ELECTRIC FIELDS
and
ELECTRIC POTENTIAL
PRELAB
PURPOSE
To investigate the electric field and potential field around simple source configurations, and to understand the relationship between them.
EQUIPMENTgraphite paper with electrode contacts, field probes connected to DataLogger interface, 5 volt dc supply, knife switch.
RELEVANT EQUATIONS
Electric Field:
Electric Potential:
Field-Potential Relationship:
DISCUSSION
A. The Electric Field
The electric field is a measure of the electric force per unit charge that a given arrangement of charges (the field source) exerts on a tiny positive test charge in the vicinity (tiny because it should be so small that the test charge itself does not affect the result).
Figure 15-1: The Electric Field Surrounding a Positive and a Negative Charge
It is therefore a vector quantity because the electric force as defined above has both direction and magnitude. This relationship can be expressed as:
The direction of the electric field is indicated by the arrows. Notice that the direction of at a given point in space is always the same as what the direction of the electric force would be on a positive charge if it were located there. What isn't so apparent from the field diagram is that the relative strength of the field is indicated by the density of spacing of the lines. Thus the force on a positive test charge gets weaker as the charge is moved farther away from the point charge source. The common unit of measure for the electric field is volts / meter.
An important property of the electric field is the fact that the resultant field produced by more than one source charge is simply the vector sum of the fields produced by each individual charge at the location of interest. There exist for example, multi-charge source configurations where the electric field remains constant throughout a given region of space. In this experiment you will investigate the electric field produced by two different source arrangements.
B. Electric Potential
Another quantity that is often an important consideration when dealing with charge arrangements is the energy associated with the configuration. Once again, it is convenient to take a tiny positive test charge and determine how much potential energy it has at a given location.
Figure 15-2: Equipotentials Surrounding a Positive Point Charge
The electric potential at a given point in space is defined to be the potential energy per unit charge possessed by the test charge due to the presence of the source charge(s). This relationship can be expressed as: