Chapters 32 & 33: Electrostatics NAME: Text: Chapter 32 Chapter 33 Think and Explain: 1-6, 8 Think and Explain: 1, 4, 5, 8, 10 Think and Solve: Think and Solve: 1-2 Vocabulary: electric forces, charge, ion, conservation of charge, Coulomb s law, coulomb, conductors, isulators, semiconductors, superconductors, charging by friction, charging by contact, charging by induction, grounding, charge polarization Equations: Q = ne F e = kq 1q 2 d 2 F E = q kq E = 2 d V = PE q e Constants: k = 9 x 10 9 Nm 2 /C 2 e = +/- 1.6 x 10-19 C 1 µc = 1 x 10-6 C Key Objectives: Concepts Charged objects have gained or lost electrons. Distinguish between charging by friction, charging by contact, and charging by induction. Electric charge is conserved. Like charges repel and unlike charges attract. Electric forces exist between charged objects and that force can be calculated using Coulomb s law. Compare and contrast electrostatic forces and gravitational forces. Understand how charge polarization allows for a charged object to be attracted to a neutral object. Be able to state the units of charge, electric force. Understand the concept of electric field as the space around every electric charge. State the direction of the e-field Sketch e-field diagrams for a single charge, a pair of charges, and between two oppositely charged parallel plates. State the direction of force on a proton and on an electron placed in the field. How would you move a charge (positive or negative) in an e-field in order to increase electric potential energy. Distinguish between electric potential energy and electric potential. Distribution of charge on a conductor Strength of the e-field inside of a conductor Problem Solving: Convert between µc and C Convert between total charge and number of electrons. Use Coulomb s law to calculate the force between two charged objects. Use Coulomb s law to find the unknown value when all other values are given. Calculate the strength and direction of the electric field given the a force on a test charge and the charge Calculate the strength and direction of an electric field some distance away from the charge. Convert between electric potential, potential energy and charge. 2014-15
Lab 32-1a: Electrostatics NAME: Taking off a sweater can sometimes make your hair stand on end. If you rub a balloon in your hair, it will stick to the wall. Socks cling to everything coming out of the drier. Scuff your feet on a rug and you can give someone a shock.. These are all common examples of what is now called static electricity. But what exactly is static electricity and electric charge? You can start to answer this by examining the behavior of some objects that are easily charged by rubbing. In all of these activities, you will be charging up various objects. You will then be asked to make specific observations. In addition to any other observations you may find, note whether objects are attracted or repelled. Activity 1: Single piece of Scotch Tape a. Stick a single piece of tape on the table and tear it off quickly. What happens when you bring the tape near your hand? Near the table? Near your partner? Try it other places. b. Make a second piece of tape like the first one and repeat your observations. What happens? c. What happens when the two pieces of tape are brought close? Activity 2: Scotch Tape Sandwiches a. Place a piece of tape on the table as you did before. At one end, write "bottom." Place a second piece of tape on top of the first and mark it "top." Quickly tear off the sandwich, and then rapidly pull apart the top and bottom pieces. b. What happens to the bottom piece of tape when it is brought near something? c. What happens to the top piece of tape when it is brought near something? d. What happens when a top is brought near a bottom? e. Prepare a second sandwich like the first one. Remember to mark the two pieces top and bottom. What happens when a top is near a top? f. What happens when a bottom is near a bottom? g. Compare your results with your classmates. Make a general statement of how the charged tape interacts with something. h. How could you determine if a single piece of tape (from activity one) is charged like a top or bottom piece? side 1
i. Is a single strip like a top or like a bottom? Lab 32-1a: Electrostatics NAME: Activity 3: Charging by Friction a. Rub a rubber rod with a piece of fur. Bring it near some little pieces of paper. What happens? b. What is the charge of the rubber rod? What is the charge of the fur? rubber rod = fur = c. Rub a piece of acrylic with a paper towel. Bring it near some little pieces of paper. What happens? d. What is the charge of the acrylic? What is the charge of the paper towel? acrylic = towel = e. Rub a piece of styrofoam with a piece of fur or paper towel. What is the charge of the styrofoam? What is the charge of the fur or paper towel? styrofoam = fur/towel = Questions These are important! 1. How do like charges interact? 2. How do opposite charges interact? 3. How does any charged object interact with an uncharged (or neutral) object? 4. Imagine that something is attracted to a top piece of tape. What is the charge of the object? 5. Imagine that something is repelled by a bottom piece of tape. What is the charge of the object? 6. Exactly how do you tell if a random object is "top", "bottom" or neutral? 7. Give some other, everyday examples of thigns being charged by friction. side 2
Lab 32-1b: Electrostatics NAME: Top = Bottom = Activity 4: Charging by Contact. a. Suspend a graphite-coated sphere from a stand so that it can swing freely in any direction without touching anything. b. Give a rubber rod a charge through friction. Bring it near the sphere. What happens? Be careful, there are couple things that happen! c. The sphere should end up getting charged from part b. What are the charges of the rod and sphere? rod = sphere = d. Hold the sphere for a moment to remove any charge it may have. Charge up a piece of acrylic by friction. Bring it near the sphere. What happens? e. The sphere should end up getting charged from part b. What are the charges of the strip and sphere? strip = sphere = f. Hopefully, in both cases above, the sphere was initially attracted to both of the objects. Why? g. Hopefully, in both cases above, the sphere was suddenly repelled by both of the objects. Why? h. Again, remove any excess charge the sphere may have. Charge up a piece of acrylic, and bring it close to the sphere. After the sphere has been charged, bring a charged rubber rod close to it. What happens? i. Describe what happened to the charge of the sphere in doing part h. j. What is meant by the phrase charging by contact? side 1
Lab 32-1b: Electrostatics NAME: Activity 5: Charging by Induction. Try and actually follow the directions. a. Charge up the piece of styrofoam by rubbing it lightly with a piece of fur. What are the charges of the styrofoam and fur? (Test the fur first, as it will lose its charge faster than the styrofoam.) styrofoam = fur = Were these things charged by contact or by friction? b. Place a pie tin on top of the charged Styrofoam. (Use the coffee cup holder when handling the tin.) Briefly touch the pie tin. What happened? c. Now pick up the pie tin, but make sure you only hold on to the coffee cup holder. What is the charge of the pie tin and the styrofoam? styrofoam = pie tin = d. Bring it near a graphite-coated sphere. What happens? e. What do you think happened to the charges of the pie tin and the sphere? What would you call this process? f. Again, holding on to the coffee cup holder, place the pie tin on the Styrofoam, and then touch the pie tin. Now pick up the pie tine by holding on to the pie tin itself. Bring it near the graphite coated sphere and also determine the charge of the pie tin. What happened? pie tin = g. Holding on to the pie tin itself, place the pie tin on the Styrofoam. Still holding onto the pie tin itself, pick up the pie tin and determine the charge of the pie tin and the effect on the graphitecoated sphere. pie tin = h. Lastly, holding onto the pie tin, place the pie tin on the styrofoam. If you now touch the pie tin, nothing should happen. (Like in part g.) Pick up the pie tin by holding onto the coffee cup. What is the charge of the pie tin now and what is the effect on the graphite-coated sphere? pie tin = j. Can you charge the pie tin by (holding onto the cup) bringing it close to, but not touching, the styrofoam and then briefly touching the pie tin? k. What happened to the charge on the styrofoam during this process? Did the pie tin ever do anything to the styrofoam? side 2
Lab 32-1c: Electrostatics - Notes NAME: Atomic Structure 1. All atoms are made of three smaller particles. What are they, what are their charges, and what are their relative masses? 2. How are these particles arranged in an atom? 3. A normal atom has no net charge. What is true about the numbers of electrons, protons and neutrons in an atom? 4. If you change the number of protons in an atom, what happens to the atom? 5. If you change the number of neutrons in an atom, what happens to the atom? 6. If you change the number of electrons in an atom, what happens to the atom? 7. It is relatively easy to change the number of only one of the particles that make up an atom which one is it and why? 8. In this lab, you charged up a variety of objects, making them positive and negative. You were actually causing some of these small particles (electrons, protons and/or neutrons) to go back and forth between the objects. a. To make something negative, what happened to the object in terms of electrons, protons and neutrons? b. To make something positive, what happened to the object in terms of electrons, protons and neutrons? 9. Define and give examples of the following terms: Insulator Conductor Which of those has all its electrons tightly bound to each atom? Which has some "free electrons" that are not tightly bound to each atom? 10. What is meant by the phrase Conservation of Charge? side 1
Lab 32-1c: Electrostatics - Notes 11. We usually talk about charge being quantized. What does this mean? NAME: 12. What is the charge of each of the following: electron = proton = neutron = 13. Because really small charges can have large electrical forces on them, we often use units of µc for the charge of an object. The µ stands for, and can be read as a. Problems 14. If an object has 5 extra electrons, what is its total charge in C? 15. What is the charge of an object that has 20 x 10 11 extra electrons? 16. How many electrons would it take to make a total charge of 6.4 x 10-19 C? 17. How many electrons would it take to make a total charge of 30 µc? 18. How many electrons would it take to make a total charge of 2 µc? 19. How many electrons would an object have to be missing to have a charge of +5 µc? 20. Why didn t I ask for how many extra protons it would take to make a total charge of +5 µc? 21. What is the total charge on an atom that has 12 electrons and 12 protons and 12 neutrons? 22. What is the total charge on an atom that has 14 electrons and 12 protons and 12 neutrons? 23. What is the total charge on an atom that has 12 electrons and 14 protons and 14 neutrons? 24. What is the total charge on an atom that has 13 electrons and 13 protons and 15 neutrons? Answers: 14) 8 x 10-19 C 15) 3.2 x 10-7 C 16) 4 17) 1.88 x 10 14 C 18) 1.25 x 10 13 C 19) 3.13 x 10 13 C 20) b/c p + don't move! only e move 21) 0 C 22) 3.2 x 10-19 C 23) 3.2 x 10-19 C 24) 0 C side 2
Lab 32-1d: Electrostatics - Notes NAME: There are three main ways to get something charged up: through friction, through contact or through induction. How you charge something largely depends on whether the object is a conductor or insulator, and whether you already have another charged object to use. Charging by Friction Explain: 1. Why do electrons get transferred from one object to the other? 2. Why don t any protons get transferred in this process? 3. Why can you not rub two identical objects to charge them by friction? 4. Assuming that you started off with two neutral insulators, and you charged them by rubbing them together, what is true about the charges on the two objects? Charging by Contact Explain: 5. Why can you not (easily) charge insulators through simple contact? 6. Imagine you have two identical metal spheres on insulating stands, labeled A and B. For each of the questions below, you are given what the charges are on each sphere tell what will happen to the charges if you touch them together. a. A = +4 & B = 0 A = & B = b. A = 0 & B = 6 A = & B = c. A = +5 & B = +5 A = & B = d. A = +2 & B = 2 A = & B = e. A = +4 & B = 2 A = & B = A B 7. If the two conductors are identical, they must have the exact same charge after touching each other. What would happen if one of the conductors was a lot smaller than the other? side 1
Charging by Induction Explain: Lab 32-1d: Electrostatics - Notes NAME: 8. Imagine you have a large positive charge (labeled +Q in the diagram.) You bring a conductor near the positive charge. The conductor is still neutral, but diagram what happens to the charge distribution in the conductor. Then show what happens when you touch the conductor, and then show the final result. +Q +Q +Q bring conductor close to charge touch the conductor final result 9. Do the same thing, but instead use a large negative charge (labeled Q in the diagram.) -Q -Q -Q bring conductor close to charge touch the conductor final result 10. In both cases, what charged particles were doing ALL the moving? Why? 11. In both cases, how did the charge of the conductor compare to the original charge you started with? 12. In both cases, did anything happen to the original charge? 13. Only conductors can be charged through induction. Why? 14. From the lab, you actually placed the pie tin on top of the charged styrofoam. Since the styrofoam was negatively charged, why didn t the pie tin simply become negatively charged because of the contact with the styrofoam? 15. What is meant by the following terms: free electron charge polarization side 2
Coulomb s Law I NAME: General Electrostatic Questions 1. Charge is measured in units of. 1 µc is equivalent to C. 2. Two like charges will and two opposite charges will. 3. The charge of a proton is. 4. The charge of an electron is. 5. Why is it impossible for an object to have a charge of 1 x 10-19 C? 6. When a vinyl strip is rubbed with fur, 1,000,000 extra electrons are deposited on the strip. What is the charge on the strip? Coulomb s Law Questions 7. What is Coulomb s Law? 8. A sphere with a charge of +0.0004 C is placed 1 m away from a sphere with a charge of 0.00004 C. Will these charges attract or repel? What is the force between them? 9. How far apart are two identical charges of 2.5 x 10-6 C if the force between them is 0.5 N? 10. A 3.2 µc charge is placed 25 cm away from a charge of +5.0 µc. a. What is the force on the 3.2 µc charge? b. What is the force on the +5.0 µc charge? 11. Two Styrofoam peanuts have somehow become charged. The peanuts each have a charge of -5 x 10-8 C and they are 3 cm apart. a. What is the force of repulsion between them? b. How many electrons are on each peanut? Side 1
Coulomb s Law I NAME: 12. Two raindrops have become charged. One raindrop has somehow gained one million extra electrons. The second raindrop has gained 3 million extra electrons. If the two raindrops are only 0.01 meters apart, what is the force of repulsion between them? 13. The radius of a hydrogen atom is about 5 x 10-11 meters, and can be thought of as an electron orbiting a proton nucleus. What is the electrostatic force of attraction between the two particles? 14. Two metal spheres have identical charges on them. They are separated by 0.25 meters and the force of repulsion them is 0.007 N. What is the charge on each sphere? 15. If the distance between 2 charges were doubled, what would happen to the force between them? 16. If the distance between 2 charges were cut in half, what would happen to the force between them? 17. If the magnitude of one charge were doubled, what would happen to the force between them? 18. If the magnitude of one charge were doubled and the magnitude of the other charge tripled, what would happen to the force between them? 19. If the distance between two charges were tripled and the magnitude of one of the charges quadrupled, what would happen to the force between them? Answers 1) Coulombs; 10-6 2) repel; attract 3) +1.6 x 10-19 C 4) 1.6 x 10-19 C 5) not an integer multiple of #3 or #4 6) 1.6 x 10-13 C 8) ( )144 N, attract 9) 0.34 m 10. a) -2.3 N b) 2.3 N 11. a) 0.03 N b) 3.13 x 10 11 electrons 12) 6.9 x 10-12 N 13) ( )9.2 x 10-8 N 14) 2.2 x 10-7 C both + or both 15) 1/4 x 16) 4x 17) 2x 18) 6x 19) 4/9 x Side 2
Purpose: Lab 32-2: Coulomb's Law NAME: 1. To determine the quantity of charge on a graphite-coated sphere. 2. To determine the number of extra or missing electrons the sphere. Procedure: 1. Mass the two graphite-coated spheres and record in the data section. (This is probably written on the board.) 2. Charge up the items by friction (this will depend on the class.) Then, charge up the two graphite coated spheres by contact. 4. The two spheres should now be repelling one another. There are two things you have to measure while the spheres are charged: the distance between the spheres and the angle the string makes with the wooden rod Data: Mass of one sphere: kg Angle of string: º Distance between spheres: m d Calculations: First, let's figure out the sign of the charges on the spheres. 1. What did you charge by friction? 2. After charging by friction, what were the signs of the charges on those items? 3. After you charged the spheres with contact, where the spheres positively or negatively charged? How do you know? Now, let's figure out the magnitude of the charge on the spheres. 4. There are three forces acting on each sphere. Make a diagram showing these forces on the sphere on the right. You do not need to use numbers, but make sure you label the forces. 5. Why must these forces add up to zero? side 1
6. Calculate the weight of each sphere. Lab 32-2: Coulomb's Law NAME: F g = N 7. The diagram shows the three forces acting on the sphere adding up to zero. You just calculated the weight (F g ) and measured the angle earlier. Using trig, find the magnitude of the electric force on the sphere. (Hint: which trig function should you use if you know the opposite side, and want to know the adjacent side? Even More Hints: Look on the board.) F g T F e F e = N 8. Calculate the magnitude of the charge on the sphere by using Coulomb's Law. (Assume that you charged each sphere an equal amount, so that q 1 = q 2.) C 9. How many missing or extra electron are on each sphere? (Remember that the magnitude of the charge of one electron is 1.6 x 10-19 C.) missing/extra electrons side 2
Coulomb s Law II NAME: 1. How many electrons make up a charge of 30 µc? 2. What is the magnitude of the electric force between two protons if the distance between them is 1.5 x 10-15 m? 3. Two identically charged objects experience a force of 2.3 N when placed 40 cm apart. What is the charge on each of these objects? 4. Two charged particles exert a force of 4.2 x 10-2 N on each other. a. What will be the force if the charges are moved so that they are eight times as far apart? b. What if they were 1/2 as far apart? 5. Three charges are arranged as shown in the diagram: 10 cm 14 cm A B C 4 µc +5 µc 8 µc a. Find the force between charge A and charge B. b. Find the force between charge A and charge C. c. Draw a free body diagram for the forces acting on charge A. d. What is the net force on charge A? e. What is the net force on charge C? (Note: this is a few steps, but remember Newton s Third Law to cut down on your calculations.) Side 1
Coulomb s Law II NAME: 6. Two charges are located as shown in the picture. +5 C -3 C a. Where should a small positive charge be placed so that the net force on it is zero? b. Where should a small negative charge be placed so that the net force on it is zero? 7. Two identical spherical shells on insulating stands are 2 meters apart. The charge on one of the spheres is 25 µc. The two charges are attracted to each other with a force of 4.22 N. a. What is the second charge? b. If you then pick up one of the spheres by the insulating stand, then let the two spheres touch each other, and then put the sphere back where is was originally, what will be the new electric force between the charges? (Hint: what will happen to the charges of the two spheres after they come in contact?) Answers 1) 1.88 x 10 14 electrons 2) 102 N 3) 6.39 x 10-6 C, both positive or both negative (6.39 µc) 4. a) 6.56 x 10-4 N b) 0.168 N 5. a) (-)18 N (attract) b) 5 N (repel) 5 N 18 N c) d) 13 N to the right e) 13.4 N to the left 6. a) to the right (somewhere) of the 3 µc charge b) same spot 7. a) -7.5 x 10-5 C b) 1.41 N (repel) (hint: new charges on both spheres 2.5 x 10-5 C) Side 2
Electric Field Part 1 NAME: Action at a Distance By the 1800s, people had discovered the mathematical rules that governed gravitational, electric and magnetic forces between objects. While the rules worked fabulously, there was an everpresent concern over what is called "Action at a Distance": exactly how do objects exert forces on each other when they are not touching? How can there be an action caused over a distance? How does one object "know" that there is a second object some distance away from it? The answer came in the concept of the "field." The existance of an object creates a field that spreads out through space, getting smaller the further away it goes. A second object doesn't know about the first directly, it simply knows that it is in the field created by the first object (and vice versa.) Objects that have mass are affected by a gravitational field and have their own gravitional field around them. An object that has charge is affected by electric fields, and in turn creates its own own electric field that affects other charged objects. [We have actually used this idea for most of the year when dealing with gravity; we are all in a gravitational field of 9.8 m/s 2, which is the same as 9.8 N/kg. The further away from the earth you are, the less you weigh because the gravitational field is smaller.] Definition If a charge (q) is experiencing an electric force (F), then the charge is in an electric field (E) defined by E = F q Problems 1. What are the units for electric field? 2. These are unrealistic numbers (because the charges are way too big), but will help you get a feel for the relationship between the variables: a. A charge of 3 C experiences a force of 27 N because it is in an electric field. How strong is the field? b. A charge of 6 C experiences a force of 27 N in an electric field. How strong is the field? c. What is the force on a 0.2 C charge in a 300 N/C electric field? d. What is the force on a 2 C charge in a 300 N/C electric field? e. A charge experiences a force of 15 N when it is in a 5 N/C electric field. What is the charge? f. What charge would experience a 175 N force in an electric field of 25 N/C? 3. What is the strength of an electric field at a location where a 1.6 x 10-19 C test charge experiences a force of 3.2 x 10-16 N? Side 1
Electric Field Part 1 NAME: 4. An electron (1.6 x 10-19 C & 9.1 x 10-31 kg) is in an electric field of 600 N/C. a. What is the force acting on the electron? b. What is the acceleration of the electron? 5. An unknown charge experiences a force of 0.4 N when it is in an electric field of 8000 N/C. a. What is the charge? b. If the unknown charge were doubled, what would happen to the force on the charge? c. If the unknown charge were doubled, what would happen to the electric field? 6. It turns out rubbing a balloon in your hair creates an electric field of about 50,000 N/C near the surface of the balloon. a. What would be the force on an electron close to the surface of a charged balloon? b. What would be the force on a proton close to the surface of a charged balloon? c. What would be true about the accelerations of a proton and electron near the surface of a charged balloon? (Think about magntiudes and directions.) 7. An electron is accelerated at 3 x 10 12 m/s 2. What is the strength of the electric field? 8. A proton (1.7 x 10-27 kg) is accelerated at 3 x 10 12 m/s 2. What is the strength of the electric field? Answers: 1) N/C 2. a) 9 N/C b) 4.5 N/C c) 60 N d) 600 N e) 3 C f) 7 C 3) 2000 N/C 4.a) (-) 9.6 x 10-17 N b) 1.1 x 10 14 m/s 2 5. a) 5 x 10-5 C b) double (0.8 N) c) same (8000 N/C) 6. a) 8 x 10-15 N b) ) 8 x 10-15 N c) opposite directions. electron bigger accleration because less mass 7) 17.1 N/C 8) 31,900 N/C Side 2
Electric Field Part 2 NAME: Source of Electric Field So far we have dealt with what happens when a charge is caught in an electric field with the equation E = F/q. Now we will figure out where the electric field comes from. We began this unit with Coulombs Law, which gave us the force that two charges exerted on each other. The force depended on the two charges and how far apart they were, and in fact the equation was very similar to the equation that described gravitational forces. q 1 q 2 d From Coulomb's Law, we know the force acting on q 1 (the charge on the left) is F = k q 1q 2 d 2 Now we can also say that the reason there is a force on q 1 is that it is in an electric field of E = F q 1 Notice what happens when we combine these two ideas: the electric field at the location of q 1 is the Coulomb force between the two charges divided by the q 1. In equation form: E = F q 1 = k q 1q 2 d 2 d 2 Hey! The term q 1 canceled out! That equation tells us the strength of the electric field a distance d away from the charge q 2. We typically write it this way: q 1 = k q 2 The electric field (E) a distance (d) away from a charge (Q) is given by E = k Q d 2 Problems 1. These are unrealistic numbers (because the charges and fields are way too big), but will help you get a feel for the relationship between the variables: a. What is the electric field 3 meters away from a charge of 15 C? b. What is the electric field 6 meters away from a charge of 30 C? c. How much charge would be needed to make a field of 100 N/C from a distance of 4 meters? d. How far away from a 0.003 C charge is the electric field 6000 N/C? 2. A fly accumulates 3.0 x 10-10 C of charge as it flies through the air. What is the strength of the electric field 0.02 m away from the fly? Side 1
Electric Field Part 2 NAME: 3. What is the electric field 30 cm away from a 33 µc charge? 4. The electric field 75 cm away from the center of a van de Graaff generator is 50,000 N/C. What is the charge on the van de Graaff generator? 5. A balloon has a charge of 8 x 10-7 C. a. What is the electric field 45 cm away from the center of the balloon? b. Where is the electric field 8000 N/C? 6. A charged balloon has radius of 10 cm. The electric field on its surface is 4000 N/C. a. What is the charge of the balloon? b. How many electrons would this represent? 7. What is the electric field 1 Å (10-10 m) away from a helium nucleus? (A helium nucleus has 2 protons in it.) 8. A 3.2 µc charge and a 4.5 µc charge are 60 cm apart, as shown in the diagram below. What is the net electric field half way between the charges? 0.6 m +3.2 µc 4.5 µc Answers: 1. a) 15,000,000,000 N/C b) 7,500,000,000 N/C c) 1.78 x 10-7 C d) 67.1 m 2) 6750 N/C 3) 3,300,000 N/C 4) 3.13 x 10-6 C 5. a) 35,600 N/C b) 0.95 m 6. a) 4.44 x 10-9 C b) 27,800,000,000 electrons 7) 288,000,000,000 N/C 8) 130,000 N/C Side 2
The Electric Field 1. Draw the electric field around the following charges: a. Single Positive Charge b. Single Negative Charge NAME: c. Two Positive Charges d. Two Negative Charges e. Two Opposite Charges 2. The diagram to the right represents a random electric field. a. At which point would the electric field be the greatest? b. At which point would the electric field be the least? B C E c. At point B, draw an arrow that would represent the force on a proton placed at B. A D d. At point C, draw an arrow that would represent the force on an electron placed at C. side 1
The Electric Field NAME: It turns out that when you put a charge on a conductor, the charge will distribute itself on the surface of the conductor so that there is no electric field inside the conductor even if there is a huge electric field just outside the conductor. In order to do this, charge tends to concentrate on edges and especially points. 3. What was the demo to show that charge piles up on edges and points? 4. Imagine that a sphere and a cube each have a large negative charge on them. Show how the extra electrons would be distributed on each shape. 5. Why is the van de Graaff generator a big sphere? 6. Charge will build up on the van de Graaff generator until the electric field just outside the generator is about 400,000 N/C, which is a pretty big field. What is the field inside the generator? 7. Air is actually a pretty good electrical insulator. However, if there is an electric field bigger than about 400,000 N/C, what happens to air? 8. Putting a big conductor in an electric field causes the free electrons on the conductor to move around and rearrange themselves. After they move around, what is true about the electric field inside the conductor? 9. What is meant by the term Electric Shielding? 10. At the Museum of Science, the person running the lightening show is in a metal cage. Why does that keep them safe? 11. If you are in an accident, and live electric wires fall on your car, what should you do? 12. Why can t an insulator shield out electric fields? side 2
Name Class Date Concept-Development Practice Page 33-2 Electric Potential 1. Just as PE (potential energy) transforms to KE (kinetic energy) for a mass lifted against the gravitational field (left), the electric PE of an electric charge transforms to other forms of energy when it changes location in an electric field (right). When released, how does the KE acquired by each compare to the decrease in PE? 2. Complete the statements. A force compresses the spring. The work done in compression is the product of the average force and the distance moved. W = Fd. This work increases the PE of the spring. Similarly, a force pushes the charge (call it a test charge) closer to the charged sphere. The work done in moving the test charge is the product of the average and the moved. W =. This work the PE of the test charge. Pearson Education, Inc., or its affi liate(s). All rights reserved. If the test charge is released, it will be repelled and fly past the starting point. Its gain in KE at this point is to its decrease in PE. At any point, a greater quantity of test charge means a greater amount of PE, but not a greater amount of PE per quantity of charge. The quantities of PE (measured in joules) and PE/charge (measured in volts) are different concepts. By definition: Electric Potential = PE/charge. 1 volt = 1 joule/1 coulomb 3. Complete the statements. CONCEPTUAL PHYSICS Chapter 33 Electric Fields and Potential 149
4. When a charge of 1 C has an electric PE of 1 J, it has an electric potential of 1 V. When a charge of 2 C has an electric PE of 2 J, its potential is V. 5. If a conductor connected to the terminal of a battery has a potential of 12 volts, then each coulomb of charge on the conductor has a PE of J. 6. If a charge of 1 C has a PE of 5000 J, its voltage is V. 7. If a charge of 0.001 C has a PE of 5 J, its voltage is V. 8. If a charge of 0.0001 C has a PE of 0.5 J, its voltage is V. 9. If a rubber balloon is charged to 5000 V, and the quantity of charge on the balloon is 1 millionth coulomb, (0.000001 C) then the PE of this charge is only J. 10. Some people get mixed up between force and pressure. Recall that pressure is force per area. Similarly, some people get mixed up between electric PE and voltage. According to this chapter, voltage is electric PE per. Pearson Education, Inc., or its affi liate(s). All rights reserved. CONCEPTUAL PHYSICS 150 Chapter 33 Electric Fields and Potential
Electric Potential Problems NAME: 1. It takes 6 J of work to move a charge of 2 C from point A to point B. What is the electric potential between these points? 2. A charge of 45 µc has 9 x 10-4 J of potential energy. What is the electric potential where the charge is? 3. A 3 C charge is at a spot with a potential difference of 120 V. How much potential energy does it have? 4. A 6 C charge is at a spot with a potential difference of 120 V. How much potential energy does it have? 5. You pick up and move a charge of 3 µc from point A to point B. The voltage at point A is 50 V and at point B is 200 V. How much work did it take to move the charge? 6. A certain charge has 30 J of potential energy and a voltage of 3 V. a. What is the charge? b. If the charge were doubled, what would be the potential energy? c. If the charge were doubled, what would be the voltage? 7. How much work would it take to push a charge of 15 µc to a potential of 120 V? 8. What is the charge of an object if it has an electric potential of 45 V and an electric potential energy of 90 x 10-6 J? 9. An electron is in a 12 V electric potential. a. How much potential energy does it have? b. If a second electron joined the first one, what would be the new electric potential? c. If a second electron joined the first one, what would be the new electric potential energy? Answers: 1) 3 V 2) 20 V 3) 360 J 4) 720 J 5) 4.5 x 10-4 J 6. a) 10 C b) 60 J c) 3 V 7) 0.0018 J 8) 2 µc 9. a) 1.92 x 10-18 J b) 12 V c) 3.84 x 10-18 J