Table of Contents - San Diego Miramar Collegefaculty.sdmiramar.edu/faculty/sdccd/sokumoto/physics196/lab/week2... · Charles Schuler, California ... Charles Stone, North Carolina

Module 8: Electrostatics -i-

Module 8

Electrostatics A Module on the Properties of Charges at Rest

Principal Author: Joshua Phiri

Florence-Darlington Technical College Florence, South Carolina

Version 7, 5/2005

The materials contained herein were developed under Grant DUE9553665 in the Advanced Technological Education Program of the National Science Foundation.

The National Science Foundation 4201 Wilson Boulevard

Arlington, Virginia 22230, USA Tel: 703-306-1234

Copyright 2003 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any

means, electronic, mechanical, photocopying, recording, or otherwise, without written permission.

Module 8: Electrostatics -ii-

Acknowledgments The Introductory College Physics: 21st Century (ICP/21) modules were produced through the efforts of a number of people in addition to the principal authors. The ICP/21 staff was made up of the following individuals:

Alexander Dickison, Seminole Community College .............……...............Editor

Sherry Savrda, Seminole Community College .……..............….Associate Editor

Pearley Cunningham, Community College of Allegheny County.Assistant Editor

Marvin Nelson, Green River Community College .................…...Assistant Editor

Cynthia Hauptner……………………………………………………Technical Writer

Wilma Lopez Hodges ……………………………………..............Technical Writer

Martin Wheeler ...................................................…………............ Graphic Artist

Tonya Bryant ...................................................………... Administrative Assistant

The National Review Committee appointed by the National Science Foundation has played an important role in the development and revision of these modules. Members of this committee were:

J.D. Garcia, University of Arizona, NSF Representative

Guillermina Damas, Miami-Dade Community College

Ruth Howes, Ball State University

Charles Schuler, California University of Pennsylvania

Betty Windham, Harper Community College

Several consultants to ICP/21 have been very valuable in providing the feedback that was necessary to make these modules stronger. The responsibility for any errors rests with the editor; however without the help and support of our consultants, these modules would have lacked a great deal.

Jack Hehn, American Institute of Physics .....………………… Project Consultant

Karen Johnston, North Carolina State University…...……Field Test Coordinator

Max Kurtz, Ztek Co. .........................................………....…. Software Consultant

David Maloney, Indiana-Purdue Fort Wayne Univ. ...…. Educational Consultant

Susan Marie Marsh, U.S. Dept. of Education.………....… Educational Evaluator

Module 8: Electrostatics -iii-

ICP/21 Editors

Alexander Dickison, PI of ICP/21, Seminole Community College

Sherry Savrda, Co-PI of ICP/21, Seminole Community College

Module Authors

Motion Sherry Savrda, Co-PI of ICP/21, Seminole Community College

Forces Alexander Dickison, PI of ICP/21, Seminole Community College

Torque Leo Takahashi, Penn State University, Beaver Campus

Work and Energy Brian Box, Northern Oklahoma College

Waves and Sound Roger Edmonds and John Terrell, Middlesex Community College

Heat and Temperature Pearley Cunningham, Co-PI of ICP/21, Community College of Allegheny County

Fluids Mark Davenport, San Antonio College

Electrostatics Joshua Phiri, Florence-Darlington Technical College

Electric Circuits Marvin Nelson, Co-PI of ICP/21, Green River Community College

Magnetism Todd Lief, Cloud County Community College

Geometrical Optics Charles Robertson, University of Washington

Physical Optics Leo Takahashi, Penn State University, Beaver Campus

Toolkit Bobbie Lang, Edgewater High School Charles Lang, Omaha Westside High School

Solution Guide Authors

Motion Karim Diff, Santa Fe Community College Sultan Parvez, Lousiana State University at Alexandria

Forces Ali Yazdi, Jefferson State Community College

Torque Jon Anderson, Centennial Senior High School Rob Hagood, Washtenaw Community College

Work and Energy Christos Valiotis, Antelope Valley College Gail Wyant, Cecil Community College

Waves and Sound Regina Bochicchio, San Diego Miramar College Steven Thedford, Redan High School

Heat and Temperature Susan Cable, Central Florida Community College Gordon Shepherd, Greensboro College

Fluids Marc Cullison, Connors State College

Electrostatics Todd Leif, Cloud County Community College

Electric Circuits Bob Boeke, William Rainey Harper College Todd Leif, Cloud County Community College

Magnetism Karim Diff, Santa Fe Community College Martin Mason, Mt. San Antonio College

Physical Optics Karim Diff, Santa Fe Community College Paul Williams, Aims Community College

Module 8: Electrostatics -iv-

The ICP/21 project benefited from the input of a great many people. The editors wish to thank the following contributors for their time and effort toward improving the ICP/21 material. Any errors in the ICP/21 modules are strictly the responsibility of the ICP/21 editors and authors. Brian Anderson, Cloud County Community College, Concordia, KS Donald Anderson, Ivy Tech State College, Kokomo, IN Jon Anderson, Anoka Ramsey Community College, Minneapolis, MN Meredith Anderson, Holkessin, DE

Marvin Artman, Del Mar College, Corpus Christi, TX Gina Borchicchio, San Diego Miramar College, San Diego, CA

Bob Breum, Lyman High School, Longwood, FL Otis Gene Byrd, Jr., Lamar State College, Port Arthur, TX Robin Byrne, Northeast State Tech Community College, Blountville, TN Susan Cable, Central Florida Community College, Ocala, FL Keith Clay, Green River Community College, Auburn, WA Karim Diff, Santa Fe Community College, Gainesville, FL Ghada Elaqad, Maricopa Community College, Mesa, AZ Robert Gramer, Lake City Community College, Lake City, FL Robert Hagood, Washtenaw Community College, Ann Arbor, MI Louis Hart, West Liberty State College, West Liberty, WV Steve Iona, Horizon High School, Brighton, CO Havis Johnson, Jefferson State College, Birmingham, AL Munawar Karim, St. John Fisher College, Rochester, NY Patrick Keefe, Clatsop Community College, Astoria, OR James Knowles, North Lake College, Irving, TX Louis Lee, Barstow College, Barstow, CA Todd Leif, Cloud County Community College, Concordia, KS John Lindberg, Seattle Pacific University, Seattle, WA Martin Mason, Mt. San Antonio College, Walnut, CA Henry Merrill, Fox Valley Technical College, Appleton, WI Tobias Moleski, Nashville State Tech, Nashville, TN Mary Beth Monroe, Southwest Texas Junior College, Uvalde, TX Michael Myhrom, Dunwoody College of Technology, Minneapolis, MN Tom O'Kuma, Lee College, Baxtown, TX Christine O'Leary, Wallace State College, Hanceville, AL M. Sultan Parvez, Lousiana State University at Alexandria, Alexandria, LA Joshua Phiri, Florence-Darling Technical College, Florence, SC Euguenia Peterson, Daley College, Chicago, IL Periasamy Ramalingam, St. Cloud State University, St. Cloud, MN Jane Repko, Lansing Community College, Lansing, MI Patricia Robbert, University of New Orleans, New Orleans, LA John Sammler, Dunwoody College, Minneapolis, MN Rihab Sawah, Moberly Area Community College, Moberly, MO M. Gordon Shepherd, Greensboro College, Greensboro, NC Purnima Sharma, West Virginia Northern Community College, Wheeling, WV Gene Skluzacek, St. Petersburg Junior College, St. Petersburg, FL John Splett, Erie Community College, Buffalo, NY Ray Stevens, Iowa Western Community College, Council Bluffs, IA James Stewart, Central Piedmont Community College, Charlotte, NC Charles Stone, North Carolina A & T State University, Greensboro, NC Seetha Subramanian, Lexington Community College, Lexington, KY Jack Taylor, Baltimore City Community College, Baltimore, MD Steven C. Thedford, Redan High School, Stone Mountain, GA David Ting, Houston Community College, Houston, TX Christos Valiotis, Antelope Valley College, Lancaster, CA William Waggoner, Creighton University, Omaha, NE Arthur Ward, Nashville State Tech, Nashville, TN Myra West, Kent State Tech, Kent, OH Paul Williams, Aims Community College, Greenley, CO Gail Wyant, Cecil Community College, North East, MD Ali Yazdi, Jefferson State Community College, Birmingham, AL Richard Zajac, Kansas State University at Salina, Salina, KS

Module 8: Electrostatics -v-

Table of Contents Page

Introduction 1

Part One: Properties of Charge 3

1.1 Exploration 1: Presence of Charge 4

1.2 Exploration 2: Electrostatic Attraction 8

1.3 Exploration 3: Interactions Between Charged Objects 11

1.4 Dialog 1: The Definition of Charge 16

1.5 Exploration 4: Deducing the Type of Charge 18

1.6 Dialog 2: Charge Separation 21

1.7 Extension 1: Generating Charge 25

1.8 Demonstration 1: The Van de Graaff Generator 26

1.9 Application 1: Antistatic Bags and Gasoline Fires 27

Part Two: The Electric Force 29

2.1 Exploration 5: The Force Between Charges 31

2.2 Dialog 3: Coulomb Forces 35

2.3 Extension 2: Calculating Electrostatic Forces 39

2.4 Dialog 4: Electrostatic Forces in Two Dimensions 41

2.5 Extension 3: Coulomb’s Law in Two Dimensions 44

Part Three: The Electric Field 47

3.1 Exploration 6: Visualizing Electric Fields 49

3.2 Exploration 7: Electric Fields of Extended Objects 54

3.3 Demonstration 2: Faraday’s Cage 59

3.4 Dialog 5: The Electric Field 60

3.5 Extension 4: Calculating Electric Fields 66

Module 8: Electrostatics - 1 -

Module 8: Electrostatics

Introduction to Electrostatics Have you ever been “zapped” as you touched a metal

doorknob after walking across a carpeted floor on a cold, dry winter day? Have you ever seen clothes affected by “static cling” as you remove them from a dryer? These are among the many examples of electrostatic discharge (ESD), or static electricity, that you might encounter in everyday life. An understanding of electrostatics is important not only to medical personnel, physicists, and electronic engineers, but also to the average citizen whose most frequent experience with electrostatic discharge may be the discomfort felt when touching a metal doorknob on a winter day.

Even though electrostatic discharge is responsible for such nuisances as static cling and dirty computer screens, it can also be found in many useful applications. These applications include photocopiers, laser printers, defibrillators, and electrostatic precipitators. While these technological tools have made our lives easier, and even saved lives, there are also dangers associated with electrostatics. One of the most dramatic examples of electrostatics is lightning. Electrostatic discharges have also been associated with the ignition of flammable gas vapors and explosions in grain elevators. For example, in hospital operating rooms, synthetic fabrics are not allowed, as they could generate electrostatic discharges that would be dangerous around the oxygen that is stored to help patients breathe.

Electrostatic charge also forms the basis for the study of the nature of electricity, the use of which has dramatically changed our world in the last 100 years. Understanding the behavior of charged objects in different conditions is fundamental to understanding both electricity and magnetism. The work in this module will be beneficial when you study the Electric Circuits and Magnetism modules. As you work through the Explorations in this module, ask yourself what type of electrostatic model is needed to explain the behavior of the charged objects you observe.


Industrial Applications A company called Electronics Today produces electronic

components. The company has discovered that 30% of its printed circuit boards undergo unexplained failures. The customer service department has been flooded with complaints stating that the electronic products purchased from the company will not function upon arrival. John, the company’s electrician, has been called to investigate the problem. He suspects the problem may be caused by electrostatic discharge due to poor handling procedures in the assembly department.

John has called you in to assist in the solution to this problem. In your role as technician, you will need to call on your knowledge of electrostatics to help John. One possible solution John suggests is for the assemblers to use antistatic cuffs while working on the circuit boards, and to make sure the components are packed in anti-static bags (shown below) before leaving the assembly area.

What are anti-static bags and anti-static cuffs, and how do these things work to reduce the risk of electrostatic discharge? In this module you will develop the knowledge to answer this question. You will also learn about the importance of protecting delicate electronic components from the common occurrence of electrostatic discharge.


Part One

Properties of Charge

Our goal in Part One of this module is to develop a model that explains the most basic electrical charge phenomena, such as the attraction and repulsion of objects. This model will help you understand how electric charges act differently depending on whether the material they are interacting with is a conductor or an insulator. We will complete several Explorations to develop this model.

We will address several basic questions in this section: Why don’t some materials retain charge for a long period of time even in low humidity conditions? Why are some materials able to get charged easier than others? Are all materials capable of developing excess charge on their surfaces when they are rubbed or brought into contact with another charged object?

Please note that even though the experiments proposed in this module may work admirably in low humidity environments, some may perform poorly or even fail to work in high humidity weather. To increase your chances of success with the Explorations in this module, make sure the materials you use are clean and dry, and the humidity is low. If you do not have a low humidity environment in which to complete these explorations, your instructor may have you watch videos of electrostatic phenomena.

Part One: Properties of Charge


1.1

Exploration 1: The Presence of Charge

You have probably heard and used the term “charge” many times. What exactly is electric charge? How do charged objects behave? In this Exploration you will investigate the properties of charged objects. You have probably also heard and used the terms “conductor” and “insulator” in relation to electricity. This Exploration will also help you start developing a better understanding of those terms.

Equipment

Electroscope (gold leaf or other) Copper rod or pipe

PVC pipe Glass rods

Wool cloth Plastic and wooden rulers

Large steel nails Inflated balloons

Plastic drinking straws Pens

Silk cloth Any other materials you and your instructor are interested in testing

Rubber rod or comb

An electroscope, shown in the illustration at right, is used to detect the presence of charge. You will begin this Exploration by investigating the effect of rubbing different materials with silk and bringing the rubbed material to the top of the electroscope.



1. Before testing the materials with an electroscope, predict what you think will happen in each of the following situations. Write your prediction and explain your reasoning for each of the following questions:

a) Are all materials able to become charged by rubbing? Why or why not? _________________________________________________________ __________________________________________________________________________________________________________________

b) Which object will become charged if you rub one object against another (for example, if you rub a glass rod with wool), or will the rubbing process charge them both? Explain. ___________________________ ___________________________________________________________________________________________________________________________________________________________________________

c) What do you think will happen to the leaves of the electroscope if you touch the knob of the electroscope with the material that has been rubbed (the test material)? Explain. ____________________________ ___________________________________________________________________________________________________________________________________________________________________________

d) What do you think will happen to the leaves of the electroscope if you move the test material away from the knob? Explain. ______________ ___________________________________________________________________________________________________________________________________________________________________________

2. You have probably heard the words “conductor” and “insulator” before. What do you think the properties of insulators and conductors are? Explain carefully. ____________________________________________________ ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________



3. Now test your predictions. Use the silk cloth to rub each of the test materials. Record your observations in the table below, and include your opinion as to whether the test material is a conductor or an insulator. You will need to touch the knob of the electroscope after each test with a clean dry finger. Why do you think this is necessary? ________________________________________________________________________________________________________________________ ____________________________________________________________

Material Effect on leaves of

electroscope Conductor or

insulator

4. What criteria did you use to base your classification of the test materials as either conductors or insulators? __________________________________ ____________________________________________________________________________________________________________________________________________________________________________________

5. Can you find any common characteristics for the materials that you tested that were conductors? Can you find any common characteristics for the materials that you tested that were insulators? Explain. ________________ ____________________________________________________________ ________________________________________________________________________________________________________________________ ____________________________________________________________

6. Repeat the testing of your materials with a wool cloth. Record your results in the table on the next page.



Material Effect on leaves of

electroscope Conductor or

insulator

7. How do the results of your tests compare with your answer in Step 2? ____________________________________________________________ ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

8. Based on the results of this Exploration, how many types of charge do there appear to be? __________________ What evidence did you use to answer this question? __________________________________________ ____________________________________________________________ ____________________________________________________________



1.2

Exploration 2: Electrostatic Attraction

You have probably experienced an effect called “static cling” in the past, and may know that the effect is somehow related to electric charges. What causes this attraction? In this Exploration we will investigate the forces between charged objects such as those you used in Exploration 2.

Equipment

Small bits of paper, aluminum foil, pepper, cotton threads, wheat flake cereal, salt, steel wool, or other objects as available

Wood

Rubber rod or comb

Glass rod

Pieces of wool and silk cloth

Aluminum foil

An inflated balloon

A faucet capable of producing a fine stream of water

1. You are probably aware of the two different types of magnetic poles, which are called north and south. Are there also different types of charges? What evidence do you have to support your answer? ______________________ ____________________________________________________________ ________________________________________________________________________________________________________________________

2. You have probably also seen that opposite magnetic poles attract each other, and like poles repel each other. Do you think the same effect applies charged objects? In other words, when you rub a rubber rod, does one end attract while the other end repels the same objects? Explain your answer. _____________________________________________________ ____________________________________________________________________________________________________________________________________________________________________________________

3. Sprinkle small bits of paper, aluminum foil, pepper, cotton threads, wheat flake cereal, salt, and steel wool on a piece of paper. Rub one end of a rubber rod with the silk cloth and bring the charged rod over the materials



on the paper. What do your observations suggest about the kinds of materials that are attracted to a charged object? _____________________ ____________________________________________________________ ____________________________________________________________

Are any of the materials repelled from the charged rod? _______________ If so, which ones? _____________________________________________ ____________________________________________________________

4. Now repeat your observations, this time rubbing the rod with the wool cloth. What do you observe? _________________________________________ ____________________________________________________________________________________________________________________________________________________________________________________

5. If you turn the rod around so that the end that you were originally holding is held near the mixture of materials, what happens? __________________ ____________________________________________________________________________________________________________________________________________________________________________________

6. Can you deduce from these observations that the rod was charged? _____ Why or why not? ______________________________________________ ____________________________________________________________ Did you observe any attraction or repulsion? ________________________ Explain. _____________________________________________________ ________________________________________________________________________________________________________________________

7. Repeat your observations with an inflated balloon rubbed with wool cloth. Does the balloon cause the same interactions as the charged rod? Describe any differences you noted. _______________________________ ____________________________________________________________________________________________________________________________________________________________________________________

8. Open a faucet and adjust it to create a fine stream of water. Bring a charged rubber rod near the stream. What happens? _______________________________ ________________________________________________________________________________________________________________________________________________________________________________



9. Charge a glass rod with silk and bring it near the stream. How does the effect on the stream compare that when the rubber rod was used? ____________________________________________________________ ____________________________________________________________ ____________________________________________________________

10. Rub the inflated balloon with the wool cloth and bring it close to the stream of water. Does it have the same effect on the stream of water that the charged rods did? _____________________________________________ ____________________________________________________________

11. Do you think the glass rod and the balloon have similar types of charge? How about the rubber rod and the balloon? What evidence do you have to support your answer? __________________________________________ ____________________________________________________________________________________________________________________________________________________________________________________



1.3

Exploration 3: Interactions Between Charged Objects

So far we have observed the effects of charges and electrostatic attraction, but we have not yet been able to deduce whether or not different types of charges are involved in these processes. In this Exploration we will begin to address this problem.

Equipment

Transparent tape Table top

Meter stick

Lab pole and clamp

Various test objects, such as pencils, pens, metal rods, paper, etc.

Barely damp sponge

1. Make a horizontal bar by clamping the meter stick to the vertically mounted lab pole.

2. Follow the steps below to create a charged piece of tape.

a. Place a piece of tape, about 10 cm in length, sticky side down on a smooth unpainted surface such as an unpainted tabletop.

b. Make a “handle” at one end of the tape by folding about 1 cm of the end over onto the sticky side of the tape.

c. Make sure the tape is firmly in contact with the table by running your hand over the top of the tape after it is placed on the table.

d. Peel the tape carefully but rapidly off the table.

e. Hang the tape from the horizontal meter stick by attaching the tip of the tape to the meter stick.



3. Describe the behavior of the tape as you bring various test objects toward it. __________________________________________________________ ____________________________________________________________ ____________________________________________________________

4. Charge another piece of tape by repeating the procedure in Step 2. Bring the second piece of tape toward the tape that is hanging from the meter stick. Describe your observations. ________________________________ ____________________________________________________________________________________________________________________________________________________________________________________

Would you call this interaction attraction or repulsion? _________________

5. As you perform the experiment above, it is important that you keep your hands and other objects away from the tape hanging from the meter stick. Why? _______________________________________________________ ____________________________________________________________________________________________________________________________________________________________________________________

6. How does the distance between the tapes affect the interaction between them? ______________________________________________________ ________________________________________________________________________________________________________________________ ____________________________________________________________

7. How could you make an uncharged piece of tape? Record your procedure. ____________________________________________________________________________________________________________________________________________________________________________________At this point, have your instructor check your procedure for making an uncharged tape. Instructor’s initials _______________________________

8. Prepare your uncharged tape and attach it to the meter stick.

9. Prepare two more pieces of tape by pressing them on a tabletop and making one handle on each piece, but do not peel either tape from the table.

a. Write “l” (for lower) on the handles of both pieces of tape.



b. Lay a second piece of tape over each of the pieces already on the table, taking care to align them so that the new pieces of tape are directly on top of the original pieces of tape.

c. Write “t” (for top) on the handles of these new pieces of tape. You should now have two pairs of tape on the table.

10. Pull both pairs of tape off the table and hang them from the meter stick. Are the tape pairs charged? ________ How do you know? _______________ ________________________________________________________________________________________________________________________

11. Carefully wipe the non-sticky side of the tape pairs with the barely damp sponge. Now are the tapes charged? _____________ How do you know? ________________________________________________________________________________________________________________________

12. Carefully separate one set of the “t” and “l” tapes by pulling them apart by the handles. Hang one of the “t” tapes and one of the “l” tapes from the meter stick. Are the tapes charged? _______________ How do you know? ____________________________________________________________ ____________________________________________________________

13. How do you think the two tapes, “t” and “l,” will interact with each other? ____________________________________________________________ ____________________________________________________________ Why? _______________________________________________________ ____________________________________________________________

14. Test your prediction by bringing the “t” tape close to the “l” tape. Describe your observations. _____________________________________________ ____________________________________________________________ ____________________________________________________________ Would you call this interaction attraction or repulsion? _________________

15. How do you think the uncharged tape from Step 8 will interact with the “t” tape and the “l” tape? __________________________________________ ____________________________________________________________Why? _______________________________________________________ ____________________________________________________________



16. Test your prediction by observing what happens when you bring the “t” tape close to the uncharged tape. Next, observe what happens when you bring the “I” tape close to the uncharged tape. Describe your observations. _____ ____________________________________________________________________________________________________________________________________________________________________________________

Now we are ready to expand our Exploration to more than one “t” tape and more than one “l” tape.

17. Without using the tapes yet, predict what interaction you think will occur when you bring the following pairs of tape close to each other. Describe the effect you predict as attraction, repulsion or no interaction. a) Two “t” tapes _______________________________________________ b) Two “l” tapes _______________________________________________ c) One “t” and one “l” tape _______________________________________

18. Pull the second pair of tapes off the table and discharge the combination by wiping with the sponge as in Step 9. Separate the tapes so that you now have another set of “t” and “l” tapes. Check your predictions from Step 13 and record your observations. a) Two “t” tapes _______________________________________________ b) Two “l” tapes _______________________________________________ c) One “t” and one “l” tape _______________________________________

19. What can you tell about the charges of tapes “t” and “l” from the results of Steps 14 and 16? _____________________________________________ ________________________________________________________________________________________________________________________

20. Based on the results of this experiment, how many types of charge do there appear to be? __________ What evidence can you provide to support your answer? _____________________________________________________ ________________________________________________________________________________________________________________________

21. Why do you think that both charged tapes (“t” and “l”) attracted the uncharged tape? ______________________________________________ ____________________________________________________________________________________________________________________________________________________________________________________

22. Obtain a rubber rod and a piece of wool. Make a new set of “t” and “l” tapes using the same process as described in Steps 8-11. Rub the rod with the wool, and then hold the rod near newly made “t” and “l” tapes on the meter stick. Compare the interactions of the rod with the tapes to the interactions between the tapes in Step 16. What similarities or differences do you



notice? Record your observations below. __________________________ ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

23. Charge a balloon by rubbing it on your hair or with a piece of wool. Check the interaction of the charged balloon with the “t” and “l” tapes. Record your observations, noting any similarities and differences. __________________ ____________________________________________________________ ____________________________________________________________________________________________________________________________________________________________________________________

24. Which pieces of tape do you think have the same type charge? _________ Which pieces of tape have different types of charge? __________________ Explain. _____________________________________________________ ____________________________________________________________________________________________________________________________________________________________________________________

25. What are the names usually given to these types of charge? ____________ ____________________________________________________________ Hint: You may want to look very carefully at the “t” and “l” you have written on your tape handles!



1.4

Dialog 1: The Definition of Charge

In this Dialog we will briefly review the atomic structure of matter to help you gain a basic understanding of electric charges. Here we will describe an atom as consisting of a collection of protons, electrons, and neutrons. Describing the atom in this way is an oversimplification of the atom, but the model is useful for helping to understand the effects you observed in Exploration 3. Remember what you have learned about scientific models in earlier modules: they are useful representations that help us understand observations.

In the model of the atom that we will use, neutrons and protons form the core of the atom, called the nucleus, with electrons surrounding the nucleus in circular orbits. Protons are positively charged, neutrons have no charge, and electrons are negatively charged. A neutral atom has an equal number of protons and electrons, which makes the net charge of the atom zero—the atom is electrically neutral.

A force of attraction exists between protons and electrons. This force

weakens as the negative electron orbits (also called shells) are further away from the positive nucleus. Hence, the electrons in the outermost shells can, under certain conditions, break loose from the atom. Adding or removing electrons from the outer orbits causes the atom to have a net charge, so an atom with excess electrons is negatively charged, while an atom with a deficit of electrons is positively charged. This principle can be also applied to an object. For example, an object with excess electrons is a negatively charged object, and an object with a deficit of electrons is a positively charged object. If an object has equal numbers of electrons and protons, it is electrically neutral, or uncharged.

Note that even though all matter consists of electrically charged particles,

an object is not considered charged unless unequal numbers of positive and negative charged particles are in (or on) the object. The natural state of any material is to be electrically neutral. The addition (or remaining charges) from a body is called a separation of charge.

Related to the concept of separation of charge is the principle of conservation of charge. The principle of conservation of charge states that charge is never created nor destroyed; it is simply redistributed. For example, if



two objects are rubbed together and one object takes electrons from the other, the object gaining electrons becomes negatively charged. Since the other object has lost electrons, it becomes positively charged.

The magnitude of the property we call charge is measured in units called coulombs. This unit is named after Charles Coulomb, who did research into the properties of interactions between charges in the 1780s. We will further discuss Coulomb’s ideas in Part Two of this module. However, note that protons have the same magnitude of charge as electrons and the value of this charge is 1.6 x 10-19 C: The proton is +1.6 x 10-19 C, and the electron is -1.6 x 10-19 C.

Insulators and Conductors All materials are classified based on their ability to allow the movement of

charges through the material. Materials that allow the movement of charge carriers are classified as conductors, and materials that do not allow the ready movement of charge carriers are classified as insulators. Semi-conductors are materials that are average to poor conductors. How the materials are classified depends on their atomic structure.

A neutral atom has the same number of negative electrons in orbits around the nucleus as positive protons in the nucleus. There is an established pattern of what orbits the electrons go into depending on the number of electrons the neutral atom has. For the purpose of our discussion it is only important to realize that there is a difference among atoms as to how many electrons are in the outermost orbit.

If the atom has one or two electrons in the outermost orbit it is easier to strip the electrons away from the atom. The materials formed by these atoms are called conductors. Most conductors are also classified as metals.

If an atom has a filled outer orbit or an orbit that only needs one or two electrons to fill it, the atoms form materials that are insulators. It is very hard to strip electrons away from these atoms.

All other atoms that have semi-filled outer orbits are materials that are semi-conductors. They do not conduct well or insulate well.



1.5

Exploration 4: Deducing the Type of Charge

Up to this point you have seen experimentally that there are two types of charge. However, you have not been able to tell for sure which charged object is positive and which is negative. In this Exploration you will build a device, the electrophorus, to help you deduce the sign of a charged object.

Equipment

Styrofoam cup

Aluminum pie pan

Styrofoam square or foam picnic plate

Tape

Wool fabric

Electroscope

Neon bulb or tube

Slightly damp sponge

1. To make an electrophorus, tape a Styrofoam cup to the center of the inside of the aluminum pie pan. This cup will serve as the handle of the electrophorus and a Styrofoam square will be the unattached base of the electrophorus.

2. Follow the steps below to charge the electrophorus:

a. Rub the Styrofoam base with wool to charge it.

b. Place the aluminum pie pan on the base, and then touch the pie pan with your finger.

c. Lift the plate off the base with the handle.

d. Bring your electrophorus close to an electroscope.

Is the electrophorus charged? How do you know? ____________________ ____________________________________________________________ ________________________________________________________________________________________________________________________



3. An interesting feature of the electrophorus is that you can recharge the plate simply by placing it back on the base and touching it with your finger, without rubbing the base. Explain how this can happen. _______________ ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

4. Lower the electrophorus back onto the foam base, touch the electrophorus gently with the tip of your finger, and repeat Step 3. What do you observe? Did touching the electrophorus cause it to become charged again? Explain. ____________________________________________________________________________________________________________________________________________________________________________________

Could you charge the electrophorus by touching it with your finger if it was not in contact with the base? Explain. _____________________________ ____________________________________________________________________________________________________________________________________________________________________________________ Try this and record your observations. _____________________________ ________________________________________________________________________________________________________________________

5. Hold the neon bulb by one lead and test the charged electrophorus by bringing it toward a neon bulb or tube. What do you observe? __________ ____________________________________________________________________________________________________________________________________________________________________________________ Reverse the bulb by holding the other end and repeat this observation. Is there any difference in the behavior of the bulb? _____________________ ____________________________________________________________________________________________________________________________________________________________________________________

6. Look carefully at the neon bulb. You will see two electrodes inside the bulb. The bulb will flash or glow around the electrode that is losing electrons to the neon gas in the bulb. Knowing this, determine whether the pie pan becomes negatively or positively charged. Record your observations and how you determined the sign of the charge on the plate. What is the sign of the charge on the Styrofoam base? _______________________________ ____________________________________________________________________________________________________________________________________________________________________________________



7. Repeat the tests in Steps 5 and 6 with a charged balloon. Describe any differences and similarities you observe. ___________________________ ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

8. Make a set of charged “t” and “l” tapes as you did in Exploration 3. Test both tape pairs with the charged electrophorus. Which tape is attracted to the electrophorus? Which is repelled? _____________________________ ________________________________________________________________________________________________________________________

9. Using the known charge of the electrophorus, can you now determine which tape, “t” or “l”, was positively charged and which was negatively charged? If so, explain your answer below and indicate which charge is on each tape. ____________________________________________________________ ____________________________________________________________________________________________________________________________________________________________________________________



1.6

Dialog 2: Charge Separation

We have explored transfer of charge, or the “charging” of various objects, using different methods of charge separation. The methods we used are described below.

Charging by Friction

In this method, a transfer of charge is caused by contact between objects, where the friction between two surfaces being rubbed together facilitates the transfer of charge. An example of this would be when you rubbed the rubber rod with silk. The rubber rod gave the silk a net positive charge. Since the silk gave up electrons to the rod, the rod acquired a net negative charge.

The type of charge acquired roughly follows what is called the triboelectric sequence, which is a relative measure of how easily an object gives up its electrons. A portion of this sequence is seen in the table at right. When any two objects shown in the table come into contact with each other, the one that is highest in the table will become positively charged, while the one that is lower in the table becomes negatively charged. Care must be taken, however, when using this table. Dirt, impurities in the material and surface imperfections can all affect the charge acquired by an object.

Charging by Conduction

The transfer of charge in this process is also through contact between objects. In this method one of the objects has an excess of charge. Since all of the excess charge has the same sign, the charges repel each other and try to get as far apart from each other as possible. When the object with this excess charge touches another object that is a conductor, some of the excess charge will escape into this second object. For example, if a charged rod is brought into contact with the metal ball of an electroscope, charges move from the charged rod to the leaves of the electroscope, as seen in the diagram at the top of the next page.

The Triboelectric Sequence

Glass Human hair

Nylon Wool

Quartz Cat fur Lead Silk

Human skin, aluminum Paper Cotton Steel Wood Amber

Hard rubber Copper, brass

Polyester Styrofoam

Saran wrap Polyethylene (Scotch tape)

Vinyl (PVC) Teflon



Charging by Induction

The transfer of charge in this process requires a charged object and an uncharged object. Charging by induction happens without contact between the charged object and the uncharged object. As you should remember from previous Explorations, an uncharged conductor can become charged when it is brought near—but does not touch—a charged conductor. If the two conductors come in contact they will charge by conduction. Below we discuss how an electroscope can be charged by induction.

When uncharged, the electroscope has an equal number of positive and negative charges (see Diagram A below). But, if a positively charged rod is brought near the ball of the electroscope, the positive charges in the rod repel the positive charges in the ball, forcing the positive charges in the ball to go down to the leaves (see Diagram B). The ball will then have a negative charge and the leaves will have a positive charge. The leaves will diverge. At this stage, the electroscope has not yet been charged because it still has an equal number of positive and negative charges on the ball and leaves. Also, the positive and negative charges on the electroscope will immediately return to their uncharged state if the charged rod is moved away, so the electroscope has not yet been charged by induction.



To charge the electroscope by induction, you would need to bring a charged rod near the ball (as in Diagram B) while you touch the ball with your finger (see Diagram C). The positive charges in the electroscope can get further away from the positively charged rod, and each other, if they travel through your finger to your body, rather than go down to the leaves. As the charges disperse throughout your body the repulsive forces essentially become zero, an effect that is called grounding. In this experiment your finger grounds the electroscope. If you remove your finger and the charged rod is moved away, the electroscope will be left with an excess of negatively charged electrons, so the electroscope is given a negative charge by induction. The leaves will remain separated even after the positively charged rod is removed (see Diagram D).

Charging by Polarization

Polarization is the mechanism that allows a charged object to attract an object that has zero net charge. Through charging by polarization, both positively and negatively charged objects are able to attract uncharged objects.

Charging by polarization occurs between a charged object and an

uncharged object, and it occurs without contact between these two objects. In important difference between charging by polarization and the other methods we have discussed in this section is that charging by polarization does not involve a transfer of charge; rather it is a realignment of the charges within an uncharged object.

An object becomes polarized

when its individual atoms or molecules align so the positive charges face one side of the object and the negative charges face the other side. As an example, we will consider a negatively charged balloon and an uncharged ball. When the two objects are brought near each other, the positive charges on the uncharged ball are attracted to the negative charges on the balloon and move closer to the balloon. The negative charges on the ball are repelled by the negative charges on the balloon and move further from the balloon.

The attractive forces between

the balloon’s negative charges and



the ball’s positive charges are stronger than the repulsive forces that occur between the balloon’s negative charges and the ball’s negative charges. The attractive forces in this situation are stronger because they are closer together than the repulsive charges; the ball’s charges have polarized, so it is attracted to the balloon.

Examples of Charging from Previous Explorations

In Exploration 4 you experimented with an electrophorus. You produced a net negative charge on the base of the electrophorus (the Styrofoam square) when you rubbed it with wool, thus charging the base by friction. The electrophorus itself, the metal plate with an insulating handle, was originally electrically neutral, but when you placed the metal plate on the charged Styrofoam square, some negative charges were repelled to the upper surface of the metal plate. When you touched the edge of the metal plate with your finger, the negative charges escaped through your body to the earth. Since electrons escaped from the metal plate, the plate was left with a net positive charge, thus charging the plate by induction. Lifting the metal plate by the insulating handle isolated the charged plate, which is why you found the presence of electrostatic charge when you tested the plate.

The process of creating a net charge on the metal plate can be repeated again and again just by placing the metal plate on the Styrofoam square and touching the edges of the metal plate with your finger. No charges are taken from the Styrofoam base, so the negative charges on the base continue to repel negative charges in the metal plate whenever it is placed on it.

Previously in this module you saw other examples of the different kinds of charging. In Explorations 1 and 2, you charged objects by the “charging by friction” method, where you rubbed a glass and a rubber rod with wool and silk. You should have found that both the rod and material became charged, and that they had opposite charges. This happens because one type of charge is rubbed off one object and goes into the second object.



1.7

Extension 1: Generating Charge

1. As you and a friend are getting out of a car, you touch the car’s metal door and get “zapped.” Your friend says what you experienced was “just static charge.” What does she mean?

2. What causes static cling?

3. You may have heard cracking sounds as you removed your clothes from a clothes dryer. Is this an example of static cling? Would the phenomena be more likely to occur if the all the clothes in the dryer were made of the same material or different materials? Explain.

4. The rubbing process did not charge some of the materials you tested. Why do you think this occurred? Think carefully about this before answering!

5. What are the properties that make a metal a good conductor and a non-metal a good insulator?

6. What type of charge will an object have if electrons are removed from it?

7. In your own words, name and describe four methods of charging an object. Which of these will leave the object only temporarily charged? Explain.

8. Describe lightning in your own words. Where does the charge come from?

9. What type of charge, positive or negative, do you acquire when you shuffle your feet across a synthetic carpet? Explain.

10. A positively charged rod is brought near a fine stream of water. The positively charged rod attracts the fine stream of water. Is the water negatively charged? What would be the effect if the rod were negatively charged? Explain.

11. Explain why a balloon that has been charged by rubbing it on your hair will stick to a wall for a short time.

12. A piece of tape is hanging from a wooden support rod. When a negatively charged glass rod is brought close to the tape, the tape is attracted to the glass rod. What type of charge is on the tape?



1.8

Demonstration 1: The Van de Graaff Generator

The Van de Graaff generator is, in effect, a charge pump. A frictional process separates charges, and one type of charge is pumped onto the metal sphere on the top of the generator by a rubber belt. Once the metal sphere is charged, we can do a number of experiments to detect the presence of charge. Some of these experiments include the following:

Strapping a wig to the surface of the sphere.

Standing a volunteer on an insulated surface and having him or her touch the sphere while it is charged.

Strapping a plume to the sphere.

Bringing a fluorescent bulb or neon tube close to the sphere while the generator is operating.

Your instructor will carry out one or more of these demonstrations. Record your observations in the space provided below. Pay close attention and try to interpret what you see in terms of the electrostatic charges.



1.9

Application 1: Antistatic Bags and Gasoline Fires

You may have bought some electronic components such as computer boards that were delivered to you enclosed in antistatic bags like the one shown above. Use the results of previous Explorations in this module to answer the following questions.

1. How do these bags protect electronic components?

2. When are static charges generated? How much energy do you think is generated (electric potential energy found in Part 3)? Under what conditions do you think this amount of energy will become large enough to damage the components? Go to the Web to find possible answers to these questions.

3. Are there precautions the end-user should take to avoid damage to the electronic components?

4. Your instructor may provide you with an antistatic bag to test using the methods we have developed so far. After testing the bag, write a paragraph describing your observations and conclusions regarding the bag’s ability to protect electronic components.



Many filling stations display safety stickers on the gasoline pumps similar to the one shown above. Gasoline is a volatile substance, meaning that it evaporates easily, putting gasoline vapors into the air. These vapors are highly flammable, and can easily be ignited by a spark.

1. Some filling stations have removed the metal latches from the gas pump handles, so that you have to hold the nozzle while pumping gas. Why do you think they have done this?

2. Why do you think the sticker shown above warns drivers to not enter their vehicles while refueling?

3. Why is it important not to fill portable gas containers while they are sitting inside a vehicle (or in the bed of a truck)?

Additional information on precautions to be taken when refueling vehicles can be found at http://www.pei.org/static/

Table of Contents - San Diego Miramar Collegefaculty.sdmiramar.edu/faculty/sdccd/sokumoto/physics196/lab/week2... · Charles Schuler, California ... Charles Stone, North Carolina

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