TOPIC 4.2: ELECTRIC FIELDS - Manitoba Education and · PDF fileDemonstrate electric fields with a Van der Graff machine and a collection of threads, streamers, or light strips of paper

TOPIC 4.2: ELECTRIC FIELDSStudents define the electric field strength, E, as a force per unit charge (in N/C).The electric force is given by Fe = qE, and lines of force are used to draw the field.The field between two parallel plates is described. Students examine problemsinvolving both the gravitational and electric forces, and describe Millikan’sexperiment and the elementary charge.The student will be able to:S3P-4-14: Define the electric field qualitatively as the region of space around a

charge where a positive test charge experiences a force.S3P-4-15: Diagram electric fields using lines of force with respect to a positive test

charge. Include: single point charges (positive and negative), near two like charges, near twounlike charges, between a single charge and a charged plate, between two oppositelycharged parallel plates

S3P-4-16: Define the electric field quantitatively as a force per unit charge(E = F/q) and solve problems using the unit field concept (F = qE).

S3P-4-17: Solve problems for the motion of charges between parallel plates whereFnet = Fe + Fg .

S3P-4-18: Describe a simplified version of Millikan’s experiment for thedetermination of the elementary charge (solve for charge whenFe = Fg).

S3P-4-19: Define the elementary charge and convert between elementary chargesand coulombs.Include: q = Ne

Topic 4: Fields • SENIOR 3 PHYSICS

Topic 4.2 – 22

SPECIFIC LEARNING OUTCOMES

S3P-4-14: Define the electric fieldqualitatively as the region ofspace around a charge where apositive test charge experiences aforce.

S3P-4-15: Diagram electric fieldsusing lines of force with respectto a positive test charge. Include: single point charges (positiveand negative), near two like charges,near two unlike charges, between asingle charge and a charged plate,between two oppositely chargedparallel plates

GENERAL LEARNING OUTCOMECONNECTION

Students will…Employ effectivecommunication skills and utilizeinformation technology togather and share scientific andtechnological ideas and data(GLO C6)

Entry-Level KnowledgeStudents are introduced to electrostatics inSenior 1 Science.

Notes to the TeacherDescribe the electric field as a force per unitcharge, and compare to the concept of thegravitational field and mass. Emphasizethat an electric field can result from either anegative or a positive charge. Distinguishcarefully between the charge thatestablishes the field and the test chargethat experiences the force. The direction ofthe field can be confusing as it is alwaysdescribed as if the test charge is positive.Therefore, the electric field vector and theelectric force vector are in the samedirection if the charge brought into the fieldis positive, and in opposite directions if thecharge is negative. Note that a line of forceindicates the direction the force would acton a positive test charge in the field. Notealso that the number of lines per unit arearepresents the intensity of the field.

Demonstrate the electric fields aroundcharged objects with grass seeds, “fuzzyfur,” or fibre clippings. Fine pieces of fibreclippings can be suspended in mineral oilbetween a pair of electrodes and the patternprojected on the overhead.

Teacher DemonstrationDemonstrate electric fields with a Van derGraff machine and a collection of threads,streamers, or light strips of paper.

Senior Years Science Teachers’Handbook ActivitiesStudents compare and contrast electric andgravitational fields. While gravitationalfields always exert attractive forces on othermasses, an electric field may exert repulsiveforces as well. (See Senior Years ScienceTeachers’ Handbook, Compare and ContrastFrame, page 10.24.)

SUGGESTIONS FOR INSTRUCTION

Topic 4.2 – 23

SENIOR 3 PHYSICS • Topic 4: Fields

SKILLS AND ATTITUDES OUTCOMES

S3P-0-2a: Select and useappropriate visual, numeric,graphical, and symbolicmodes of representation toidentify and representrelationships.

S3P-0-2c: Formulate operationaldefinitions of major variablesor concepts.


Students will…Understand how stability,motion, forces, and energytransfers and transformationsplay a role in a wide range ofnatural and constructedcontexts(GLO D4)

SUGGESTIONS FOR INSTRUCTION SUGGESTIONS FOR ASSESSMENT

SUGGESTED LEARNING RESOURCES

Lapp, D.R. (1996) “Field Lines Flying Fur.”The Physics Teacher 34.4: 252.

Software Exploring Electrodynamics; Explorations 1-2: Electric Field

EM Field 6, Physics Academic SoftwarePublishing Organization

MultimediaVideodisc: Video Encyclopedia of PhysicsDemonstrations: Demo 17-10, Electric Field

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Topic 4.2 – 26

SPECIFIC LEARNING OUTCOME

S3P-4-17: Solve problems for themotion of charges betweenparallel plates where Fnet = Fe + Fg .


Students will…Demonstrate appropriatescientific inquiry skills whenseeking answers to questions(GLO C2)

Notes to the TeacherIdentify the constant field between parallelplates by first considering a line of chargeand a single point charge placed nearby.The horizontal components of correspondingcharges cancel and the vertical componentsadd. For oppositely charged plates, theeffect is enhanced. The field is constant andis directed from the positive to the negativeplate. The edges of the field bulge slightlysince there are no charges outside the platesto balance the horizontal effects of thenearby charges.

Since the electric field between parallelplates is constant, we may use all of thekinematic relations derived in 3.1.7 andNewton’s Second Law to solve problems forcharges that are moving between the plates.In Senior 3 Physics, students should onlyconsider charges that are moving in astraight line between the plates. Problemscan include solving for charge, electric fieldstrength (E), electric force (FE), mass,gravitation force (Fg), net force, acceleration,final velocity, or displacement.


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Topic 4.2 – 27


SKILLS AND ATTITUDES OUTCOMES

S3P-0-2c: Formulate operationaldefinitions of major variablesor concepts.

S3P-0-2h: Analyze problems,using vectors.Include: adding and subtractingvectors in straight lines and atright angles, vector components


Students will…Understand how stability,motion, forces, and energytransfers and transformationsplay a role in a wide range ofnatural and constructedcontexts(GLO D4)

Student ActivitiesStudents predict the path of a particle in anelectric field, using ExploringElectrodynamics Explorations 1 and 2.

Senior Years Science Teachers’Handbook ActivitiesSee Compare and Contrast, Senior YearsScience Teachers’ Handbook, Building aScientific Vocabulary, page 10.15.



Bonham, S. (1999) “Using Physlets to TeachElectrostatics.” The Physics Teacher 37.5:276.

Concept Overview, Attachment 19, Successfor All Learners: A Handbook onDifferentiating Instruction.


Topic 4.2 – 28


S3P-4-18: Describe a simplifiedversion of Millikan’s experimentfor the determination of theelementary charge (solve forcharge when Fe = Fg).

S3P-4-19: Define the elementarycharge and convert betweenelementary charges andcoulombs.Include: q = Ne


Students will…Identify and appreciatecontributions made by women andmen from many societies andcultural backgrounds towardsincreasing our understanding ofthe world and in bringing abouttechnological innovations(GLO A4)

Notes to the TeacherMillikan’s experiment can be diagrammedand described, or demonstrated usinganimations.

Define the elementary charge. Convertbetween elementary charges and coulombs.The coulomb is actually definedoperationally by the force of attractionbetween two current-carrying wires.However, at this point, introduce thecoulomb as a fixed number of elementarycharges (1C = 6.25 x 1018 elementarycharges). Therefore, in coulombs, oneelementary charge is:

q = N x e1 C = 6.25 x 1018 e

and

e = C

e = 1.6 x 10–19 C

16.25 x 1018

An elementary charge is the charge of anelectron (–e) or a proton (+p).

Solve for the quantity of charge (q), electricforce (Fe), mass, gravitational force (Fg),number and type (proton/electrons) ofelementary charge (N) for Millikan-typeproblems.

In terms of Newton’s Second Law, for asmall sphere placed between the plates, wehave:

Fnet = Fapplied + Ffriction.

such thatFfriction = 0 (negligible) and Fapplied = Fe + Fg

There are several cases to address forcharges between parallel plates:Fe = –Fg (the sphere is stationary between

the plates)Fe < Fg (opposite to Fg such that a < g)Fe > Fg (opposite to Fg such that a > 0

[upwards])Fe > 0 (in the same direction as Fg such

that a > g)


Topic 4.2 – 29


SKILLS AND ATTITUDES OUTCOME

S3P-0-1b: Describe theimportance of peer review inthe evaluation andacceptance of scientifictheories, evidence, andknowledge claims.


Students will…Describe scientific andtechnological developments,past and present, andappreciate their impact onindividuals, societies, and theenvironment, both locally andglobally (GLO B1)

Illustrative ExampleA negatively charged 2.0 x 10–5 kg dropletof oil is placed between two oppositelycharged plates. The oil drop is balancedwhen the electric field is adjusted to4.2 x 10–7 N/C.

a) Diagram to indicate the charge on eachplate.

b) How many elementary charges are on theoil drop?

m = 2.0 x 10–7 kg E = 4.2 x 105 N/C

- - - - - - - - - - -

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Fg

Fe Fe

Fg

Pencil-and-Paper TasksWritten Test: Students solve for chargewhen Fe = Fg.

Visual DisplaysStudents diagram the different possibilitiesfor charge, plates, forces, and acceleration.



SoftwareMillikan Oil Drop Experiment: Vernier CBLsystem.MultimediaVideo: The Mechanical Universe:Introductory Physics: #12: The MillikanExperiment Videodisc: Video Encyclopedia of PhysicsDemonstrations: Demo 24-24: Millikan OilDropWebsitesSearch for “Millikan applets” using thekeywords: Millikan’s experiment + applet


Topic 4.2 – 30


S3P-4-18: Describe a simplifiedversion of Millikan’s experimentfor the determination of theelementary charge (solve forcharge when Fe = Fg).

S3P-4-19: Define the elementarycharge and convert betweenelementary charges andcoulombs.Include: q = Ne


Students will…Identify and appreciatecontributions made by women andmen from many societies andcultural backgrounds towardsincreasing our understanding ofthe world and in bringing abouttechnological innovations(GLO A4)

Since the drop is balanced,Fe = Fg

qE = mg

q =

q = 4.7 x 10–13 CSince q = Ne,

N =

Senior Years Science Teachers’Handbook ActivitiesAnticipation Guide: Students read DouglasAllchin’s article “Flirting with Fraud:Millikan, Mendel and the Fringes ofIntegrity” in Appendix 4.4. Use thequestion, “Was Millikan’s selective use ofdata ‘good’ science?” to address the Natureof Light outcome S3P-0-1b (scientificknowledge claims).

See Appendix 4.4: Student ArticleAnalysis—Scientific Fraud?

4.7 x 10–13 C1.6 x 10–19 C

2.0 x 10–7 (9.8)4.2 x 105 N/C


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Topic 4.2 – 31


SKILLS AND ATTITUDES OUTCOME

S3P-0-1b: Describe theimportance of peer review inthe evaluation andacceptance of scientifictheories, evidence, andknowledge claims.


Students will…Describe scientific andtechnological developments,past and present, andappreciate their impact onindividuals, societies, and theenvironment, both locally andglobally (GLO B1)

Understanding the Major ExplanatoryStories of ScienceAccess the following URL for an articlerelated to the integrity of earlier work inelectric fields that could be suitable for astudent article analysis: <http://www1.umn.edu/ships/updates/fraud.htm>

The article, “Flirting with Fraud: Millikan,Mendel and the Fringes of Integrity” byDouglas Allchin, is also reprinted inAppendix 4.4.


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Topic 4.2 – 32

NOTES

Documents

TOPIC 4.2: ELECTRIC FIELDS - Manitoba Education and · PDF fileDemonstrate electric fields with a Van der Graff machine and a collection of threads, streamers, or light strips of paper