Leader: _____________________________ Recorder: ___________________________
Skeptic: _____________________________ Encourager:
_________________________
Electric Field Mapping Apparatus with rectangular, triangular and
circular plates
3 x Point Probes
DMM
2 Sheets Quadrille Graph paper (one laminated)
4 x Banana plug cables
15” Ruler
Introduction
In this activity we will use
the same apparatus and procedure as in the previous lab to map the electric
field for some additional geometries of conducting plates. In particular instead of two parallel plates,
we will look at the field pattern created when a potential difference is placed
between a rectangular plate and a circular plate, and a rectangular plate and a
triangular plate. The first geometry we will
explore is shown in Q1.
Q1) You will connect the +
terminal of the power supply to a round plate and the – terminal to the
rectangular plate. On the figure sketch
what you think the electric field lines will look like for this geometry and
then sketch what you think the equipotential surfaces will look like. Use a solid line for the field lines and a
dotted line for the equipotential surfaces.
Q2) Explain your reasoning for
your answer to Q1).
Procedure
1. Set-up the Apparatus
To set up the experiment,
first determine the scale of the graph paper by measuring the spacing in cm for
10 grids and divide by 10. Record the
spacing between the grids on the graph paper in the space below.
Q3) Spacing per grid = ________
cm/grid
Next, place the graph paper
underneath the dish so that the grid lines run parallel to the edges of the
dish. Place the stainless strip with its
long edge parallel to the short edge of the dish, and so that its inner edge is centered on the axis of
the graph paper. Place the circular disk
so that it is centered on the same axis and its closest edge is 20 grid lines
from the edge of the strip. Place the tip of two of the probes on each of the
conductors. Fill the dish with water
until the conductors are just covered by the water.
Using the provided banana
plug cables, connect the + terminal of the power supply to one the probe
touching the circular plate and the - terminal to the other probe. We will
refer to the conducting plate connected to the + terminal of the power supply
as the + conductor and the conducting plate connected to the – terminal of the
power supply as the – conductor. It is
customary to use a red wire for the connection to the positive side, and a
black wire for the connection to the negative side. Set the power supply to 6 V
and turn it on.
2. Setup the DMM
Connect the COM lead from
the digital multimeter (DMM) to the remaining point probe. The connectors on the back of the probe will
unscrew revealing a hole in the connector that you can put the probe into. Tighten down the connector to hold the probe
and make a good contact. Turn the dial
on the DMM to the 20
3. Preliminary Measurements
We will alter the procedure
slightly from the previous lab activity.
Place the point probe connected to the COM lead on the DMM on the -
conductor. Move the lead connected to
the VΩ terminal on the DMM toward the circular from
the rectangular plate along the axis of the graph paper until you find a point
that reads a potential difference of 1.2 V.
Record the location of the point in figure 1. Next find the location along the axis at
which the potential difference is 2.4 V to the rectangular plate. Repeat until the table is completed. Fill in the measured potential difference
between the two plates in the last row.
Figure 1 Data Table for
Potential Difference
Position of COM Probe |
Position of VΩ Probe |
ΔV (V) |
0 (- conductor) |
|
1.2 |
0 |
|
2.4 |
0 |
|
3.6 |
0 |
|
4.8 |
0 |
20 (+ conductor) |
|
From the data you recorded in figure 1, record the points you
determined between which there is a constant potential difference of 1.2
V. Fill in the appropriate value for the
last row.
Figure 2 Data Table for positions
Potential Difference steps of 1.2 V
Position Lower Potential Point |
Position of Higher Potential Point |
ΔV (V) |
0 (- conductor) |
|
1.2 |
|
|
1.2 |
|
|
1.2 |
|
|
1.2 |
|
20 (+ conductor) |
|
Q4) Do the points between which
there is a constant potential difference seem fairly evenly spaced or do they
tend to bunch up towards one conductor or the other? Explain.
Q5) If the equipotential
surfaces are closer together, does that indicate a stronger or weaker field
than if they are further apart? Explain.
4. Record Equipotential lines
Use the same procedure as in
the previous lab activity to find 4 equipotential surfaces in the region
between the plates. Place the COM probe
at the points you recorded in figure 1.
Be sure to include points near the edge of the conductors as well.
Q6) On your graph paper, draw
electric field vectors along the equipotential surfaces. Be sure they point in the correct direction
and label them with the magnitude of the average electric field.
Replace the circular plate with the triangular plate so that its vertex
is along the axis of the graph paper and 20 grids from the inner edge of the
rectangular plate.
Q7) You will connect the +
terminal of the power supply to the triangular plate and the – terminal to the
rectangular plate. On the figure sketch
what you think the electric field lines will look like for this geometry and
then sketch what you think the equipotential surfaces will look like. Use a solid line for the field lines and a
dotted line for the equipotential surfaces.
Q8) Explain your reasoning for
your answer to Q7).
Repeat the series of potential difference measurements that you made
above for this geometry.
Figure 3 Data Table for
Potential Difference
Position of COM Probe |
Position of VΩ Probe |
ΔV (V) |
0 (- conductor) |
|
1.2 |
0 |
|
2.4 |
0 |
|
3.6 |
0 |
|
4.8 |
0 |
20 (+ conductor) |
|
From the data you recorded in figure 3, record the points you
determined between which there is a constant potential difference of 1.2
V. Fill in the appropriate value for the
last row.
Figure 4 Data Table for positions
Potential Difference steps of 1.2 V
Position Lower Potential Point |
Position of Higher Potential Point |
ΔV (V) |
0 (- conductor) |
|
1.2 |
|
|
1.2 |
|
|
1.2 |
|
|
1.2 |
|
20 (+ conductor) |
|
Q9) Do the points between which
there is a constant potential difference seem fairly evenly spaced or do they
tend to bunch up towards one conductor or the other? Explain.
Q10) Is this effect more
pronounced for the circular plate or the triangular plate?
On the flip side of your graph paper, map out the equipotential
surfaces for the points you identified in figure 4.
Q11) On your graph paper, draw
electric field vectors along the equipotential surfaces. Be sure they point in the correct direction
and label them with the magnitude of the average electric field.
Q12) Where does the field appear
to be strongest?
Q13) If you have a conductor
with a sharp point, how does the strength of the field near the point compare
to points near flat places on the conductor?