Equipotential lines
Equipotential lines provide a quantitative way of viewing the electric potential in two dimensions. Every point on a given line is at the same potential. Such maps can be thought as topographic maps.
Rules for equipotential lines:
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For instance consider the
map of the Rawah Wilderness on the right. All
points on the same line are at the same
elevation, just as all points on the same
equipotential lines are at the same voltage.
Water will always flow downhill,hence the rivers
are always perpendicular to the lines on the
topographic map, similar to the way electric
field lines are always perpendicular to
equipotential lines. When lines are close
together, the slope is steep, e.g. a cliff, just
as close equipotential lines indicate a strong
electric field. Lakes are at the same elevation,
in the same way conductors are at the same
potential.
The figure to the left
shows equipotential lines. The electric field at
point a can be found by calculating the
slope at a:
is the voltage difference
between the two lines near a, and 
is the distance between the two lines
nearest a.