Electrostatic Field Mapping
Introduction
The purpose of this lab is to draw a map of the electric field produced by a charge distribution and to compare the results to those predicted by electromagnetic theory. Since equipotential surfaces are always at right angles to the electric field lines when they cross, you will be able to draw a map of the electric field lines once the equipotential surfaces have been drawn.
Experimental Procedures
The Electric Field Produced by a Point Charge
1) Set the voltmeter function to DCV and the range to 20. Place the red lead in the hole marked V/Ω. Place the black lead in the hole marked COM.
2) Tack the conducting paper to the peg board using plastic push pins. Tack the large conducting ring to the paper using lots of pins. Place a metal pin in the center of the ring. Push all pins very firmly and check periodically to assure good connections.
3) Touch the red wire of the power supply to the metal pin in the center. Attach the black wire to the ring. Do not allow the head of the pin to lose contact with the paper.
4) Attach the black wire of the voltmeter to the ring. Touch the red wire of the voltmeter to the center pin. Adjust the power supply so that the voltage is approximately 10 Volts.
5) Touch the red voltmeter lead to the conducting paper. Measure and record the voltage.
6) Move the red lead such that the voltage doesn't change. Lightly trace this path on the conducting paper using a pencil until one equipotential surface has been drawn. Label the path with the measured voltage.
7) Repeat steps 5 and 6 until about five equipotential surfaces have been drawn. When you are finished, be sure to turn off the power supply.
8) Remove the paper from the peg board and draw several electric field lines. The electric field lines always go from higher to lower potential (by convention) and must be perpendicular to the equipotential surfaces. Qualitatively compare your results to the theory that the field lines radiate straight outward from the point charge.
9) Select several points on your paper and calculate the magnitude of the electric field |E| = ΔV/Δs in the vicinity of that point. Theory predicts that the magnitude of the electric field will decrease with distance from the point charge.
10) Present your results to your instructor before moving to the next configuration.
The Electric Field Produced by Two Parallel Lines (2-d Capacitor)
Use the other side of the conducting paper and repeat the above procedures for two long, straight lines. Place the lines about half their lengths apart. Theory predicts that the electric field is uniform between the lines and that there will be a fringing effect (weaker field of changing direction) near the edges.
The Electric Field Produced by a Dipole
Obtain another sheet of conducting paper and repeat the above procedures for two points. Consult your textbook for a picture of the expected electric field and compare your results qualitatively. You need not calculate the magnitude of the electric field.
The Electric Field Produced by another Configuration
Use the other side of the conducting paper and create your own configuration. Repeat the above procedures. You need not calculate the magnitude of the electric field.
Your raw data will include the sheets of paper. Write the names of the members of your group on the sheets, staple the sheets together, and give them to your instructor before you leave the lab.