Download Mapping Electric Fields

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Mapping Electric Field Lines
Purpose and Objectives
Any charge source (q) is surrounded by an electric field that is represented by lines called electric field
lines extending out from the charge source. If the charge source is positive, the direction of the field
lines points away from the source. If a charge is placed in an electric field, the charge will be acted
upon by a force. Therefore we can say that within the electric field there is the potential for work to
be done. This potential is dependent upon the electric field strength and the distance (d) covered.
This distance must be in the direction of the field lines. A charge will not experience a change in
potential (also called potential difference) if it is moved perpendicularly to the field and maintains the
same distance from the source because the electric field strength doesn't change. We can draw lines
representing these areas of no potential change. These lines are called equipotential lines. They are
always parallel to the surface of the source, never intersect each other but always intersect the electric
field lines at right angles.
In our experiment, electric fields will be generated in an electrolytic solution, which is slightly impure
water. (It comes from our tap naturally impure).
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To map electric field and equipotential lines between 2 point sources.
To map electric field and equipotential lines between 1 point and 1 planar source.
Determine the relationship between work and energy.
Materials
clear water trays
clip leads
digital multi-meter (DMM)
variable power supply or batteries
conducting cylinders and plates
graph paper
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Figure 1 Experimental Setup
PROCEDURE
1. Put about a centimeter of tap water in the clear glass tray.
2. Mark a coordinate system on a piece of graph paper. The origin of coordinates can be
anywhere on the paper, but it might be best to put the origin at the center of the paper.
3. Place the tray on the paper in a symmetric way. Then place two "point sources" (represented
by short metal cylinders) on the horizontal axis of your coordinate system and at equal
distances from the center. This symmetry will facilitate plotting field lines.
4. On a second piece of graph paper, set up another coordinate system just like the one on the
paper that's under the tray. Also, copy the outlines of the sources on your second paper.
5. Connect the two sources to the two terminals of the power supply using the wires provided.
Set the supply to 6 volts. Use enough batteries to provide a minimum of 6 volts if using
batteries instead of a variable power supply.
6. Using the digital multi-meter, attach the "com" terminal to the source that is attached to the
negative terminal of the power supply. Turn the DMM on to DC volts.
7. Turn on the power supply. You are now ready to take data. Immerse the second DMM probe
in the water somewhere roughly between the sources and note the DMM reading. The secret
to this technique is to stick the probe into the water vertically and read the voltage quickly.
Otherwise, because of electrolysis, the reading on the DMM will change rapidly.
8. The reading might be 2.54 V, or some other number. Also, the probe will have x, y
coordinates. Record this point on your second piece of paper. MAKE SURE TO TURN
OFF THE POWER SOURCE WHEN NOT COLLECTING DATA.
9. Move the probe and find another nearby point with the same potential. Note its coordinates
and record those coordinates on your graph paper. Repeat this procedure until you have
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about 6 points with the same potential. Make sure that the points are spread out, not bunched
up close together.
10. Now immerse the probe, choose another potential, and generate another equipotential line.
Keep going till you get a total of 5 lines between the sources. To make sure that the lines are
spread sufficiently far apart, you might want to choose potentials like 1, 2,3,4,5 volts, and
draw one line for each.
11. Connect all of the points for each equipotential line with a smooth curve.
12. Then, using a different color pen or pencil (if possible), draw lines perpendicular to the
equipotential lines. These will be the electric field lines. At the surface of the sources, the
electric field lines are always perpendicular to the surface. Use this fact when attaching the efield lines to the sources. Draw enough e-field lines so that you can clearly see the structure of
the field.
13. Label the sources with their polarity (+/-). Also, attach directions to the field lines by putting
arrows on them. The arrows point from positive to negative.
14. Repeat steps 1-13 for another set of sources-maybe a point and a plane or two planes. Use a
bar of steel to represent the plane.
ANALYSIS
1. Define potential difference. What is the difference between electric potential and electric
potential energy? What is the unit for potential difference?
2. What two units are used for electric fields? Write out the units of both to show that they are
equivalent.
3. Why are the electric field lines always perpendicular to equipotential lines?
4. Why can electric field lines never cross?
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