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 (through the week)• 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 on individual whiteboards, share with neighbors, class

Why study E&M?• Chemistry, Biology, Geology• Technology• Bridge to conceptual understanding

(conceptual technology)• Limits? (Sure: Classical theory, one

interaction, within current scope of understanding) There’s unanswered questions!

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 involve charge? 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• As a more general understanding of

interaction as the exchange of different “particles” understood as quantum mechanical states of corresponding fields

• In quantum electro-dynamics (as an exchange of photons)

• See diagram next for lowest order terms.

An EM higher order term

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

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

• (macroscopically) Discrete q1, q2, . . . and continuous charge distributions ρ, σ, λ

• Represented in terms of monopole, dipole, quadrupole, 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

• Electroscope demonstration• Tape on table-top demonstration

BUT WAIT!!!physics ≠ revealed knowledge

experiment disproves (not proves) • Consider alternate one charge theory

• Design an experiment to disprove this.• Write up for next class; see handout

charge force+ + repel+ o attracto o neutral

Math Review• Introduction and comments• Review (from MIT, posted)• Mostly at end of week, but you may look

ahead

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 provide 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 (handout)

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 of chapter 21 subject 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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