Purpose: To draw lines of equal electrostatic potential and electric field lines for various charge configurations ( a sharp corner and a point charge near a straight line).

Discussion: The electrostatic potential difference is defined as DV = DU/q, where q is a "test charge". In this experiment, the battery establishes a fixed potential difference between two fixed objects, the "positive" and "negative" electrodes, by setting up a charge distribution on them. By connecting the battery to the different parts of the "objects" on the conductive paper, you can establish a charge distribution on these objects. The voltmeter can measure the potential difference between any two points on the conductive paper.

Curves along which the potential is constant, are known as "equipotential lines". The electric field is always perpendicular to the equipotential lines. In a region where the potential is changing, there must be an electric field given by E = -DV/Dx. The negative sign indicates that the electric field points in the direction of decreasing potential.

Procedure:

1. Connect the positive and negative terminals of the battery to the electrodes on the black paper with a point charge and a line. Connect the positive terminal to the point charge, and the negative terminal to the line. Measure the potential difference between the electrodes. Note that this potential does not depend on where you touch each electrode. This is due to the fact that the electrodes are "conductors".

2. By keeping one voltmeter lead on the negative electrode and placing the other lead at various points on the black paper, find lines of equal potential difference. Transfer these points onto a drawing on a blank sheet of paper.

3. From the shape of the equipotential lines, make a qualitative sketch of the electric field lines. Compare this to the electric field lines of a positive charge near a negative charge (Figure 15.13a, page 496 in your textbook). Sometimes it is said that the line connected to the negative battery terminal creates a negative "image charge" as if it where a mirror. This is not really true, but you should be able to see why it is a useful idea. Can you see this?

4. Repeat steps 1 and 2 for the sharp corner near a line. Connect the sharp corner to the positive battery terminal, and the line to the negative terminal.

5. Notice that the equipotential lines are closer together near the corner than anywhere else. The electric field must be largest there! Using your drawing as a guide, can you see why lighting strikes sharp objects such as lightning rods and people? This is discussed in pages 498-499 of your text.