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)