Chapter 27 The Electric Field



Chapter 27 The Electric Field

1. The electric field of a point charge is _____________________, but real-world charged objects have vast numbers of charges arranged ___________________________.

2. Look at figure 27.1, Notice that no matter the size or shape of the charged object, charges seem to be evenly spaced.

3. The electric field of a point charge is defined by formula 27.1. Write that formula here and define all variables:

4. Equation 27.2 ( F= Eq), tells us that the net electric field is the vector sum of the electric field due to each charge. In other words, electric fields obey ______________________________.

5. Looking at figure 27.3, it is clear to see that the net electric field is the result of _______________________________________.

6. Copy down the “Typical electric field strengths” from table 27.1. These numbers should help you identify if the answers you get for problems in chapter 27 are logical!!!

7. Use example 27.1 to support the steps of the electric field of multiple point charge strategy from page 820.

8. Two equal but opposite charges separated by a small distance form an ___________________________.

9. Explain the difference between a permanent and induced electric dipole:

10. Look at figure 27.7,explain in your own words the electric dipole created on each point charge due to the + and – charges. You may want to include sketch and colored pencil lines to help with your explanation.

11. It is useful to define the dipole moment shown as the variable __________. Look at equations 27.11 and 27.12, notice the difference not only in the numerator of these to equations. Note each equation here, define the variables and describe under which situations each equation is used.

12. In chapter 26 we used vectors to show “fields” at different points in space. Now we will start using “electric field lines.” The four steps in the tactics box on page 824 will help you draw/understand electric field lines. Copy the four rules here:

13. Look at figures 27.9b and 27.10, how do these sketches differ? How are they similar?

14. Complete the Stop and Think form page 825 here:

15. If a charged object contains a large number of excess electrons, it is not practical to track every electron. So…the charge is considered ____________ and ____________________.

16. A charged rod has a linear charge density which is defined by the equation:

17. A charged two dimensional surface has a charge density which is defined by the equation:

18. The equations above assume that the object is uniformly charged which means ______________________________________________________________.

19. Complete the Stop and Think from page 825 here:

20. Use example 27.3 to help you understand the steps in the strategy box on page 827. I suggest writing each step in the strategy with figure 27.13!!

21. Notice in example 27.4 the equation for electric field of a rod. Copy that formula here and define each variable:

22. Read the section titled “An Infinite Line of Charge” and then complete the Stop and Think 27.3 here:

23. Complete Conceptual Questions 2 and 3 here:

24. Complete Exercises 1,3,6,7,8 and 10 from pages 844-845 here:

25. The electric field of a ring can be calculated just like two point charges placed on an axis separated by a distance equal to the radius of the ring. Example 27.5 follows the five step strategy of 1) choose axis set up 2) identify the point at which to calculate the field 3) divide the segments into segments 4)draw the field vectors and 5) make any cancellations that are possible.

This example also clearly shows the z axis for the third dimension. Notice that the field in the y cancels leaving only the Ez to calculate.

Complete #11 from page 845 here. Use example 27.5 to guide you.

26. Looking at figure 27.16, explain why there is no electric field in the center of the ring:

27. A charged disk needs to be thought of as a series of rings which conbine to make a solid or continuous surface.

The surface charge of a disk is described by equation 27.16. Write that equation here and define all variables:

28. The electric field created by a charged disk is closely related to the surface charge through equation 27.22. Write that equation here and define each variable:

29. Use example 27.6 to help you complete # 13a from page 845:

30. An electrode is a _________________________ which is used to “steer” electrons along desired paths.

31. Electric field strength of a charged plane is directly proportional to ___________, which means the more charge, the ______________the field. The other important fact is that the field strength is the __________at all pints in space around the plate independent of the distance to z.

32. Looking at figure 27.18, a positively charged plane has an electric field on both sides which points ___________________ from the plate. If the plate where negatively charged, the field lines would _______________________ on both sides of the plate.

33. A sphere charge distribution can be compared to gravitation fields of planets!! It will not matter if the object is a hollow or solid sphere, as long s the charge distribution is uniform. The electric field created by a sphere is calculated via equation 27.28. Write that equation here and define each variable:

34. Complete the Stop and Think on page 833 here:

35. A capacitor is made of ________ electrodes placed a distance apart. These plates are so equally but _______________ and known as a _________________________.

36. The net charge of the capacitor is ____________ and charged by transferring electrons from one plate to the other. Because opposite charges attract, all of the charge is on the ____________ surfaces of the two plates and can be modeled as charged planes with opposite surface charge densities.

37. Inside the capacitor the field points from the _____________ plate to the _______________plate. BUT outside of the capacitor the field is 0. Explain in your own words why this happens:

38. The electric field within the capacitor can be defined by equation 27.29. Write both variations of this equation and define each variable:

39. What is a fringe field?

40. Does the shape of the electrodes matter? Explain your response:

41. Read the section on “uniform electric fields” and then complete the Stop and Think 27.5 from page 835.

42. Now it is time to define the motion of a charged particle in an electric field calculated through the equation F=qE. Notice in figure 27.24 that if a proton is placed in the electric field it moves ______________________ but an electron placed in the electric field moves ____________________________________.

43. If F is the only force action on the charged particle, the particle can accelerate due to Newton’s ____________ Law. Acceleration can be defined as force divided by mass OR (charge times electric field) divided by mass.

44. Will a proton and a sodium ion experience the same acceleration when placed in an electric field? Explain:

45. A charged particle in a uniform electric field will move with constant ________________________ but the direction of motion must be determined.

46. What are some uses for particle acceleration between capacitor gaps?

47. The force created by an electric force is (qE) OR charge times electric field force. Remember from objects in circular motion that F= mv2/r. So looking at figure 27.27, an electron moving in a circular path around a proton is driven by the force of attraction.

qE= mv2/r

48. Complete the Stop and Think from page 838 here:

49. In a uniform electric field the net force on a dipole is ___________________. Also, the electric field will cause the dipole to rotate until the net torque is _____________________________ and the dipole is at its equilibrium position. According to figure 27.28b, the ______________ end of the dipole points in the direction of the electric field.

50. As a dipole turns into its equilibrium position it will “feel” torque forces. Torque is defined by the angle which it turns through as it moves into its equilibrium position. Torque is defined in several terms via equations 27.34 and 27.35…but notice these are basic trig relations.

51. If a dipole is placed by a point charge, such as in figure 27.32, the net force is toward the strongest area of the field…where the lines are closest together. The dipole will just “flip” so that it will move into that part of the field.

52. Complete the following exercises from pages 845-847 here: #17,19,21,22,25 and 27.

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