LAB 2
Electric Field & Potential Mapping
OBJECTIVES
1. Determine the location of equipotential surfaces in the region around two electrode
arrangements, dipole and parallel plates.
2. Construct electric field lines from the measured equipotential surfaces.
EQUIPMENT
Field mapping apparatus, electrode plates, templates, paper, DC Power Supply, Digital
Multimeter (DMM), Deflection tube apparatus
PROCEDURE
Field Mapping for different Electrode Arrangements
a. With the help of an instructor, setup the field mapping apparatus to measure
equipotentials for two electrode arrangements. Set the power supply to 15 V using
your voltmeter.
b. In order to make a contour map of equipotential lines on the paper, connect the V-
lead from the meter to the probe on the field-mapping board. Systematically search
for a number of points whose potential is about 9 V. Mark them, and draw a smooth
line connecting them (don't connect the dots with straight lines!). Then repeat this
process for increments of 3V for equipotentials ranging between 3V through 12V.
Each smooth line is the “equipotential” for its voltage. Label each one right after you
create it.
c. For each of the two electrode arrangements, perform the following steps:
 Draw in the equipotential curves for each electrode arrangement. Study the
spacing between your equipotential lines and, with your partners, identify regions
of strong and weak electric field. Explain your reasoning for each electrode
arrangement.

The E-field lines, which go from the positive conductor to the
negative conductor, are always perpendicular to the equipotential
lines. Draw in the E-field lines using dashed lines. Clearly indicate
the direction of the electric field at several positions on eac h
plot as indicated below. This will produce a map of the E-field for
your conductors.

Make sure to draw the correct spacing (approximately) between the field lines.
The E-field is strong where the field lines are close together and weaker where
they are far apart.
Electrode Arrangements
Part 1: Parallel Plate
a. Locate the equipotentials at voltages 3V, 6V, 9V and 12V. Make sure that the
fringe fields are also mapped out (i.e. the edge fields).
b. Measure the distance ∆s between each of the following pair of potentials.
c. Calculate the E-field using E = ∆V/∆s between adjacent pairs of equipotentials
around the center of the parallel plates.
d. Compare the averaged E-field value with each individual value using a percent
difference. How do they compare? Record your data in a table.
Questions:
 Is the electric field around the center of the parallel plate’s constant (i.e. not
considering the edges)? Use your data values to answer this question.
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

Identify regions where the field is strong and weak on your mapped field and
explicitly show these on your plot.
Starting with your mapping results, is there symmetry in the parallel plate electric
field? Is there symmetry in the equipotential lines? Explain.
Part 2: Electric Dipole
Repeat part (1a) for the electric dipole arrangement.
Questions:
 By looking at your equipotential curves, how does the electric field strength vary with
distance from one of the charge particles?
 Identify regions where the field is strong and weak on your mapped field and
explicitly show these on your plot.
 Starting with your mapping results, is there symmetry in the electric field for the
dipole charges? Is there symmetry in the equipotential lines? Explain
Part 3: Deflection of Electrons in a Potential Field
a. A deflection tube in its special stand is setup. Studying the deflection tube and its
diagram. Explain the role of the (i) heater filament, (ii) cathode, (iii) anode, and (iv)
parallel plates. From these four elements, explain how an electron beam generated
and deflected in the deflection tube using
 Electric field model
 Electric potential model
b. Using conservation of energy, predict and explain whether an electron beam will be
deflected more or less as the potential energy of the parallel plates is either
increased or decreased.
c. With the help of an instructor, starting from 0 V, increase the voltage (∆Vplates) to 5 kV
in 0.5 kV increments. You should now see a curved trajectory due to the vertical
electric force applied to the electron beam. CAUTION: do not increase the voltage
greater than 5 kV.
After you have completed your observations and without turning off the power
supply, turn the HIGH VOLTAGE ADJUST to zero. The light from the heater
filament will stay on.
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