Download Chapter 21 - Interactive Learning Toolkit

Chapter 21
Electric Charge and Electric Field
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
• 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
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Why does charge exist?
What is charge?
In what forms does charge exist?
What interactions does charge cause? and
what are their characteristics?
• How is the electrical interaction related to
other kinds of interactions? (demo:
strength of the electrical interaction)
The electrical interaction
(charges at rest)
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•
•
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
• 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/vis
ualizations/electrostatics/chargebyinduction/c
hargebyinduction.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/d
estroyField_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/c0
2_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_vi
sualize.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 charges
in 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/PithCr
eate_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/vis
ualizations/electrostatics/MovingChargePosEl
ec/MovChrgPosElec_640.mpg
• … a negative charge goes by in a straight
line near the point?
– http://web.mit.edu/8.02t/www/802TEAL3D/vis
ualizations/electrostatics/MovingChargeNegEl
ec/MovChrgNegElec_640.mpg
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