Chapter 21

Electric Charge and Electric Fieldhttp://m.wmmr.com/2012/02/video-water-droplets-orbiting-a-needle-in-space

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Our approach• Consider some initial questions and

interest

• Outline ideas of the electrical interaction

• Consider the form charge takes

• Work on visualizing charge

• Do some math review

• Look at the charging processes

• Grasp Coulomb’s Law (on electric force)

Our approach (cont’d.)• Grasp the Electric Field

• Work on visualizing the Electric Field

• Understanding the Electric Field in and around conductors and insulators

• Understand the dynamics of charged objects in an Electric Field

• Look at electric diploes (+ other moments)

• Master Electric Field and Electric force calculations

Your ideas, experience

• What are some phenomena involving electric charge and the electrical interaction?

• Make a list, share with neighbors, class

Why study E&M?

• Chemistry, Biology, Geology

• Technology

• Bridge to conceptual understanding (conceptual technology)

• Limits?

Do we have good answers to

• Why does charge exist?

• What is charge?

• Why is charge conserved?

• How is the electrical interaction related to other kinds of interactions? (demo: strength of the electrical interaction)

• In what forms does charge exist? (later)

• What interactions does charge cause? and what are their characteristics? (next)

The electrical interaction(charges at rest)

• As force at a distance

• As force from a field

• The Electric Field

• In quantum electro-dynamics (as an exchange of particles/particles as fields)

In what form is charge found?• (microscopically) The Standard Model

– http://www.pha.jhu.edu/~dfehling/#chart

• (macroscopically) Discrete and continuous charge distributions ρ, σ, λ

• Represented in terms of monopoles, dipoles, quadrupoles, etc.

Visualizing charge

• Explanation activity (groups, white boards, equipment, notes, write up to hand in)

• Charged balloon– http://phet.colorado.edu/en/simulation/balloons

• “John Travoltage”– http://phet.colorado.edu/en/simulation/travoltage

Math Review

• Introduction and comments

• Review (from MIT, posted)

Charging(Use whiteboards)

• Describe (using diagrams) three ways of charging objects: charging by friction, charging by contact, charging by induction.– http://physci.kennesaw.edu/physlets/electricity

/tm-cond-insu-17.htm

– http://web.mit.edu/8.02t/www/802TEAL3D/visualizations/electrostatics/chargebyinduction/chargebyinduction.htm

Coulomb’s Law(use whiteboards)

• What simple situation does the scalar form of Coulomb’s Law describe? (sketch the physical situation for two positive charges and include vector arrows)

• Among what quantities does Coulomb’s Law provides a relation? (make a list)

• Make the sketch (above) for the vector form of Coulomb’s Law?

Coulomb’s Law (cont’d)(use white boards)

• Sketch a more complicated situation involving discrete charges in which Coulomb’s Law could be applied.

• Sketch a more complicated situation involving a continuous charge distribution in which Coulomb’s Law could be applied.

• Ranking tasks

The Electric Field

• What, precisely, is a vector field?• How is the electric field defined

operationally at a point in space?• What is the underlying physical reason the

electric field can be defined this way?• How is the force on a specific charge

figured out from the electric field?• Why does the electric field satisfy the

superposition principle?

The Electric Field representation• How is the electric field represented

mathematically? Give an example.

• How is the electric field represented by vector arrows? Question

• How is the electric field represented by lines (of oriented grass seed)? http://web.mit.edu/8.02t/www/802TEAL3D/visualizations/electrostatics/DestroyField/destroyField_640.mpg

… Electric Field representation

• The electric field represented by the motion of test charges http://www.falstad.com/vector3de/index.html

• How is the electric field represented by field lines? http://physics.bu.edu/~duffy/semester2/c02_visualize.html

The Electric Field (cont’d)

• What is an example of a complicated situation involving discrete charges in which the electric field could be defined?

• Set up the sum. (Use whiteboard.)• What is an example of a complicated

situation involving a continuous charge distribution in which the electric field could be defined?

• Set up the integral. (Use whiteboard.)

Electric field lines

• What do the electric lines look like for a variety of simple charge distributions?– http://physics.bu.edu/~duffy/semester2/

c02_visualize.html– More

• What, in general, is the relation between electric field lines and charges (in a static situation)?

E-fields & conductors

• What is implied about E inside a conductor in a static situation?

• What is implied about E at the surface of a conductor in a static situation?

• What is implied about E inside a closed conducting box with no charge inside?

Movement of chargesin an E-field

• What is the starting point for determining the motion of a charge in an E-field?

Electric Dipoles• How is the dipole moment vector defined?

• What is the torque on an electric dipole in an E-field?

• What is the potential energy of an electric dipole in an E-field? (How is the zero of potential energy defined?)

• What is the E-field produced by an electric dipole?

– http://web.mit.edu/8.02t/www/802TEAL3D/visualizations/electrostatics/PithBallsCreate/PithCreate_640.mpg

E-field calculations

• along the axis away from a line of charge

• along a mid-line perpendicular from a line of charge

• off a mid-line perpendicular from a line of charge

• along the middle perpendicular from a square of charge

the end

• review these slides to refresh your memory of what we did, what you learned during our classes, and what is left to understand on chapter 21subject matter

• What would the electric field at a point look like as a positive charge goes by in a straight line near the point?– http://web.mit.edu/8.02t/www/802TEAL3D/

visualizations/electrostatics/MovingChargePosElec/MovChrgPosElec_640.mpg

• … a negative charge goes by in a straight line near the point?– http://web.mit.edu/8.02t/www/802TEAL3D/

visualizations/electrostatics/MovingChargeNegElec/MovChrgNegElec_640.mpg

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