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Transcript
The Electric Field
The electric field E at a point in space is
defined as an electric force F, acting on a
positive test charge q divided by the
magnitude of the test charge

 F
E
q
1
Q
E
2
4 0 r
Units of Electric Field
1 N/C
Superposition Principle
for Electric Fields
Field Lines
1. At every point of electric field line,
electric field E is tangent to this line. No
two field lines can cross!
2. The line must begin at positive charge and
terminate on the negative one unless go to
infinity.
3. The number of line per unit area is
proportional to the magnitude of electric
field.
Electric Fields and Conductors
Important: electric field inside of good
conductor is zero!!!!
Important: any net charge on a good
conductor distributes itself on the
surface!!!!
Electric field is always perpendicular to the surface of
conductor:
Conceptual Example
A hollow metal box is placed between two parallel charged
plates. What’s the field like inside the box?
P
d
q
d
Q
Is it possible to place a charge at point Q such
that the electric field produced at point P by
the two charges will be add to zero?
Two positive charges are a fixed distance apart. The sum
of their charges is QT. What charge must each have
in order:
(a) Maximize the electric force;
(b) Minimize it.