Ph272- General Physics II (Electricity and
Magnetsism)
Lecture 2: Ch. 22 - Electric Field
Key Topics
Overview
- Electric Field and Electric Field Lines - Coulomb's
Law tells us that two charges can exert a force on each other
even when the charges are not in contact. To help visualize the
action of one charge on the other, Michael Faraday invented the
concept of electric field (a force field concept).
For example, q1 produces an electric (force) field everywhere
in space. When a second charge, q2, is placed in this force field,
it will experienced a force, F12.
In this lecture, we will quantify the concept of electric
field and learn how to sketch electric field lines.
- Electric Dipole - In last lecture, we see that a charged
comb can pick up pieces of paper (which is neutral).
How does a charge object attract a neutral object?
This will lead to the concept of polarization
and (induced) electric dipole.
Electric Field
- Definition of electric field: We say that a charge
q1 produces an electric field everywhere in space and is given
by Eq. 18-7 (I have a bettter notation, see lecture)
- Given the electric field, calculate the force on a charge
q0: When a charge q0 is placed at the position r, then the
force on q0 is given by:
F10 = q0 * E1(r)
-> Q. What about the electric produced
by q0?
-> A. The electric field produced by q0 does not act on
itself.
So the force on q0 = q0 * E-field produced by all other charges
at position r.
- Electric field produced by more than one charge: Use
superposition principle, Eq. 18-8.
-> Do problem 18-13, 18-14, P.620.
Electric Field Lines
- Method 1: Draw arrows at many locations
- Method 2: Draw continous lines (method used in this
text book)
- Rules for drawing electric field lines: See P.613.
Try to understand the electric field patterns for Figure 18-16
to 18-19.
-> Do Problems 18-17 and 18-20
- Motion of a point charge in an electric field: Find the force
and the acceleration. If the eelctric field is uniform in space
then the acceleration is constant (in space and hence in time)
then you can find the trajectory from what you have learned in
Ph170. If the acceleration is not constant, then it alot more
complicated. We will not go into details. Do.
Problem 18-23.
-> Q. What arrangement of charges
produce an uniform electric field?
Electric Dipole
- Electric dipole moment: A neutral oject has zero net
charge but it can have an electric dipole moment when its positive
charges are separated from its negative charges, see p.616.
- Induced dipole moment: A electric can induced a diople
moment in a neutral object. When the el;ectric field is removed,
the electric dipole moment returns to zero.
- Permanent diople moment: "Polar" molecules
possess permanent dipole moments.
- Alignment of electric dipole in an electric field:
Torque and energy, Eq. 18-11 and 18-12.
-> Do Problems 18-25,18-26.
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