Electric Field
Definition:
A test charge
is a charge which feels the force of other charges, but is too
small to exert any appreciable force on them.
This is, of course, just a mathematical abstraction. By
convention, a test charge is always taken as positive.
Definition:
The electric field, 
(
),
at a point of observation, ,
is the net Coulomb force acting on a test
charge q placed at that point ,
per unit value of test charge, q:
()
= 
()
/ q
- The SI-unit of
the electric field therefore has to be N/C. Later in this chapter,
when we discuss potential (SI unit Volt(V)) we will see that this
is equivalent to the more commonly used V/m.
1 N/C = 1 V/m.
- The electric field is a vector, in the same direction as the
net Coulomb force vector at that point.
Electric field of a point
charge:
- The magnitude of the force felt by test charge q due to point
charge q1 at a distance r from it is the Coulomb
force:
|
Fc = k
|
q1 q
|
|
|
|
r2
|
- Thus the
magnitude
of the E-field of point charge q1 is:
|
E1(r) = Fc / q =
k
|
q1
|
|
|
|
r2
|
- Direction
of the field: If q1 is positive, it repels the
test charge q. Therefore the E-field due to a positive point
charge q1 points away from the charge, along the line
joining it with the point of observation (where the test charge q
is placed).
- Clearly, if q1 were negative, the field due to it
would point towards it, along the line joining it and the point of
observation.
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MultiMedia Physics
2000