We shall consider in this Section systems of charges
fixed positions. First, let us briefly review how the concept of
Field is formalized.
2.1 Coulomb's Law
Coulomb's Law describes the interaction between two
It is taken as the customary starting point of Electrostatics, which in
turn uses to be the first topic in Electromagnetics.
In terms of forces, Coulomb's Law states that the
upon q' due to q is
Some relevant features are worth to be remarked:
as the force on a unit charge at every point in space.
The force is proportional to the charge, as
be observed in the following simulation
Three charges are shown. A central one with
units), and two charges equidistantly placed with respect to the
one, with Q=-20 charge units (left) and Q=+10 charge units (right),
In this way, the interaction between the two charges at the ends is
Notice also that the force between charges of opposite sign is
while between charges of the same sign is repulsive.
- It may also be noticed that, due to its symmetry,
Law is consistent with Newton's Third Law, as it is seen in the
The two charges considered have the values -20 and
- The force is inversely proportional to the
square of the
distance separating the charges
In this case, a central charge with value 10000
set of charges with values +1 or -1 located at different distances of
first one are shown. The data corresponding to each charge can be
in the particle parameter panel by clicking on the charge with the
button of the mouse. The values of force and distance to the central
can be known in this way.
(At present, the value of 2 for the exponent in
Law is known to be exact up to 16 digits. Of course this has not been
from measurements of forces between charges, but from some consequences
deduced from Coulomb's Law such as the zero field within a conductor
the Concept of Metal below).)
- Coulomb's Law is also consistent with the isotropy
space: all directions are equivalent in the absence of charges (and
any other physical system). When two charges are placed at given
a direction is being distinguished from the rest, the one corresponding
to the line joining the charges, and this is the direction of the
- q' is static (q can move)
- Vacuum and material media
- Relativistic and quantum-mechanical limits of
- Electric Field: By putting together all
not depend on q in Coulomb's Law we arrive at the definition of the
associated to the charge q'