Physics
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Lab 4: Electric Potential
Objectives In this lab you will use PhET’s simulation Charges and Fields to study the electric potential due to
discrete charge distributions.
Part 1: Electric Potential Due to a Point Charge 1. Uncheck the box for Electric Field.
2. Check the boxes to display Values and the Grid. Make sure these are the only boxes checked.
3. Place a +1-nC charge toward the center of the simulation.
4. Use the potential meter to measure the electric potential at 8 different field points around
the point charge. The field points should all be a distance of 1 m from the point charge, but
they should be in different directions from the point charge. Complete the following table.
5. Based on your measurements, does the magnitude of the electric potential depend on the
direction of the field point relative to the point charge?
6. We now turn to investigate the dependence of the electric potential on distance from the
point charge. Use the tape measure to place the potential meter at each of the distances
shown in the table below and record the corresponding electric potential.
7. Make a scatter plot of the electric potential versus distance from the point charge. Plot
electric potential along the vertical axis and distance along the horizontal direction. Include
the best-fit curve in your graph and the equation of the best-fit curve. Decide the type of
curve to fit the data with based on theoretical expectation. Are your results consistent with
theoretical expectation?
8. An equipotential surface, as the name suggests, is a collection of all field points that are at
the same potential. Use the pen tool of the potential meter to draw equipotential surfaces.
Investigate the equipotential surfaces of a point charge and describe them.
9. Now check the box to display Electric Field. What is the relationship between the direction
of the electric field and the equipotential surfaces? Make a hypothesis and test it for the
other charge distributions in the other parts of the lab.
10. Use the simulation to compare and contrast the electric potential field of a single negative
point charge to that of a single positive point charge.
Part 2: Electric Potential Due to an Electric Dipole An electric dipole consists of a pair of equal and opposite charges.
1. Uncheck the box for Electric Field.
Field Point 1 2 3 4 5 6 7 8
Electric Potential (V)
Distance (m) 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 2.75 3
Electric Potential (V)
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2. Check the boxes to display Values and the Grid. Make sure these are the only boxes checked.
3. Place a +1-nC charge and a −1-nC charge 2 meters apart along the horizontal. The positive
charge should be to the left of the negative charge. Take the origin of the coordinate system
to be at the midpoint of the two charges, with the 𝑥-axis directed to the right and the 𝑦-axis
directed up. Let 𝑥 and 𝑦 denote the 𝑥 and 𝑦-coordinates of the field point, respectively. Let
us first investigate the electric potential at field points on the horizontal line that passes
through the two charges. This line coincides with the 𝑥-axis. Complete the following table.
Also calculate the electric potential of the dipole by adding the electric potentials of each
point charge.
4. In the above table, how do the measured values compare with the calculated values?
5. Let us now investigate the electric potential at field points on the perpendicular bisector of
the line segment that joins the two charges. This line coincides with the 𝑦-axis. Complete the
following table.
6. In the table above, how do the measured values compare with the calculated values?
7. Now complete the following table.
8. Investigate the equipotential surfaces and describe them.
9. What is the relationship between the direction of the electric field and the equipotential
surfaces?
Part 3: Electric Potential Due to a Pair of Positive Charges 1. Replace the charges in Part 2 with two +1-nC charges.
2. Repeat all the steps of Part 2 for this new pair of positive charges.
3. Compare and contrast the electric potential field of an electric dipole to that of a pair of
positive charges.
Part 4: Electric Potential Due to Three Nonlinear Charges 1. Place three point charges at locations of your choice. The three charges should not all have
the same sign, and they should not be arranged along a straight line. Choose the origin of
the coordinate system and use the same origin for both tables in this part. Indicate your chosen charges and their locations in the following table.
–2 –1.5 –0.5 0 0.5 1.5 2
Measured
Calculated
x (m)
Electric Potential (V)
–1 –1 1 2
–1 1 1 –1
Measured
Calculated
x (m)
Electric Potential (V)
y (m)
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2. Measure and calculate the electric potential at three different field points of your choice.
Complete the following table.
3. Compare your measured and calculated values in the table above.
Charge (nC) x (m) y (m)
First Charge
Second Charge
Third Charge
Measured
Calculated
y (m)
Electric Potential (V)
x (m)