Physics

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Lab 2: Electric Field

Objectives In this lab you will use PhET’s simulation Charges and Fields to study the electric field produced by

discrete charge distributions.

Part 1: Field 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. Distribute 8 sensors evenly around the point charge. Use the tape measure to place each

sensor at a distance of 1 m from the point charge. All sensors should be the same distance

from the point charge. For each sensor, the value in V/m is the magnitude of the electric

field. Note that 1 V/m stands for 1 volt per meter, and 1 V/m = 1 N/C. The angle in degrees

is the direction of the electric field relative to the +𝑥-direction, with positive angles being

counterclockwise and negative angles being clockwise. Complete the following table by

recording the magnitude of the electric field read by each sensor.

5. Based on your measurements, does the magnitude of the electric field depend on the

direction of the field point relative to the point charge?

6. Place 7 of the 8 sensors back in their bin, keeping a single sensor in the simulation.

7. We now turn to investigate the dependence of the magnitude of the electric field on distance

from the point charge. Use the tape measure to place the sensor at each of the distances

shown in the table below and record the corresponding magnitude of the electric field.

8. Make a scatter plot of the magnitude of the electric field versus distance from the point

charge. Plot electric field 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?

9. Place the sensor and the tape measure back in their respective bins.

10. Now check the box to display the Electric Field (not just its direction) and uncheck the Grid

box. How is the magnitude of the electric field represented? Describe the global structure of

the electric field (magnitude and direction) due to a positive point charge.

11. Use the simulation to compare and contrast the electric field of a single negative point

charge to that of a single positive point charge.

Sensor 1 2 3 4 5 6 7 8

Electric Field (N/C)

Distance (m) 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 2.75 3

Electric Field (N/C)

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Part 2: Field Due to an Electric Dipole An electric dipole consists of a pair of equal and opposite charges.

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 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 𝐸

be the magnitude of the electric field, and 𝜃 the angle the electric field makes with the +𝑥-

direction, with counterclockwise angles being positive and clockwise angles being negative.

4. Let us first investigate the electric field at field points on the horizontal line that passes

through the two charges. This line coincides with the 𝑥-axis. Complete the following table.

Use a sensor for the measured values. Also calculate the magnitude and direction of the

electric field by adding the electric fields of each point charge.

5. In the above table, how do the measured values compare with the calculated values?

6. Let us now investigate the direction of the electric field 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.

7. In the table above, how do the measured values compare with the calculated values?

8. Now complete the following table.

9. In the table above, how do the measured values compare with the calculated values?

10. Place the sensor in its bin.

11. Check the box to display the Electric Field (not just its direction) and uncheck the Grid box.

Describe the global structure of the electric field (magnitude and direction) due to an

electric dipole.

–2 –1.5 –0.5 0 0.5 1.5 2

Measured

Calculated

Measured

Calculated

x (m)

E (N/C)

θ (degrees)

–2 –1 0 1 2

Measured

Calculated

Measured

Calculated

y (m)

E (N/C)

θ (degrees)

–1 –1 1 2

–1 1 1 –1

Measured

Calculated

Measured

Calculated

x (m)

E (N/C)

θ (degrees)

y (m)

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Part 3: Field 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 structure of the electric field of an electric dipole to that of a pair

of positive charges.

Part 4: Field 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. Measure and calculate the electric field (magnitude and direction) 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

Measured

Calculated

y (m)

E (N/C)

θ (degrees)

x (m)