Page 1/3
ELECTRIC FIELDS AND EQUIPOTENTIALS (rev. 4/13)
[The background material presented here is similar to that on pages 241-243 of the laboratory
manual. Reading both before coming to lab will better prepare you to conduct the experiment.]
INTRODUCTION
The electrostatic force is one of the four fundamental forces controlling interactions between
matter in our universe. Of the four, only the gravitational force and the electrostatic force are
observable on a macroscopic level. But the gravitational force is only significant when a very
large mass is present, such as in the case of free fall or in the interaction between stellar
bodies. On the other hand, the electrostatic force is evident on a daily basis providing us with
such effects as static cling, electrostatic filtering, and lightning discharge.
An unbalanced charge distribution will act with an electrostatic force on any other charges that
exist in the vicinity of the distribution. The electric force per unit charge at any point in the
region near the distribution is known as the electric field E. Then, the electrostatic force F on a
test charge q0 is merely the product of the electric field (at that point in space) and the charge at
the same location:
[1]
If the electric field is due to a single point charge q, then it can be simply described by:
[2]
where k is the Coulomb constant and r is the radial distance from the charge. The direction of
this field would be spherically outward from a positive point charge. In the more general case
when E is due to a non-point charge distribution, it is often more beneficial to visualize the
strength and direction of the field rather than computing a complicated mathematical expression.
Lines which are drawn to visualize the pattern of the electric field in the region near a charge
distribution are known as electric field lines. The specific electric field at any point in space will
always be tangential to the field line at that point. The field lines also indicate the path a positive
charge would travel when released from rest in the region of space. Because of this last
characteristic, electric field lines always run from positive charges toward negative
charges. Furthermore, field lines always intersect charge distributions at right angles and are
drawn densest where the charge distribution is the strongest. Since the field lines are an estimate
of the overall field in space, there can only be one line (or tangent) for every location near a
distribution.
However, since the electric field is a vector quantity, it is very difficult to directly determine the
pattern of the electric field lines. Yet as a charged particle moves along an electric field line, its
potential energy changes due to work being done on the charge by the electrostatic
force. Energy, being a scalar quantity, is much easier to measure with ordinary laboratory
equipment.
An unbalanced charge distribution also creates a potential energy per unit charge at every
location in the vicinity near the distribution. Another name for this potential energy per unit
charge is the electric potential. The difference between the electric potential at any two points in
space is known as potential difference, or in more common usage, voltage. The SI unit for
electric potential (or potential difference) is the Volt, equivalent to a Joule per Coulomb. The
relationship between field and potential is given by:
Next Page
Find
Go to Page
Thumbnail Index
Image View
Download a Copy
Close