Lesson Plans

Physics 1st Edition ©2002

by James S. Walker

Week 19

Chapter 19: Electric Charges, Forces, and Fields

College Board Performance Objectives:

• Discuss the nature of electrical charge.
• Understand charge quantization.
• Recognize that all charges are multiples of the fundamental unit of charge.
• Describe and illustrate Millikans Oil-Drop Experiment and its significance in the history of the development of physics.
• Demonstrate that charge is conserved.
• State the Law of Conservation of Electrical Charge.
• Distinguish between an insulator and a conductor using examples.
• State Coulombs Law and express it in terms of an equation.
• Apply Coulombs Law to physical situations involving systems of point charges using the principle of superposition.
• Define the electric field in terms of an isolated point charge.
• Calculate the magnitude and the direction of the force that would act on a test charge placed at a given point in an electric field.
• Write a mathematical expression to determine the electric field at a given point in space.
• Calculate the electric field of a system of charge distributions using the principle of superposition.
• Describe the behavior of a charged particle in a parallel plate capacitor.
• Explain how to charge a body by induction.

College Board Lab Objectives:

• Discover the electrical properties of conductors and insulators.
• Devise an experiment to measure the charge on an electron.
• Determine the shape of the electrical field around a conductor.
• Sketch the electric field pattern between point charges and charged objects.

Suggested Labs:

• Mapping the Electrical Field
• Charge on an Electron

Resources:

• Student Edition — pp. 609–639
• Student Study Guide — pp. 326–343
• Instructor's Solution Manual Volume 2 — pp. 1–25
• Instructor's Solution Manual CD — Chapter 19.doc
• Instructor's Resource Guide — pp. 77–80
• Test Items File — pp. 240–256
• Media Portfolio CD — Lecture Resources Chapter 19

Pacing Guide:

• Electric Charge—day 1
• Insulators and Conductors—day 1
• Coulomb's Law—days 2 and 3
• The Electrical Field—days 2, 3, and 4
• Electric Field Lines—days 3 and 4
• Shielding and Charging by Induction—days 3 and 4
• Lab—day 5
• Block Scheduling
  Electric Charge, Insulators and Conductors, and Coulomb's Law require a block-and-a-half of study. Emphasize electrical force, the use of the principle of superposition, quantization of charge, the fundamental unit charge, separation of charge, and polarization. The Electric Field, Electric Field Lines, and Shielding and Charging by Induction require the remaining time. Stress the electric dipole, the parallel plate capacitor, calculating electric fields for various combinations of charges. Note that Gauss' Law is not required for the AP* B Physics Exam.

Key Words:

Critical Thinking Questions:

1. Three point charges, q₁ = 50.0 nC, q₂ = 40.0 nC, and q₃ = 60 nC are fixed on three corners of a square 10.00 cm on a side. Charge q₁ is placed on the lower left corner, q₂ on the upper left side, and q₃ on the upper right side of the square. (a) By the superposition principle, find the magnitude of the electric field at the vacant corner of the square. (b) If a 25.0 nC point charge with a mass of 2.00 g is placed at the vacant position, what is the magnitude of the initial force on the point charge? (c) If it is released from rest what is the magnitude of its initial acceleration?
2. Two small pith balls, each with a mass of 10.00 g, are supported from a common point by silk threads 30.00 cm in length. The pith balls are given identical charges. (a) If the pith balls make an angle of 15° with respect to one another, what is the charge on each? (b) Is it necessary to specify whether the charges are positive or negative?
3. Two point charges, q₁ = –50.0 nC and q₂ = + 30.0 nC, are separated by 10.0 cm. (a) What is the magnitude of the force acting between them? Is this force attractive or repulsive? (b) The point charges are allowed to touch and returned in their original positions. What force, if any acts between them? If there is a force, is it attractive or repulsive?
4. Compare the electrical force to the gravitational force acting between the proton and electron in the Bohr hydrogen atom with a radius of 0.511 Å. (b) What is the acceleration of the electron? (c) What is the tangential velocity of the electron? (d) What is the radial velocity? (e) What is the period of the electron?
5. A positive point charge of 3.0x10^–8 C and a negative charge of 4.0x10^–8 C are separated by 20.0 mm. Calculate the magnitude of the electric field that the positive charge experiences due to the presence of the negative charge.
6. Solve problem 36 on p. 642 in the textbook.
7. Solve problem 66 on p. 644 in the textbook.

Answers: 1. (a) 8.82x10⁴ N/C, (b) 2.21x10^–3 N, (c) 1.1 m/s²; 2. (a) 9.4x10^–8 C and (b) no; 3. (a) 1.35 mN of attraction and (b) 90 µN of repulsion; 4. (a) 2.3x10³⁹ x the gravitational force, (b) 9.66x10²² m/s², (c) 2.22x10⁶ m/s, (d) 4.35x10¹⁶ rad/s, (e) 1.44x10^–16 s; 5. 9.0x10⁵ N/C; 6. (a) 32 cm and (b) –8.3x10^–11 C; 7. (a) 1.6x10³ N/C and (b) 6.3x10^–2 N

Troubleshooting Tips/Error Traps:

• Stress converting µC, nC, and pC to proper powers of ten when making calculations. Provide many examples.
• The fundamental unit of charge is e and it is a positive quantity. Many students, incorrectly, will refer to this as the charge of the electron. Since the electron has a negative charge, the charge of the electron must be expressed as –e.
• Students may have difficulty understanding action-at-a-distance electrical force.
• Stress the properties of the electric field.
• Stress that Coulomb's Law applies to point charges.
• When two or more electrical fields of point charges overlap, the resultant electrical field at a given point in space is the vector sum of all electrical fields at that point. The vector sum is found by superposition.
• In making electrical field calculations, strongly emphasize that great attention must be paid to the signs of the interacting particles.
• Students may have difficulty in properly expressing the proper direction of electrostatic forces. Example problems will help.
• The electrostatic force on a point charge is in the direction of the electric field if and only if the charge is positive.

End of Chapter Activity:

1. One electronic charge is
   1. 1.00 C.
   2. 1.60x10^–19 C.
   3. 1.60x10¹⁹ C.
   4. 6.25x10^–18 C.
   5. 6.25x10¹⁸ C.
2. Doubling the separation of two identical negative charges causes the force of electrostatic repulsion to change by a factor of
   1. four.
   2. two.
   3. 1.414.
   4. one-half.
   5. one-quarter.
3. Electric field lines
   1. are closer together in regions of lower electric field strength.
   2. are perpendicular to the electric field vectors at each point in space.
   3. never cross.
   4. begin on a negative charge and end on a positive charge.
   5. point inward toward a negative charge.
4. The direction of the electric field vector in space is
   1. away from the position of a positive test charge.
   2. toward the location of a positive test charge.
   3. antiparallel to the direction of the electrostatic force on a positive test charge at that point in space.
   4. parallel to the direction of the electrostatic force on a positive test charge at that point in space.
   5. perpendicular to the direction of the electrostatic force on a positive test charge at that point in space.
5. In Coulomb's law, k is the
   1. Coulombic charge.
   2. Coulomb constant.
   3. permittivity of free space.
   4. conductivity of free space.
   5. dipole moment.
6. A small particle is considered to be negative when it has
   1. dipole properties.
   2. an excess of protons.
   3. a deficiency of neutrons.
   4. a deficiency of electrons.
   5. an excess of electrons.
7. The units of Coulomb's constant are
   1. C.
   2. N/(m²).
   3. (NC²)/(m²).
   4. (m²C²)/N.
   5. (Nm²)/(C²).
8. A pith ball holds a charge of +7.6 µC. What total number of deficient electrons does this charge represent?
   1. 1.6x10¹⁴ e
   2. 4.7x10¹³ e
   3. 6.1x10¹³ e
   4. 2.4x10¹³ e
   5. 3.2x10¹³ e
9. An electron is placed in an electrical field of strength 3000 N/C. What is the magnitude of the force the electron experiences?
   1. 1.9x10^–16 N
   2. 2.4x10^–16 N
   3. 3.2x10^–16 N
   4. 4.8x10^–16 N
   5. 6.4x10^–16 N
10. A particle of mass 2.4 g is given a charge of –6.0 µC and is placed in an electrical field that is directed parallel to the earth's gravitational field. What is the field strength, expressed in N/C, if it balances the weight of the particle?
    1. 1.6 x10³
    2. 2.4 x10³
    3. 3.9 x10³
    4. 4.8 x10³
    5. 6.4 x10³

Answers: 1 (B), 2 (E), 3 (C), 4 (D), 5 (B), 6 (E), 7 (E), 8 (B), 9 (D), 10 (C)

Suggested Conceptual Questions:

Suggested Problem Assignments: