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d
E   (7.3)
dx

This equation expresses the correlation of the field strength and
the potential: strength vector of electric field at any point is equal to

potential gradient taken with "-".
Thus, the field strength vector coincides with the direction of the
utmost potential change. Sign "-" in formula (7.3) indicates that the
vector E is directed towards the decrease of potential.

If the field is uniform (E = const), then the latter formula will be
in the form known from the school course of physics:

  U
E  1 2  (7.4)
l l

where l is the distance along the direction E between points with
potentials  and  .
1 2
In common case


E  grad     (7.5)

where  - Del or Nabla is a vector differential operator. The del
symbol can be interpreted as a vector of partial derivative operator

     
  i  j  k (7.6)
x  y  z 

  
where i , j, k are the unit vectors in their respective directions.
According to the correlation of the field strength and potential in

SI system, unit of strength of electric field is measured in volt per
V
meter  .
m
8. The Principle of Experimental Research of Electric Field
So, the equipotential and force lines of the electric fields are
mutually perpendicular therefore knowing the pattern of the
equipotential lines we can get the force lines and vice versa. For

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