324 Chapter 12 Electricity © 2002 Nelson Thomson Learning

12.2 Electric Fields and Electric Charge

Students study the properties of electric fields, the electric force, and the elementary charge and learn how to calculate the quantity of charge.

Achievement Chart Categories

Assessment Opportunities/Specific Expectation Addressed

Assessment Tools

Knowledge/Understanding Practice questions Understanding Concepts, q. 1–2 EM1.01 Section 12.2 Questions Understanding Concepts, q. 1–7 EM1.01

Rubric 1: Knowledge/Understanding

Inquiry Section 12.2 Questions Applying Inquiry Skills, q. 8–10 EM1.01 Lab Exercise 12.2.1 Analysis, a–b EM1.01

Rubric 2: Inquiry Skills

Communication Section 12.2 Questions Reflecting, q. 11 EM1.01

Rubric 3: Communication

Making Connections Lab Exercise 12.2.1 Analysis, b EM1.01

Rubric 4: Making Connections

Expectations Addressed Overall Expectations—EMV.01 Overall Skills Expectations—SIS.04, SIS.05, SIS.06, SIS.07, SIS.08, SIS.09 Specific Expectations: • EM1.01 define and describe the concepts and units related

to electricity and magnetism (e.g., electric charge, electric current, electric potential, electron flow, magnetic field, electromagnetic induction, energy, power, kilowatt-hour)

BACKGROUND INFORMATION The notion of an invisible field of force is difficult for students to grasp but is absolutely essential for future understanding of electrical and magnetic phenomena. Reference and analogies should be made to the gravitational field previously studied.

Charles Augustin de Coulomb (1736–1806) was a French physicist who, after serving as a military engineer in the West Indies, returned to France to do research. There, he devised a torsion balance that measured the amount of force acting on it by the resulting twist of its thin suspending fibre. The value of the constant in Coulomb’s law is

k = 8.99 × 109 N·m2/C2 (much larger than G in the equation for the force of gravity).

Robert Andrews Millikan obtained his doctorate in physics from Columbia University in 1895. After postdoctoral work in Germany, he was appointed Professor of Physics at the University of Chicago in 1910. He initially used charged water droplets in his famous experiment but switched to oil to overcome the problems of evaporation. He also performed the very delicate experiments necessary to verify Einstein’s interpretation of the photoelectric effect and named and identified the origin and nature of “cosmic radiation.”

© 2002 Nelson Thomson Learning Unit 5 Electricity and Magnetism 325

ADDRESSING ALTERNATIVE CONCEPTIONS

Many misconceptions are associated with electric fields, the most common being that field lines indicate the direction of motion of any charged particle in the area. The field lines indicate the direction of the force on a small positive test charge only. The test charge must be small so as not to change the field being mapped.

Other problems may result from the calculation of the quantity of charge, especially due to negative signs. There are many ways of addressing the problem, but the best way is to present the idea consistently and clearly to the students with an emphasis on understanding rather than on getting the answer.

Related Background Resources

Nelson Web site: www.science.nelson.com for specific Web links

PLANNING

Suggested Time Narrative/Practice—25 to 30 minutes Lab Exercise 12.2.1—10 to 15 minutes Section Questions—20 to 30 minutes

Core Instructional Resources • Solutions Manual • Colour Transparencies

Supplemental Resources • Lab and Study Blackline Masters

TEACHING SUGGESTIONS • Have students practise drawing electric fields, and help

them understand how to draw these diagrams by considering the net force on a small positive charge.

• Students should work individually on the lab exercise at first and later discuss in small groups or with the whole class what they have found.

• The electric fields depicted in this section may be demonstrated with dry grass seeds floating on the surface of a mineral oil bath. Spherical and rectangular electrodes attached to the terminals of a large 12-V storage battery

provide the field, and the grass seeds will align themselves with the field lines. This demonstration gives visual evidence of a field of force that otherwise can only be imagined. • The extremely small size of the charge on an electron is

difficult for students to comprehend; so is the notion of an arbitrary unit for electric charge, the coulomb. However, the idea that every electric charge is a multiple of a fundamental unit is easy for most students to grasp, especially if they complete “An Experiment Similar to Millikan’s” (see Extensions/Modifications of Lab Exercise 12.2.1 below). The actual details of the experiment should be left to a later course, but the ingenuity of the technique is worth emphasizing.

• Provide calculations involving both positive and negative charges, showing clearly how to handle the negative signs.

Lab Exercise 12.2.1 • Students should be aware of the basic procedure used by

Millikan as outlined in the text.

LAB EXERCISE 12.2.1 Investigating Data from Millikan’s Oil Drop Experiment • Students investigate data similar to values measured by

Millikan and look for patterns.

BEFORE Teacher Preparation Time: 10 to 15 minutes Materials and Equipment: • None required. Safety and Disposal: • None required. Assessment: • Hold a class discussion of the patterns students have

found. • Have students complete “An Investigation Similar to

Millikan’s” (see below, under Extensions/Modifications). Student Preparation • None required.

DURING • Try not to give the students hints, but push any who are

headed in the wrong direction.

AFTER • Hold a class discussion of patterns. • Discuss why more data were used by Millikan.

326 Chapter 12 Electricity © 2002 Nelson Thomson Learning

(Sample answers follow.) • (a) See the observation chart that follows (answers are in

bold).

Drop number

Charge

1 3.2 × 10–19 C = 1.6 × 10–19 C × 2

2 16.0 × 10–19 C = 1.6 × 10–19 C × 10

3 17.6 × 10–19 C = 1.6 × 10–19 C × 11

4 6.4 × 10–19 C = 1.6 × 10–19 C × 4

5 8.0 × 10–19 C = 1.6 × 10–19 C × 5

6 12.8 × 10–19 C = 1.6 × 10–19 C × 8

7 11.2 × 10–19 C = 1.6 × 10–19 C × 7

8 4.8 × 10–19 C = 1.6 × 10–19 C × 3

9 1.6 × 10–19 C = 1.6 × 10–19 C × 1

10 9.6 × 10–19 C = 1.6 × 10–19 C × 6

11 19.2 × 10–19 C = 1.6 × 10–19 C × 12

12 14.4 × 10–19 C = 1.6 × 10–19 C × 9

• (b) Two patterns are readily observable:

• All of the measured charge values are an integer multiple of the value 1.6 × 10–19 C (ranging from 1 to 12, in this representative sample).

• No charge values are smaller than 1.6 × 10–19 C. Although these data are a small representative sample of Millikan’s actual data, it is assumed that the remaining data satisfy these patterns. From these data, a possible conclusion that could be drawn is that the smallest possible charge that exists is 1.6 × 10–19 C. Since Millikan postulated that the charge was due to either an excess or a deficit of electrons, it can be concluded that the charge on one of these electrons is –1.6 × 10–19 C (negative, by convention) and the charge on one proton is +1.6 × 10–19 C (positive, by convention) in order to balance the charge on a neutral atom. Millikan called this smallest possible charge value the “elementary” charge, e (e = 1.6 × 10–19 C).

Extensions/Modifications: • An Experiment Similar to Millikan’s. Using different

numbers of equal-mass coins in plastic film canisters (or different numbers of equal-mass marbles in opaque bags), students can devise and carry out an investigation to determine the mass of each coin (or marble). In this investigation, be sure the containers cannot be opened.