2: What is the magnitude of the electric charge of an electron? 3: What is the law of conservation of electric charge?

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1 Chapter 18 Discussion January :58 PM Electric Forces and Electric Fields Reading Review 1: What is the SI unit of electric charge? 2: What is the magnitude of the electric charge of an electron? 3: What is the law of conservation of electric charge? 4: What is a conductor? What is an insulator? Give some examples of each. 5: Explain charging by induction. Give an example of how this works. 6: What is Coulomb's law? What can you use it for? What is its formula? 7: What is an electric field? What can you use it for? What is the formula for an electric field due to a "point charge?" 8: What is an electric field line? 9: Sketch the electric field line pattern for a positive point charge. a negative point charge. an electric dipole. a parallel-plate capacitor. 10: What is the principle of superposition? How does it make your life easier? Ch18DClassSP2015 Page 1

2 11: What are the properties of a conductor placed in an electric field when equilibrium is reached? (That is, no electric current flows.) 12: What is electrostatic shielding? What are some applications? Discussion Questions 1: Two metal rods have identical size, shape, and composition. Rod A has a net charge of +5 units, and Rod B has a net charge of 1 unit. The rods are originally separated, but then they are brought into contact with each other, while they are insulated from the rest of the world. After being in contact for some time, the rods are separated, and still insulated from the rest of the world. What is the final charge on each rod? 2: Repeat Question 1 if the rods are plastic. 3: You have three metal rods with identical size, shape, and composition. Rod A has a charge of +6 units, and Rods B and C are neutral. Explain how to get a negative charge on one of the rods using only the three rods present. 4: Two particles with positive charges A and B, where A > B, are held in place a distance r apart. Ch18DClassSP2015 Page 2

3 a: Draw a diagram showing the magnitude and direction of the electric force that each particle exerts on the other. Which force is greater? Explain. b: How would the magnitude of the force on each charge change from the situation in Part (a) if the magnitude of one of the charges were doubled? c: How would the magnitude of the force on each charge change from the situation in Part (a) if the distance between the charges were doubled? 5: Consider the same situation as in Question 4: And now include two additional particles so that the three "B" particles have the same charge and lie on the arc of a circle centred at the position of particle A: Ch18DClassSP2015 Page 3

4 Now consider the following two explanations of the new situations: Alice: "The net force on particle A due to the other three particles is now three times as great as before, because there is now three times as much charge producing the force, and the distances of the other three charges from charge A have not changed." Basil: "The force exerted on A by the top B particle cancels the force exerted on A by the bottom B particle, because they are symmetrically placed. Thus, the force exerted by the three B particles on A is the same as in Question 4, where there was only one B particle." Which person is correct? Explain. 6: Consider the following two situations. All of the particles are positively charged, and the magnitudes of all the charges on the particles labelled B are identical. Alice: "The force exerted by the three B particles on Particle A in Situation 2 is three times as great as in Situation 1, because the charge Ch18DClassSP2015 Page 4

5 of the three B particles is three times as great in Situation 2." Basil: "The force exerted on Particle A is the same in the two situations, because the forces from Particles B 2 and B 3 in Situation 2 are blocked by Particle B 1." Who is correct? Explain. 7: Consider the following situation. Particle A has a positive charge. Alice: "The force exerted by the two right particles on Particle A is zero, because if you think of the two right particles as a unit, their net charge is zero." Basil: "Even if you think of the two right particles as separate particles, one repels A and the other attracts A, and since they are the same distance from A, the two forces cancel out, and so the net force exerted by the two right particles on Particle A is zero." Who is right? Explain. 8: Consider the following situation. Particle A has a positive charge. Ch18DClassSP2015 Page 5

6 Alice: "The force exerted by the two right particles on Particle A is zero, because if you think of the two right particles as a unit, their net charge is zero." Basil: "Even if you think of the two right particles as separate particles, one repels A and the other attracts A, and since they are the same distance from A, the two forces cancel out, and so the net force exerted by the two right particles on Particle A is zero." Who is right? Explain. 9: The following diagram represents the electric field pattern for a particle that has a positive charge (i.e., a "point charge"). Is the magnitude of the electric field at position 1 greater than at point 2? Explain how you can tell. Ch18DClassSP2015 Page 6

7 10: The following diagram represents the electric field pattern for an electric dipole. Is the magnitude of the electric field at position 1 greater than at point 2? Explain how you can tell. 11: Is the magnitude of the electric field at position 1 greater than at point 2? Explain how you can tell. Ch18DClassSP2015 Page 7

8 12: Is the magnitude of the electric field at position 1 greater than at point 2? Explain how you can tell. 13: Sketch the electric field vector at each of the indicated points. Ch18DClassSP2015 Page 8

9 14: Calculate the magnitude and direction of the electric field at the point indicated by an asterisk. At position A there is a particle with charge 4.2 nc, and at position B there is a particle with charge 2.7 nc. Solution: Ch18DClassSP2015 Page 9

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11 The components of the net electric field at the asterisk point are: Ch18DClassSP2015 Page 11

12 15: Calculate the net force exerted on the positively charged particle by the other two particles. Ch18DClassSP2015 Page 12

13 Solution: The net force acting on the central particle is: The negative sign indicates that the direction of the net force on the central particle is to the left. Ch18DClassSP2015 Page 13

14 16: A particle of mass kg and charge C is placed in a region of space where the electric field is constant and has a magnitude of 2400 N/C. Determine the acceleration of the particle. Solution: The force exerted on the particle by the electric field is By Newton's second law of motion, the acceleration of the particle is Is it reasonable that the acceleration is so large? 17: You have the following charged particles: +1 nc, +2 nc, and +4 nc. Is it possible to arrange all three particles so that the net Ch18DClassSP2015 Page 14

15 electric field is zero at some position? If so, give an example. If not, explain why not. Solution: Yes, there are many ways. Here's one way to do this: Place all the particles along a line, and try to have the particles with the smaller charges balance the particle with the greater charge. You can do this by having each of the smaller charges create a field exactly half as strong as the field of the particle with the larger charge, but oppositely directed. (There are many other ways to do this; just choose different percentages instead of You can also double all the distances in the diagram below, or scale them all by the same factor.) With a little bit of play, or by working with the formula for the electric field due to a point charge, you'll be able to figure this one out for yourself based on the discussion yourself. And you should certainly be able to check that the net electric field at the (*) point is zero. Try to determine other configurations that will also produce a zero field someplace, if you are in a playful mood. Ch18DClassSP2015 Page 15