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?
Electric Potential and Electric
Energy
Electric Potential and Potential
Difference
We define the change in electric potential
energy, PEa-PEb when a charge moves
from some point b to some point a, as the
negative of the work done by the electric
force to move the charge from b to a.
It’s useful to define the electric potential (or
simply the potential) as a potential energy per
unit charge:
PEa
Va 
q
Units of Electric Potential
1 V = 1 J/C
Relation between Electric
Potential and Electric Field
Vba
E
d
Important: this simple formula for E
may be used only for uniform field!!!!
Equipotential Lines
The equipotential surface must be perpendicular to the surface.
Important: the surface of a good
conductor is always an equipotential
surface!!!!
1. Even though electric forces are very much stronger than
gravitational forces, gravitational forces determine the motions
in the solar system. Why?
2. We compared the electric and gravitational forces between an
electron and a proton. Why is the result valid for all
separations?
3. When we approach another person, we are not aware of the
gravitational and electric forces between us. What are the
reason in each case?