static, is all around us. For your lab report please complete the attached lab questionnaire. The questionnaire must be handed in the appropriate locked boxes Science Center 110 before 5 PM on February 18. Equipment Scotch Magic TapeT StyrofoamTM cup AM/FM radio with external antenna TV or CRT computer monitor 2 Packing peanuts / puffs Needle, thread and scissors Ruler Lamp 2 Rods /chopsticks / mixing spoons 1 sheet of graph paper Aluminum pie-plate Styrofoam plate Reading If you are interested in learning more about Benjamin Franklin and his scientific activities, you may enjoy reading Benjamin Franklin's Science by I. Bernard Cohen (Harvard University Press, 1990). Online information on Franklin can be found at 1 John Bigelow, The Works of Benjamin Franklin, (Knickerbocker Press, NY, 1904) Vol II, p. 370 Page 1 / 8

2 I. Sticky Electrostatics Rubbing materials together can generate static electricity. You can test this by scuffing your feet across carpeting on a dry day and then touching a doorknob. ZAP! But is it the friction of scuffing your feet across the floor that causes the charge buildup, or is something else going on? When two surfaces touch (like your socks on a carpet) chemical bonds can temporarily form between surfaces, as touching atoms latch onto one another's electrons. When the surfaces are made of two different materials, the atoms in one surface often exert a stronger pull on the electrons than the other surface. As a result, when the surfaces are pulled apart, electrons are stripped out of the weaker atoms by the stronger. These stolen electrons create a negative charge on one material, leaving positive charge (actually, a lack of negative charge) on the other surface. It is strictly the act of one surface touching and then not touching another surface that causes the charge transfer. So why does scuffing your feet cause buildup of charge? Neither your socks nor the carpet is a perfectly smooth surface. When you scuff your feet across the floor you increase the number of atoms in your socks in contact with the carpet. The more atoms touch and pull apart, the more electrons will get transferred. Similarly, when you rub a balloon through your hair, you increase the number of atoms in your hair that touch atoms on the balloon. The friction between your socks and the carpet or between your hair and the balloon has no effect on charge transfer. What matters is the size of the surface area that comes in contact. If you want to test this idea, try rubbing two balloons together. You'll find they have more friction between them than between one balloon and your hair, however because they are identical materials the atoms in each balloon have an equal hold on their electrons and no charge is transferred. Most Positive Most Negative (readily lose electrons) (readily steal electrons) Rabbit's fur Lucite Bakelite Acetate Glass Quartz Mica Human hair Nylon Rayon Wool Cat's fur Silk Paper Cotton Wood Sealing wax Amber Resins Hard rubber Metals Polyester Polystyrene (Styrofoam) Orlon Saran Wrap Polyurethane Polyethylene Polypropylene Sulfur Celluloid Vinyl (PVC) Teflon The likelihood of one atom latching onto another atom's electrons is something you deal with more often than you may think. Consider the Saran Wrap in your kitchen. The cheap stuff doesn't work as well as the name brand, and for some reason none of it sticks to Styrofoam and all of it sticks to glass. What's going on? Using the Triboelectric table at right, why do you think Saran Wrap doesn't cling to Styrofoam but does cling to glass. You may want to wait to answer this question until you've completed the next set of procedures. Neutral atoms will bond together to create complete shells of electrons. For a detailed description, see: Triboelectric Series: Experimenters have established lists, called triboelectric series, of the relative affinities materials have for gaining and losing electrons. By studying these lists, you can learn that rubbing wool on Styrofoam leads to negatively charged Styrofoam (and positively charged wool). Materials with similar properties (e.g. hair, wool, fur) clump together on the list and don't interact strongly. In general, objects listed near one other, like cotton and amber, interact poorly. This list's author notes, the series is reproducible only in rare circumstances. Cleanliness, humidity, and manufacturing differences affect ordering. Adapted from Electrostatics and its Applications, edited by A.D. Moore, (Wiley & Sons, NY, 1973). Page 2 / 8

3 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 cm long pieces of tape. Create a non-sticky handle on the end of each piece by folding over a couple of cm sections. These are your working strips. 3. Stick your working strips firmly to your base tape. Make sure they are in full contact with the base tape by pressing them down firmly with your fingers. 4. Grasping their handles, briskly pull your working strips off of the base tape (imagine you are removing 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 to non-sticky) and the opposite sides facing each (non-sticky to sticky). What happens? How does the 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 tapes back together again. Is the behavior of the tapes different? 6. Carefully stick the two strips of tape together (sticky to non-sticky) so that you have a double thick 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 from this new starting configuration. Do the strips behave differently this time? Is the behavior the same 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 the tabs of the top and bottom stripes in each pair so you can track which strips started on the top and bottom. (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 top strips 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 two objects 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 and bottom pieces of tape charged? Page 3 / 8

4 II. Electrostatic Forces The electrostatic or Coulomb force between electrically charged objects is one of the four fundamental interactions of matter. Like the gravitational interaction, it has an infinite range, but unlike the gravitational 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. The electrostatic force between point charges is proportional to the product of the charges and falls off inversely with the square of the separation between the charges. This is true for spherically symmetric distributions of charge when the separation between their centers is much larger than the radius of the charge distribution. This relationship was determined quantitatively by Charles Augustine de Coulomb in 1785 and is known as Coulomb's law:! F = k Q Q 1 2 $ " # % & where k, the Coulomb constant, has a value of about 9! 10 9 N m 2 C 2 It takes 6.2! electrons to create 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 the procedure, consider what should happen. Initially, the charged packing peanut dangles straight down due to gravity. If an object with the same charge is brought near the puff, it will swing around from the charged source until the Coulomb force and gravity are balanced. In this new configuration, the puff is balanced between tension (T) from the string holding it up, gravity (mg) pulling it down, and the electrostatic force (F Coulomb ) pushing it sideways. The sum of the vertical forces are zero which gives us mg=tcos! [Eq. 1] and the sum of the horizontal forces are zero which gives us F Coulomb = Tsin! [Eq. 2] You can solve for the Coulomb force by dividing Eq. 2 by Eq. 1: F Coulomb mg = Tsin! Tcos!!!"!!F Coulomb = mg tan! [Eq. 3] r 2 For small angles tan! " sin! = x L (since for small angles y! L ). Rearranging Eq. 3 we have F Coulomb = mg x L! Now the Coulomb force is given by F c = k Q Q 1 2 # " r 2 $ where r is the distance to a second puff off to the right. Substituting the above expression for F c in Eq. 3 gives us k Q 1 Q 2 r 2 = mg x L [Eq. 4] Eq. 4 shows that the displacement of the puff, x, is inversely proportional to r 2. Page 4 / 8

5 II. Procedure 1. 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 as shown. The puffs should be centered on the thread. 3. Select one puff as your test puff, and the other as your charge source. Pull the thread tight at the base of the test puff and stab the needle through the puff so that it creates a straight pointer at the puff's bottom (Step 3a). Use the 20-cm piece of thread to tie scissors to the second puff (Step 3b). 4. Attach both puffs to long rods (chopsticks, kabob sticks, or mixing spoons are fine) to form bi-fiber suspensions as shown (Final Setup). The suspensions should be as identical as possible so that the puffs hang at the same height. 5. Tape your test puff+rod to the edge of a table or counter several inches from one edge. Place a bright light straight in front of the puff so that the puff casts a sharp shadow on the surface behind it. Tape a ruler to that surface such that the shadow from the needle touches the 0 on the ruler. 6. Tape a second ruler to the surface behind the puff so that its 0 end is lined up 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 the two rods by about 20 cm. 8. Charge the puffs by rubbing them with fur, hair, wool, or some other electron source. Page 5 / 8

6 9. After you have charged both puffs, you should notice that the test puff's needle's shadow no longer lines up with 0 on the ruler. Record the shadow's new position as well as the separation between the two rods. 10. Move the charge source puff+rod progressively closer to the test puff+rod, and repeat step 9 after each move. Your two rods should be about 1-cm apart for your last measurement. Charge can discharge ( evaporate ) from your puffs (especially on humid days), so you will need to work quickly. Working with a partner is encouraged! 11. When you are done with the initial measurements, you may want to verify that your puffs haven't discharged too much. How can you do this? 12. During the previous several steps you qualitatively measured how one charged puff moves in response to a charged source. You can use the numbers you recorded to obtain a quantitative understanding of how the two puffs repelled one another. From the earlier discussion, we know the square of the separation is related to the displacement of the test puff: k Q 1Q 2 = mg x r 2 L!!!!!!! 1 " x [Eq. 4] 2 r 13. Plot x versus 1/r 2. Include error bars. Note, plotting x versus 1/r 2 means that x is plotted on the vertical axsis and 1/r 2 is plotted on the horizontal axsis. Try to draw a straight line that passes through all of your data points (the line should pass through the error bar associated with each data point not the point itself). 14. Eq 4. can be rewritten into the standard form for a straight line, y = nx + b with b = 0, x = L mg kq Q r therfore the slope of the line n equals L 2 mg kq Q 1 2 Since you charged the two puffs the same way, it is reasonable to assume they have equal amounts of charge on them, or Q 1 = Q 2 so Q 1 Q 2 = Q 2, slope = n = L mg kq2 [Eq. 5] 15. What is the value of the slope of your line? Be sure to include units.the slope of the line is related to the amount of charge on the puffs. You can measure L and look up g and k. We have measured a handful of puffs and a handful of needles and determined the puffs to have an average weight of 0.05 ± 0.01grams and the sewing needles to typically weigh 0.12 ± 0.2 grams. Filling these values into the equation above, solve for Q 2. What is the charge on each puff? How many electrons did each puff take from the fur or hair you used as an electron source? Does this number seem surprisingly large or small 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

7 III. Charging by Induction - The Electrophorus Alessandro, Count Volta is credited with inventing the electrophorus perpetuum in This practical machine allowed the (apparent) perpetual generation of charge. The principle behind it is simple. Like charge repels like charge. When a neutral object is brought near a negatively charged dielectric, the free electrons in the neutral object flow as far from the charged dielectric as they can get. If the neutral object is than touched with a conductive object connected to ground, those electrons will actually flee the neutral object, leaving it positively charged. If the neutral object is actually touched to the charged source, the electrons on the charged object will flow onto the neutral object, making it negatively charged. In this final part of the lab, you will create your own electrophorus perpetuum in a manner similar to that used by Volta. A regular Styrofoam pie plate becomes a charged dielectric when it is rubbed against your hair or a wool sweater. Combine this with an aluminum pie plate with Styrofoam-cup handle, you're ready to create charge! Page 7 / 8

8 III Procedure 1. Tape an upside-down Styrofoam plate to a table or counter top. The tape should touch only the edges 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 insulating handle 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 charge it negatively as you did earlier. Bring it close to the Styrofoam plate. Is it attracted or repelled? What type of charge is on the Styrofoam plate? When you are done, hang your test puff back up on the side of the table. 5. Make sure the aluminum pie plate is neutral (uncharged). Touching it with your hands should work. However, you can verify its neutrality by touching it to a water faucet, which serves as an excellent ground. 6. Holding on to its Styrofoam handle, move the neutral aluminum plate as close to the Styrofoam dielectric as possible without letting them touch! While keeping the plates as close together as possible, momentarily touch a finger to the top surface of the aluminum pie plate, and then raise the aluminum plate. 7. Now while touching only the handle bring the aluminum pie plate near the test puff. Is the puff attracted or repelled by the aluminum plate? What sign is the charge on the aluminum plate? Is this the same or opposite of the charge on the Styrofoam plate? Was the process used to charge the 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 the two plates to touch. The process of charging the plate requires energy, which is introduced by the work done when the aluminum plate is separated from the charged Styrofoam. Try untaping the Styrofoam 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 press the plates together so that they are in close contact but don t touch the pie plate yourself. How does the test puff react? What do you notice about the amount of charge transferred this time? Was the process used to charge the aluminum plate induction or conduction? 10. Which process do you think is more efficient at transferring charge, induction or conduction? 11. Draw figures that illustrate the movement of charge when charging by induction in the above experiment. Show the following in your drawings: How the Styrofoam plate got its original charge? How charges moved when you held the aluminum plate near the Styrofoam? What happened when you momentarily touched the aluminum plate with your finger? What happened when you moved the aluminum plate away from the Styrofoam? Page 8 / 8

9 Name Lab Section Date Collaborators: Lab Questionnaire Electrostatics In Your Home Due Feb 18 (Except where noted, please answer these question in the space provided.) I. Sticky Electrostatics 1) Using the Triboelectric table on page 2 of the lab assignment, explain why Saran Wrap doesn't cling to Styrofoam but does cling to glass. 2a) After removing your two test strips of tape from your base tape, what happened when you brought the test strips together? b) How does the orientation of the tape affect what you see? c) What do you think is causing this effect? 3) After running the charged tape between your fingers is its behavior the same or different? Why or why not? 4) After taping the test strips together and pulling them apart, do the strips behave differently this time? How do you explain this? 5) After playing with the four charged strips, what do you discover? Page 1 of 3

10 Name Lab Section Date Collaborators: II. Electrostatic Forces 1) Please attach your table of measured x and r values, and calculated 1 r 2 values. 2) How could you verify that your puffs didn't lose their charge during the experiment? 3) Please attach your graph of x versus 1 r 2. 4) Please attach your calculations of Q. What is the charge on each puff? 5a) How many electrons did each puff take from the fur or hair you used as an electron source (please attach calculations)? b) Does this number seem surprisingly large or small to you considering the effects you observed due to the electrostatic charge? III. Charging by Induction The Electrophorus 1a) Is a test puff repelled by or attracted to the Styrofoam plate? b) What is the sign of the charge on the Styrofoam plate? 2a) After you bring the plates together without letting them touch, is the test puff attracted or repelled by the aluminum plate? b) What is the sign of the charge on the aluminum plate? c) Is this the same charge or opposite of the charge on the Styrofoam plate? d) Was the process used to charge the aluminum plate induction or conduction? Explain? Page 2 of 3

11 Name Lab Section Date Collaborators: 3) What happens when you bring the plates together a second time, this time with the Styrofoam free to move (be sure to ground the aluminum plate first)? 4a) After charging the aluminum plate by contact, is the test puff attracted to or repelled by the plate? b) What is the sign of the charge on the aluminum plate? c) Is charge the same or opposite to the charge on the Styrofoam foam plate? d) Was the process used to charge the plate induction or conduction? 5a) Compare the amount of charge transferred by conduction to the amount of charge transferred by induction. (How much does the test puff react)? b) Which process do you think is more efficient at transferring charge, induction or conduction? Why? 6) Draw and attach figures that illustrate the movement of charge when charging by induction in the above experiment. Show the following in your drawing(s): How the Styrofoam plate got its original charge? How charges moved when you held the aluminum plate near the Styrofoam? What happened when you momentarily touched the aluminum plate with your finger? What happened when you moved the aluminum plate away from the Styrofoam? Page 3 of 3