Pasco electric field mapping lab

temple university physics

Mapping the Electrostatic Potential and Electric Field

The objective of this experiment is to study the potentials, equipotential curves and electric fields produced by various two-dimensional electrostatic charge distributions. In practice, direct measurement of the electric field turns out to be quite difficult. Instead, we exploit the fact that the electric force is a conservative force, and thus can be considered to be associated with a potential – the electric potential , where the components of the electric field vector are given by the change of the electric potential in that direction,

(1)

One consequence of Equation 1 is that if one can identify a line (or surface) along which the potential has a constant value then the electric field is necessarily perpendicular to that line at all points, see Figure 1. Therefore, in order to map the electric field for a charge configuration, it is sufficient to map out the equipotential lines.

There is a technical difficulty, however, with setting up and controlling static charge distributions: it is not easy to fix charges at precise locations. For this reason we will simulate static charge distributions using a small direct current flowing through electrodes, drawn to look like our static charge distributions, and conducting paper. The electric field shapes, potential and equipotential lines will be identical to those for the simulated static charge configurations.

Figure 1. Equipotentials and electric field lines for a positive point charge (circle at center).

NOTE: You may want to reference your text or other sources to confirm your results and aid in mapping the fields accurately and expeditiously.

Learning Goals for this Laboratory:

· Practice visualizing electric fields and electric potentials around conductors of many shapes.

· Practice graphing and analyzing nonlinear relations.

· Practice connecting simple circuits.

Apparatus
Pasco field mapping board, digital voltage meter with point probes, D.C. power supply, several sheets of conducting paper with different electrode configurations, push pins

Figure 2. Setup for Part IV of this lab showing the cork board with the parallel plate electrode configuration on conducting paper, voltmeter and D.C. power supply.

Part I. Point Source and Guard Ring
Figure 3. Point source and ring guard configuration. Voltmeter probes not shown.

1. The electrodes in this experiment are made with conducting silver paint on conductive paper. Locate the conductive paper with the point source and guard ring electrode configuration (Figure 3) and pin the corners of the paper to the cork board.

2. Take a cable with a banana plug on one end and a ring terminal on the other and plug the banana end into the positive of the power supply, then pin the ring terminal end of the cable into the central point electrode using a metal pushpin as in Figure 3. Make sure there is good contact between the ring terminal and the painted electrode. Try to avoid making new holes in the electrode with the pushpin, it is sufficient to have physical contact between the terminal of the wire lead and the silver of the electrode.

3. Similarly, connect the negative of the power supply with the circle-shaped guard ring electrode.

4. Turn on the power supply and set it to 5 V by first increasing the current limit knob, then increasing the voltage to 5 V. This voltage provides a continuous source of charge to the electrodes, creating the electric field and potential we will measure.

5. Before measuring the field and potential, let’s check the electrodes for proper conductivity (a damaged electrode could skew your results).

If the electrode is a good conductor, all points on the electrode should have nearly the same potential. For our purposes, the maximum potential between any two points on a single electrode should not be more than a few mV. Use the voltmeter to probe the potential between different points along an electrode. Please do not push the voltmeter probes through the paper; simply placing the probes on the paper should give you a good reading. If you see a potential larger than a few mV between different points on an electrode, first check that the power supply wires are firmly connected to the electrode via the pushpin. If you still see a potential where there shouldn’t be one the electrodes could be damaged, so ask your lab instructor to double-check your setup and get a replacement electrode if necessary.