Electric Field Lines and Equipotential Lines
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Objectives
1. To nd equipotential lines for various static charge distributions.
2. To relate equipotential lines to electric eld lines.
3. To determine electric eld lines for various static charge distributions.
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Introduction
2.1 Rules for electric eld lines
1. Electric eld lines give the direction of force on a small positive test charge.
2. Electric eld lines point from positive charges to negative charges.
3. The density of the electric eld lines is proportional to the magnitude of the electric
eld at that location.
4. The direction of the electric eld at a particular point in space is tangent to the electric
eld line at that point.
2.2 Rules for equipotenital lines
1. The electric potential is a measure of the energy available to electric charges.
2. Equipotential lines are the collection of points that are at the same potential.
3. Electric eld lines are perpendicular to equipotential lines, and point from high potential to low potential.
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Experimental Procedure
IMPORTANT! Always keep yourself grounded throughout this experiment. Also consider
how the equipment arrangement may eect the experiment. A misplaced power supply can
change charge distributions, and the experimenter's arm can easily do the same.
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3.1 Drawing the Electrode Patterns
1. Plan and sketch the layout of four electrode patterns shown below. If you have time
at the end of the lab, you can create other patterns as well.
Note: This is the most dicult and crucial part of the experiment. Follow these steps
carefully:
2. Draw each of your pattern of electrodes on the black, conductive paper.
(a) Place the conductive paper, printed side up, on a smooth, hard surface. DO NOT
attempt to draw the electrodes while the paper is on the corkboard.
(b) Shake the conductive ink pen (with the cap on) vigorously for 10-20 seconds to
disperse any particle matter suspended in the ink.
(c) Remove the cap. Pressing the spring loaded tip lightly down on a piece of scrap
paper while squeezing the pen barrel rmly starts the ink owing. Drawing the
pen slowly across the paper produces a solid line. Drawing speed and exerted
pressure determines the path width.
(d) Once a satisfactory line is produced on the scrap paper, draw the electrodes on
the black, conductive paper. If the line becomes spotty or thin, draw over it again.
A solid line is essential for good measurements.
(e) The line will be air dry in 5 minutes at room temp. It takes about 20 minutes for
them to reach maximum conductivity, so make sure to draw out all four patterns
at this time.
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3.2 Mapping out Equipotential Lines
1. Mount the conductive paper on the corkboard, using one of the metal push pins in
each corner.
Power supply
−
+
Voltmeter
+−
Red Probe
2. Connect the electrodes to the DC power supply, using the connecting wires and the
metal push pins. Make certain that the pin holds the wire rmly to the electrode.
3. Next connect the black lead on the voltmeter (port labeled COM) to the negative
electrode using the alligator clips. You can then measure potential by placing the red
(positive) probe (port labeled V) on the conductive paper. All potential dierences
will then be measured with respect to the negative electrode (which will then be at
zero volts). Make sure to turn the voltmeter to DCV.
4. Turn on the power supply, but rst make sure that there is no short circuit! Set the
power supply for about 10 VDC. Ask your instructor how to operate the power supply
the rst time you use it.
5. For each of your electrodes, trace out equipotential lines at 1 Volt intervals. Map this
on the white graph paper provided for you. Each one of you needs to make your own
graphs. Make sure to label each line with its corresponding potential. You should have
at least six points per line (more if you are not sure exactly how it looks). Also make
sure to draw in and clearly label your electrodes, and their corresponding potentials
on your graph.
6. Sketch the electric eld lines through the equipotential lines (they are perpendicular,
remember?). Use a dierent colored pen and draw in the DIRECTION of the lines
too. You should draw in about ten eld lines for each pattern of electrodes.
7. Repeat until you have mapped out the equipotential lines and electric eld lines for
the four patterns below.
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Two like charges
Two opposite charges
+
DC
Source
DC
Source
−
DC
Source
+
−
+
DC
Source
+
−
−
Parallel Plate Capacitor
Floating Electrode
3.3 Mapping out Electric Field Lines
1. We can also use the voltmeter to measure the direction of the electric eld at a particular point on the paper. Chose one of your electrode sets again. Get a fresh sheet
of graph paper for this part, but keep your corresponding equipotential graph handy
too.
2. Connect the black probe into the ground port (COM) of your voltmeter. Tape the
red and black probes together, so that their separation is very small (about 1 cm).
Touch the black lead to the paper. Keeping the separation of the probes constant,
rotate the red probe until the potential reading in a given direction is the highest.
This corresponds to the direction of the electric eld at that point. Draw an arrow in
that direction (with the tail where the black probe was, and the head where the red
probe was). Now place the black probe at the head of the arrow you just drew and
repeat. As you keep repeating, you should begin to trace out an electric eld line. Do
this for at least ten lines, pretty equally spaced.
3. How do your electric eld lines just found compare to the electric eld lines you found
from the equipotential lines?
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