Physics 142
Laboratory 1
Electrostatic Potential
So far in class, we have learned how to calculate the electric field and electric potential produced by
various arrangements of charges. In this lab, we will explore the electric field and electric potential
produced by conductors held at constant potential. Remember:
•
Electric potential is the potential energy per charge for a “test charge” placed at a point. Electric
potential is measured in volts (V).
•
Equipotential lines are the lines along which the electric potential is constant. Drawing the
equipotential lines allows you to represent the potential graphically.
•
Electric field is the force per charge for a “test charge” placed at a point. Electric field is
measured in volts per meter (VÎm).
•
Electric field lines are lines drawn in the direction of the electric field. Electric field lines are
always perpendicular to equipotential lines, and point from high potential to low potential.
•
Conductors allow charges to move around inside of them. The electric field inside a conductor
is always zero, and the electric potential inside a conductor is constant.
Setup
Lay a piece of graph paper in a flat-bottom insulating pan, and add about an inch of water. Position two
electrodes (metal plates) on the graph paper, and attach the terminals of the electrodes to a power supply:
negative
plate
positive
plate
Set the power supply to approximately & V. This will maintain a constant potential difference between
the plates, thereby creating an electric field in the water.
Procedure
You will explore the electric field in the water by mapping out the equipotentials, using a voltmeter to
measure the voltage differences.
Some notes:
1. Fix a metal probe in the water connected to one input of the voltmeter. This determines the
location at which Z œ !. Do not move this probe during data collection.
2. Attach the other input of the voltmeter to another probe. Move this probe around to read the
voltage at various locations in the water.
3. Your goal is to map the equipotentials. You will want to map at least five different
equipotentials, e.g. the equipotentials for !Þ!! volts, "Þ!! volts, #Þ!! volts, $Þ!! volts, %Þ!! volts,
and &Þ!! volts.
4. How to Map an Equipotential: Choose a voltage, say "Þ!! volts. To map the corresponding
equipotential, find several different locations in the water where the potential is exactly
"Þ!! volts, and record the coordinates. Plot the points on graph paper as you work to check that
your data makes sense.
5. Record the coordinates of at least three or four different points for each equipotential (more if the
equipotential is particularly oddly shaped).
6. Touching either plate will ground it, and ruin the potential difference. Also, touching the metal
of the probes will ground them and ruin the reading on the voltmeter.
Other Setups
After you have mapped out the potential lines for two parallel plates, use the same procedure to map out
potential lines for some other setups. Explore at least two of the following configurations (or feel free
to make up your own).
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