Download Electric Fields

Name: _________________________
Partner(s): _________________________
_________________________
Pre-lab Questions
Electric Fields and Equipotential Lines
Read over this lab procedure and answer the following questions before coming to lab.
1) What device will do you use in this lab to find all the points that have
equal electrical potential?
2) What are the three sheets of paper you attach to the board in this lab?
3) In the space below, draw both the electric field lines and the
equipotential lines of a single negative charge. Please pay close
attention to the direction of the field lines and the space between the
equipotential lines.
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Electric Fields
and
Equipotential Lines
OBJECT: To examine the shape of electric fields; in particular to map the
equipotential lines of an electric field and be able to map the field lines.
METHOD: We know that the electric field caused by an object or a pair of
objects varies as you change your distance from the objects. Positions that
have the same electric field strength may be connected together to form
equipotential lines. Note that the direction of the field may change along
these lines, but the magnitude of the field remains the same. By mapping
out these equipotential lines, we can then map out the electric field itself.
In this lab you are to create an electric field between different objects
placed on a conducting board and map out the equipotential lines using a
probe connected to a voltmeter connected as shown in Fig. 1 below. A
voltmeter is an instrument that can tell you the value of the difference in
electrical potential between two points. In our case we will use it to find
points where there difference in electrical potential between the two leads is
zero.
You will use the apparatus shown in Fig. 1 below. The DC
power supply s connected to the poles of the apparatus using banana plugs.
Three sheets are placed on the board in the order in Fig. 2. Be sure that the
middle carbon paper sheet has the carbon side on contact with the white
tracing paper. The sheets are held in place using the magnetic rubber strips.
Power Supply
V
Figure 1
Voltmeter/Power Supply
Hookup
Conducting Paper
Carbon Paper (Face down)
Tracing Paper
Figure 2
Paper Order
Procedure: With the apparatus described above, you will be provided a
pair of metal bars that can be placed on the board under the electrodes.
Connect the DC power supply (8-10 V) to the outer poles of the electrodes.
When the voltage is applied to the terminals, charges flow between them
across the black conducting paper following the lines of force of the electric
field established.
Starting about 1 cm from one of the electrodes, press one of the
voltmeter probes onto the conducting paper hard enough to allow the carbon
paper to make a mark on the white paper. Keep this first probe at that
location. By trial and error, place the other probe onto the conducting paper
and note the voltmeter reading. Pick up this second probe and move to
different locations until you see a “null reading” on the voltmeter. (That is,
the voltmeter reads 0 V.) Once this happens, press the probes onto the
board hard enough to make a mark on the white paper. Keep the first probe
fixed and locate at least 7 more points that give a null reading and mark
them on the white paper.
After locating at least 8 points that give 0 V, the first voltmeter lead
to a new voltage potential point about 1 cm from the previous point and find
a new series of equipotential points using the above procedure (collect at
least 8 data points.) Continue repeating the above procedure as you move
the first voltmeter probe in 1 cm increments across the board from one
electrode to the other.
Once completed, remove the magnetic strips to release the papers.
On the white paper, connect all the points that gave a null reading for the
first location of the first voltmeter probe. Then connect all the point that
gave a null reading for the second location of the first voltmeter probes. Do
this for all locations of the first voltmeter probe. Each of the lines that you
make will be the equipotential lines. The electric field lines can be sketched
in by noting that the electric field lines are always perpendicular to the
equipotential lines and are directed from the positive pole to the negative
pole. In a different color, CAREFULLY sketch the electric lines on the
white paper.
The entire procedure outlined above should be completed for (a) two
point sources, (b) two linear sources and (c) a third pair of sources provided
by the lab instructor.
Homework
Be sure that you and your partner each have a copy of the data for the three
situations that you investigated.
1. Using the data that you acquired, neatly sketch in the equipotential lines
for all three of the source pairs that you investigated.
2. The electric field lines discussed in class are perpendicular to these
equipotential lines. Using a different color than used for your
equipotential lines, neatly free hand draw in these lines for each of the
source pairs studied in the lab.
3. Answer the following questions:
A. Can two different electrical field lines ever cross each other? Can two
different equipotential lines ever cross each other? Explain.
B. Is work required to move an electric charge along a line of
equipotential? Explain. (Think about the strength of the force along
these lines and which way it is pointing.)
4. Points P and Q are on the same equipotential lines in an electric field and
points R and S are on a higher equipotential line. Compare the work
required to move an electric particle with a positive charge from:
(a) P to R
(b) S to Q (c) P to S (d) R to S
For example, in which case will the greatest amount of work be done?
In which case will the work be the least? Defend your answer.
5. The drawings below show equipotenial lines for two different electric
fields.
a) How do the strengths of the two fields compare?
b) In which direction does the electric field point in both cases?
c) What type of charge distribution might cause these two fields?
Where would the positive charges be? Where would the negative
charges be?
+4V
+2V
+6V
+8V
+8V
+6V
+4V
+2V
6. Based on the results of this activity and your knowledge of the
relationship between electric field lines and equipotential surfaces,
CAREFULLY sketch a representative set of equipotential lines and
electric field lines for each of the source distributions show on the next
page. In particular, be sure to show how the electric field line intercept
the surfaces of the conductors. Also, be sure to indicate the direction of
the electric field on the electric field lines. (Assume that the positive
pole is on the right in both cases.)