Download PHYS 1212: Experiment 12 Report

Experiment 12
Date/ Time: Tues 2:00 p.m.
Stephanie Ferrera
Partner: Jasmine Fung
Introduction/ Goals:
In Experiment 12, students were asked to investigate the shapes of equipotential surfaces
that surround charged conductors. By doing so, students were able to interpret their
measurements of electric fields and electric potential lines.
There are four main goals to keep in mind throughout the lab, which students should be
able to interpret by the end of the experiment. Firstly, students should be able to explain the
difference between the concepts of electric fields and electric potentials. Secondly, students
should be able to correctly draw and identify electric field lines and electric equipotential
surfaces and curves on the same diagram. Students should also be able to explain the relationship
between both. Thirdly, students should be able to determine how electric fields and electric
potentials behave near conductors. Lastly, students should be able to explain the experiment and
the concepts within.
Overall, the experiment should allow students to see the meaning of electric fields and
equipotential lines, and how they relate.
Equipment:
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Probe
Experiment 12 (lab book)
Experiment 12 mapping board
Voltmeter
Power Source
Conducting Plates
Banana plug cables (4x)
Setup / Procedures
1. Using banana cables, attach the probe to the Voltmeter
2. Next, connect the power source to the mapping board using the banana cables. Make sure
one cable connects to the probe and the other to the board.
Experiment 12
Date/ Time: Tues 2:00 p.m.
Stephanie Ferrera
Partner: Jasmine Fung
3. With the brown side facing up, tighten the conducting plate to the bottom side of the
mapping board.
4. Align the Experiment 12 mapping sheets to the topside of the mapping board. Make sure
to align the board with the X & Y axis provided. This part is crucial so be as accurate as
possible.
5. Turn on the power source.
6. Adjust the power source to precisely 6V
7. By rubbing the probe across the surface of the mapping board, one can see different
values. Search for 5 different values of 1V, 2V, 3V, 4V, and 5V. Make sure to indicate
the points by drawing dots and labeling.
8. Connect all of the dots for 1V together to form the electric potential lines. Repeat for 2V,
3V, 4V, and 5V.
9. Draw lines from the positive to the negative point charges. Make sure each field line is
perpendicular to each equipotential surface line. Lastly, show the direction of the field
lines by adding arrowheads.
10. Turn off the power source
11. Remove conducting plate, and replace with insulator/conductor plate (Refer back to steps
3 and 4 for help)
12. Repeat steps 5-10 for the insulator / conductor plate.
13. Predict how the third map (point charge and line charge) will appear. Ask TA for
approval or further explanation.
14. Remove conducting plate, and replace with new conducting plate (Refer back to steps 3
and 4 for help)
15. Repeat steps 5-10 for the new conducting board.
Theory:
Experiment 12 uses four main equations to help explain the theory of the electric fields
and electric potential. Firstly, the electric field (E) is actually defined in terms of a force on a
positive test charge q’. Thus, E= F/q’. An electric field can also be placed at a point, with
distance r, from a point charge q. This point would be located at the origin of the coordinate
system. Thus, E = q/(4*pi*r^2* ϵ0). The electric potential, on the other hand, is defined in terms
of the electrical potential energy of a point charge at that position. It’s important to note, that the
electrical potential of any point in space can be defined in these terms. Thus, V= q/(4*pi*r* ϵ0).
The electric field produces a vector quantity where as the electric potential will produce a scalar
Experiment 12
Date/ Time: Tues 2:00 p.m.
Stephanie Ferrera
Partner: Jasmine Fung
quantity. The electric field can be represented graphically with two different methods. The first
method is by drawing a set of lines, which give the direction of the electric field at any point in
space. The next way is to draw a set of surfaces such that the electric potential has the same
value at every point on the surface. Lastly, we need to look at the physical result of the electric
field; the electric field is equal to the negative gradient of the electrical potential. This direction
is perpendicular to the equipotential and has a magnitude given by E= - ΔV / Δs.
Observation/ Discussion:
1. 2 Point Charges: There was a positive and negative point charge within the board.
Surrounding both the positive and negative points, there are circular equipotential
lines. As you moved away from the positive charge to the negative charge, the points
decreased in voltage. As the equipotential lines moved away from the positive point,
the lines became more and more vertical. However, once it passed a certain point, in
this case where the 3V points were located, the lines became curved again. Finally,
circling around the negative conductor. It’s also important to note that the electric
field lines went from positive to negative.
2. Conductor and Insulator: This board had a conductor and insulator plate and two
point charges. The equipotential lines were curved towards the positive point charge.
As you moved towards the negative point charge, however, the lines started curving
towards it. The potential lines also decreased in voltage as you moved away from the
positive charge towards the negative charge. The electric filed lines flowed from
positive to negative. As the electric field lines approached the insulator, the lines
became parallel and went around the insulator. As the field lines approached the
Experiment 12
Date/ Time: Tues 2:00 p.m.
Stephanie Ferrera
Partner: Jasmine Fung
conductor, however, the lines became normal to the conducting surface and seemed to
go into rather than around the surface.
3. Point and Line Charge Prediction: We predicted that the equipotential lines would
start out curved towards the positive charge. As it would move closer to the negative
line charge, the equipotential lines would become more and more vertical and parallel
to the negative line charge. It would also decrease in voltage as it approached the
negative line charge.
4. Point and Line Charge: Our predictions were correct. The equipotential lines started
curved towards the positive charge. As it approached the negative line charge, the
lines became more and more vertical and parallel to the negative line charge. The
electric field lines flowed from the positive point charge to the negative line charge,
and the equipotential lines decreased in voltage.