PHY 242 Laboratory for 9/2/99

Electric Field Mapping

Following the instructions provided with the Overbeck Field Mapping Apparatus, each group will map the equipotential lines of three patterns: the Plane-Plane, the Faraday ice pail, and one other pattern of your choice. Record points close enough together to insure that an accurate equipotential line can be sketched through the points. (In the regions near the conductors, where the direction is changing quickly, spacing should be about one centimeter. It may be be about twice that in areas well away from the conductors.) Each member of the group will sketch their own lines, hence each member should have their own copy of the point maps.

Report:

  1. Sketch the equipotential lines and indicate the potential of each. Sketch the electric field lines of force for each map. The lines of force should be perpendicular to conductors and to the equipotential lines at all points. Take care in making your sketch!

  2. On one of the maps, select and label two points on one of the electric field lines where it intersects adjacent equipotential lines. Estimate the average strength of the electric field between those points using DV = Ed.

  3. Find the approximate charge residing on the point conductor on one of the maps. This can be done by utilizing the estimated value of E near the point conductor, where E is found by the same method used in the previous step. The charge can be found using the Coulomb expression for the electric field of a point charge. Explain why this represents only an estimate. (What else contributes to the electric field?)

  4. Noting the spacing between the lines of the electric field you have sketched, show in detail (in a separate sketch) how the charge is distributed on the conducting surfaces of one of the maps.

  5. Answer the following questions:
    1. Why must the lines of force be perpendicular to the surface of any conductor with a static charge distribution?
    2. Is it possible for equipotential lines to cross? Is it possible for lines of force to cross? Explain.
    3. For the plane-plane pattern, approximately by what percentage does the field strength differ between the center and edge of the plates along the bisector? Explain your estimate.
    4. For the Faraday ice pail pattern, comment on the field strength inside the pail. What can you say about the charge distribution on the pail?