Experiment 21

Electric Fields and Equipotential Surfaces

Objectives

To use equipotential surfaces to map electric fields and to learn to visualize electric fields.

Introduction

This experiment is intended to help the student visualize electric fields.  Additionally, it should provide a graphic demonstration of the existence of equipotential lines.  The procedure will involve mapping equipotential lines in a thin sheet of water when electrodes of various shapes are placed in it.  This arrangement involves electric currents but the electric fields found in this manner have the same pattern as fields in the static case where currents are not involved.

There are actually two circuits in this experiment:  (a) the field producing circuit made up of a power source connected to two electrodes in the water and (b) the detector circuit made up of movable electrodes attached to a galvanometer.

Procedure 1

Connect the power supply to two point electrodes and center the electrodes at opposite ends of the dish.  Connect the galvanometer to the two movable electrodes and place one in the dish at about the middle.  The other movable electrode should be used to find about 8 positions for which the galvanometer reads zero amplitude.  Record these positions.  The two movable electrodes are then on the same equipotential line (there is no voltage between them).  Repeat the above procedure for another 7 positions of the movable electrode at the side of the dish.  Be sure to record the position of the electrodes.

Procedure 2

Repeat Procedure 1 with two line electrodes.

Procedure 3

Repeat Procedure 2 with a metal ring in the center of the dish.  Check the field inside the ring as well as that outside.

Graphs

Each procedure will yield a plot of equipotential lines.  When all lines have been drawn, sketch (neatly) 9 lines which are orthogonal (perpendicular) to the equipotential lines.  These represent force (field) lines.  They should begin on one electrode and end on the other.