Pse4 Irm Ch23

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Chapter 23: Electric Potential

Outline

23-1 Electric Potential Energy and Potential Difference23-2 Relation between Electric Potential and Electric Field23-3 Electric Potential Due to Point Charges23-4 Potential Due to Any Charge Distribution23-5 Equipotential Surfaces23-6 Electric Dipole Potential23-7 E

G

Determined from V 23-8 Electrostatic Potential Energy; the Electron Volt

* 23-9 Cathode Ray Tube: TV and Computer Monitors, Oscilloscope

Major Concepts

Electric Potential Energy

The Electron-voltElectric PotentialAs it relates to the electric fieldUnitsOf a point chargeOf charge distributions

EquipotentialLines and surfaces

Electric Potential in TechnologyCathode ray tube

Summary

The previous chapters dealt with how to determine the electric field surrounding a charged object, usually for the purpose of determining electric forces on external charges within those fields. This chapter deals with how todetermine the electric potential of a charged object, usually for the purpose of determining the electric potentialenergy when external charges are located at a potential. The concept of equipotential surfaces is introduced andthe relation between potentials and electric field is discussed. The effect of potentials and fields on conductors iscovered. Finally, application of electric potentials to technology like the cathode ray tube is discussed.

Teaching Suggestions

Within each section or group of sections are various hints and comments regarding common misconceptions, preconceptions, and difficulties for the major concepts addressed. This is followed by suggestions for:

demonstrations; concept-building explorations/exercises (which may include Ranking Task Exercises (RTE), ConcepTest Clicker Questions (CTQ), ConcepTests from Eric Mazurs Peer Instruction (CTQ-PI), Physlet

Physics (PP), E&M TIPERs (EMT), Physlet Quantum Physics (PQP)); and transparencies (T). Of course, howyou use these suggestions depends upon your particular teaching style, course structure, and syllabus.

At the end of each section or group of sections are a few reading quiz questions. These questions are intended totest knowledge of definitions and to basically determine if the students read the section(s).

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Physics for Scientists & Engineers with Modern Physics, 4 th Edition: Instructor Resource Manual

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Sections 23-1 23-2: Electric Potential Energy and Potential Difference; Relation between ElectricPotential and Electric Field;

Hints/Difficulties/Misconceptions/Preconceptions:

Since the electric force is conservative, there will be an electric potential energy associated with the force.Remind students that this was the same with the gravitational force and gravitational potential energy. You mayhave to review the definition of a conservative force.

Students will struggle with determining if the change in electric potential energy is positive or negative. Tellthem to think about the underlying nature of the particles, like when two like charges move toward each other there will be a repulsive force, which will cause them to slow down and decrease kinetic energy. The law of conservation of energy says then that the potential energy must increase. By similar reasoning, you can discusslike charges moving away from each other, as well as unlike charges moving towards and away from each other.

You will need to emphasize more than once that the potential is a scalar quantity, and that positive charges produce positive potentials and negative charges produce negative potentials.

Concept-Based Explorations/Exercises:

RTE: Three Electric Charges Electric Potential Energy (p. 124)Uniform Electric Field Change in Potential Energy of a Positive Charge (p. 152)Uniform Electric Field Change in Potential Energy of a Negative Charge (p. 153)

CTQ: Electric Potential Energy I (23.1a)Electric Potential Energy II (23.1b)Electric Potential Energy III (23.1c)Work and Potential Energy (23.2)Electric Potential I (23.3a)Electric Potential II (23.3b)Hollywood Square (23.4)

EMT: Three-Dimensional Locations in a Constant Electric Potential Work (ET6-RT1)Cart Approaching Sphere Distance (ET4-CT1)Three Charge System Electric Potential Energy and Work Done (ET6-CT1)

Transparencies :

T: 13. Figure 23-1 Work done by electric field on charged particle14. Table 23-1 Some Typical Potential Differences

Reading Quiz Questions:

1. The SI unit for electric potential energy is

a)

N b) Jc) Vd) C

2. The SI unit for electric potential isa) N

b) Jc) Vd) C

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3. A rock moving through a gravitational field is analogous to a ___________ charge moving through anelectric field.

a) positive b) negativec) neutrald) continuous distribution of

4. Which of the following has the smallest potential difference associated with it?a) Power supply for TV tube

b) Flashlight batteryc) Automobile ignitiond) Household outlet

5. When high voltages are present, a glow may be seen around sharp points, known asa) a lightning glow.

b) a electrical potential glow.c) a heliostatic discharge.d) a coronal discharge.

Key: 1-b; 2-c; 3-a; 4-b; 5-d

Sections 23-3 23-4: Electric Potential Due to Point Charges; Potential Due to Any ChargeDistribution


For multiple point charges, students will find it almost too easy to simply add the individual potentials together because they are reminded of the vector addition that was needed with electric fields and they will be confusedwith resulting negative values. Remind students that without negative numbers, there wouldnt be positive

numbers.Also remind students that the initial equation derived for the electric potential is for point charges only. For continuous distributions of charge, you will need to show them the methodology of modeling charge distributionsas a collection of infinitesimally small point charges and integrate appropriately (similar to determining theelectric field of charge distributions)


RTE: Two Electric charges Electric Potential (p. 123)Charged Conducting Spheres Electric Potential at the Center (p. 128)Charged Conducting Spheres Electric Potential at Various Points (p. 130)

EMT: Points Near a Pair of Equal Opposite Charges- Potential (ET8-RT2)Pairs of Charged Connected Conductors Electric Potential (ET8-RT3)Charged Curved Rod Electric Potential (ET8-RT4)Three-Dimensional Locations Near Point Charge Electric Potential (ET8-RT6)Six Charges in Three Dimensions Electric Potential (ET8-RT8)Spherical Conducting Shell Electric Potential (ET8-RT9)Systems of Eight Point Charges Potential (ET8-RT10)Points Near Pair of Charges Potential Difference (ET8-CT1)

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Transparencies :

T: 144. Figures 23-7 & 23-8 Electric Field and Potential Inside and Outside a Conducting Sphere


1. To determine the work done, one can always just simply multiply force time distance.

a)

True b) False

2. Which of these charge configurations was NOT an example from these sections?a) A sphere of charge

b) A ring of chargec) A disk of charged) Two opposite point charges

Key: 1-b; 2-a

Sections 23-5 23-6: Equipotential Surfaces; Electric Dipole Potential


To help students understand equipotential surfaces, draw a single point charge on the board along with theequation for the potentials due to point charges. Then draw a circle around the charge and ask the students whatthe potential is at any given location on the circle. Then draw another circle of a larger radius and repeat the

process. Be sure to ask if the potential at any point on the first circle is the same at any other point on the secondcircle. Now you can introduce the equipotential surface by having the students visualize the circles as the cross-section of spheres.

The topographical map analogy presented is always a good one to use, particularly when emphasizing that theelectric field is perpendicular to the equipotential surface everywhere and that the electric field always points from

a higher to lower potential by way of the shortest distance between two points. Also remember to tell them thatthe surface of a conductor is always an equipotential surface.

If you decided to discuss electric dipoles from the previous chapter, it will be simple to follow up with the shortsection presented here. If you did not discuss dipoles previously, then skipping this section will be fine.

Demonstrations:

The Conducting Sheet. This demonstration shows an equipotential surface. Use a conducting sheet and draw a point charge. Measure the potential at different locations and join points having the same potential. Use adifferent color pen and draw the electric field lines. Repeat for other charge configurations, such as multiple pointcharges; a point charge and a charged plate; and two charged plates.


CTQ: Equipotential Surfaces I (23.5a)Equipotential Surfaces II (23.5b)Equipotential Surfaces III (23.5c)Equipotential of Point Charge (23.6)Work and Electric Potential I (23.7a)Work and Electric Potential II (23.7b)

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EMT: Equipotential Surfaces Electric Field (ET5-RT18)Electron in Equipotential Surfaces Kinetic Energy Change (ET6-RT3)Charges and Equipotentials Work (ET6-RT4)

Transparencies :

T: 147. Figures 23-17 & 23-18 Electric Field and Equipotential Lines148. Table 23-2 Dipole Moments of Selected Molecules


1. The topographical map display was of the __________ region of California.a) San Andreas fault

b) Napa valleyc) Sierra Nevadad) Rocky mountain

2. A conductor must be entirely at the same potential in the static case.a) True

b) False

3. Which of these is a small unit for the dipole moment?a) The volta

b) The debyec) The dipolad) The faraday

Key: 1-c; 2-a; 3-b

Sections 23-7 *23-9: EG

Determined from V ; Electrostatic Potential Energy; the Electron Volt;

Cathode Ray Tube: TV and Computer Monitors, Oscilloscope Hints/Difficulties/Misconceptions/Preconceptions:

Determining EG

from V will probably be more of a mathematic challenge then anything, as most students willunderstand the underlying connections between E

G

and V by now, especially if your discussions about equipotentialsurfaces were successful.

The introduction of the electron-volt as an alternate unit for electric potential energy will be fruitful whendiscussing the application of electric potential in high-energy and medical technology.

While the last section can be skipped without any loss for fundamental knowledge for the students, discussing the

application of many of these concepts enables the students to make the connection between physics and theoutside world.

Demonstrations :

The Van de Graaff generator. This demonstration is useful for displaying many aspects of electricity. If youhave a Van de Graaff generator, it might be a good time to use it and summarize a lot of electrical concepts andquantities youve discussed up to this point, including: charge production; electrical repulsion; electric fieldgeneration; voltage; and electrical discharges (dont forget those sparks!).

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RTE: Uniform Electric Field/Potential Lines Force on Charge at Rest III (p. 139) Non-Uniform Electric Field Electric Potential (p. 145)Uniform Electric Field Potential Difference (p. 147)Uniform Electric Field / Potential Lines Strengths of the Electric Field (p. 150)Uniform Electric Field Electric Potential at Different Points II (p. 151)

EMT: Three-Dimensional Locations in a Constant Electric Potential (ET5-RT4)Potential vs Position Graphs I Electric Field (ET5-RT9)Potential vs Position Graph II Electric Field (ET5-RT10)Potential vs Position Graph III Electric field (ET5-RT11)Three-Dimensional Locations in a Uniform Electric Field (ET8-RT7)Potential Near Charges Electric Field (ET5-CT1)Potential vs Position Graph II Electric Field (ET5-CT2)

Transparencies :

T: 149. Figures 23-22 & 23-23 Cathode Ray Tube and Monitor


1. The electron-volt is an alternate unit for a) electric potential.

b) electric potential energy.c) electric current.d) electric charge.

2. An example in these section discusses disassembling a(n) __________ atom.a) hydrogen

b) helium

c)

oxygend) gold

3. A(n) _________ is a device for amplifying, measuring, and visually observing an electrical signal as afunction of time.

a) cathode b) CRTc) EKGd) oscilloscope

Key: 1-b; 2-a; 3-d

Suggested Readings

Beichner, R., Visualizing Potential Surfaces with a Spreadsheet.; The Physics Teacher (February 1997), p. 95-97.

De Jong, M., Graphing Electric Potential, The Physics Teacher (May 1993), p. 270-272.

Kristjansson, L., On the Drawing of Lines of Force and Equipotentials, The Physics Teacher (April 1985), p. 202-206.

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Moelter, M., Evans, J. and Elliott, G., Electric Potential in the Classical Hall Effect: An Unusual Boundary-Value Problem, American Journal of Physics (August 1998), p. 668-677.

Saslow, W., Voltaic Cells for Physicists: Two Surface Pumps and an Internal Resistance, American Journal of Physics (July 1999) p. 574-583.

Skeldon, K., Reid, L., McInally, V., Dougan, B. and Fulton, C., Physics of the Theremin, American Journal of Physics (November 1998), p. 945-955.

Worner, C.H., On the Teaching of the Electric Dipole, The Physics Teacher (November 2001), p., 462-463.

Zimmerman, N., A Primer on Electrical Units in the Systeme International, American Journal of Physics(April 1998), p. 324-331.

Notes and Ideas

Class time spent on material: Estimated: Actual:

Related laboratory activities:

Demonstration materials:

Notes for next time:

Pse4 Irm Ch23

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