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Physics Lab 212P-3 Template
Electric Fields and Electric Potential
NAME:
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LAB PARTNERS:
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LAB SECTION:
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LAB INSTRUCTOR: __________________________
DATE:
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EMAIL ADDRESS:
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 Penn State University
Physics 212P-3
Written by Redwing and Samarth
1
Physics Pre-lab 212P-3 Template
Electric Fields and Electric Potential
Name:__________________________
Section:_____
Date:__________
(Read this & answer the questions before coming to lab)
Summary of relevant concepts:
a) The electric potential energy U of a charge at any point in space is defined as the negative
of the work done by the electric field E when the charge is moved from infinity to that point.
Here, we define the electric potential at infinity to be 0. Note that this definition is equivalent
to saying: "electric potential energy is the work YOU have to do in bringing a charge from
infinity to that point."
b) The electric potential V at any point in space is defined as the negative of the work done by
the electric field when a charge of +1 C is brought from infinity to that point i.e.
A 

V    E  ds

c) We also talk about the potential difference between two points A & B:
B 

V    E  d s
A
d) i.e. the negative of the work done by the electric field in moving a charge of +1C from A to
B.
e) Electric potential and potential differences are measured in VOLTS; electric potential energy
is measured in joules.
f) Equipotential surfaces are a convenient way of picturing the electric potential in any region.
Most often, equipotential LINES are used to portray a cross-sectional view of equipotential
surfaces. By definition, all points on an equipotential line/surface have the same potential.
 Penn State University
Physics 212P-3
Written by Redwing and Samarth
2
Pre-lab Questions:
Q1. What is a "conservative force?" (Recall: Physics 211.)
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Q2. When a mass that is free to move is released in the presence of a gravitational field, does it
move from a region of high gravitational potential to a region of low gravitational potential or
vice-versa?
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Q3. When a charge that is free to move is released in the presence of an electric field, does it
move from a region of high electric potential to a region of low electric potential or vice-versa?
Does your answer depend on whether the charge is positive or negative? Why/why not?
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 Penn State University
Physics 212P-3
Written by Redwing and Samarth
3
The figure below shows a region of space with a uniform electric field E. You can move charges
from A to B along the three different paths shown. ACBD is a square of side L.
Q4. Suppose a positive charge +q is moved from A to B. Calculate the work W done by the
electric field on the charge, if the charge +q were taken:
(a) first from A to C and then from C to B.
(b) first from A to D and then from D to B.
 Penn State University
Physics 212P-3
Written by Redwing and Samarth
4
Q5. Calculate the work done by the electric field on the charge if the charge were moved along
the straight line AB. How does this compare with your answers to Q4?
Q6. From your answers to Q4 & Q5, what is the potential difference V = VB - VA between
points A and B ?
Q7. An equipotential surface is defined as a surface on which there is no potential difference
between any of the points. What are the equipotential surfaces for the problem above? Sketch a
cross-sectional view of a few equipotential surfaces, showing surfaces with a constant potential
difference between them.
 Penn State University
Physics 212P-3
Written by Redwing and Samarth
5
Lab Activity 1: Visualizing the electric potential created by a dipole.
Q1. Use the program to make a plot that shows equipotentials for the electric dipole with the
following values: V = -3.2 V, -1.5 V, -0.7 V, -0.3 V, 0 V, +0.3 V, +0.7 V, +1.5 V, +3.2 V.
(In case you cannot match these values exactly, don't worry about it. Get as close as you
can.) In this plot, also show a few electric field lines. Print this figure and include it with
your report.
 Penn State University
Physics 212P-3
Written by Redwing and Samarth
6
Now, refer to your equipotential and electric field line plot, as well as to the exercises you
carried out above, and answer the following questions.
Q2. Do you notice any obvious geometrical relationship between equipotentials and electric field
lines? Describe this relationship and why it makes sense. (Think about the direction of the
electric field and the work done in moving a charge along an equipotential.)
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Q3. If you were to bring a positive charge from infinity to any point on the perpendicular
bisector of the electric dipole following an arbitrary path, how much work would you do?
Justify your answer using your equipotential plot.
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Q4. Is it true that wherever the electric potential V = 0, the electric field E is also 0?
Justify/support your answer using information on your plot.
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 Penn State University
Physics 212P-3
Written by Redwing and Samarth
7
Q5. Is it true (in general) that wherever the electric field E = 0, the electric potential V is also 0?
Suggest an arrangement of charges that would help justify your answer.
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Q6. Suppose you released a positive charge from a point located on the +0.3 V equipotential.
Would it move to a point of higher or lower potential?
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Would it move to a point of higher or lower potential energy?
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Justify your answers based on the electric field lines in your plot.
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Q7. Describe how your answers to Q6 would change if you used a negative charge instead.
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 Penn State University
Physics 212P-3
Written by Redwing and Samarth
8
Lab Activity 2: A Real Experiment & A Return to (Messy) Reality.
Q8. How does the potential vary when you probe different locations ON the silver ring?
What we expected and why:
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What we observed:
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Q9. How does the potential vary at different positions INSIDE the silver ring?
What we expected and why:
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What we observed:
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 Penn State University
Physics 212P-3
Written by Redwing and Samarth
9
Q10. How does the potential vary with position outside the silver ring? Use sketches to show the
expected and observed equipotentials. Make sure you also indicate in the sketch below the
positions of your pushpin connectors.
expected
Sketches of
and
equipotential patterns:
observed
Creative exercise: comment on discrepancies between your expectations and observations:
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 Penn State University
Physics 212P-3
Written by Redwing and Samarth
10
Physics Post-lab 212P-3
Electric Fields and Electric Potential
Name:__________________________
Section:_____
Q1. In the figure does the electric potential increase
toward the right or the left?
Toward the right
Date:__________
Equipotential surface
Field line
Toward the left
Q2. If the adjacent equipotential surfaces differ by
10 V and the right-most one is at an electric
potential of –100 V, what is the electric potential of
the left-most one?
Q3. If we move an electron toward the right, is the work done on the electron by our force
positive or negative?
Positive
Negative
Q4. If we move an electron toward the right, is the work done on the electron by the electric field
positive or negative?
Positive
Negative
Q5. Is the electric potential energy of the electron increased or decreased as a result of the move
to the right?
Increased
Decreased
 Penn State University
Physics 212P-3
Written by Redwing and Samarth
11
Q6. If the surface of a charged conductor is an equipotential surface, does that mean that the
charge is spread uniformly over the surface?
Yes
No
Why or why not?
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 Penn State University
Physics 212P-3
Written by Redwing and Samarth
12
 Penn State University
Physics 212P-3
Written by Redwing and Samarth
13