Plotting equipotential lines of electric

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Plot the equipotential lines patterns of electric fields set up by electrodes of different shapes.



The experimental results were get by setting up the apparatus as shown below first.

– A paper was placed under the conducting plate.

– Three electrodes were dipped into the conducting plate

– One of the electrodes and a flying probe were connected to a galvanometer

– A 3V battery box was connect to the other two terminals of the equipotential plotting kit.

Then, the flying probe was move on the plate slowly until at a point where the reading of the galvanometer was zero. The position of the point was marked. With several marked points, the equipotential lines could be plotted by drawing a line through all these point.

Here are the 3 sets of equipotential lines by using electrodes of different shapes.

A. Two points

B. Point and line

C. Two parallel lines


1. The role of the conducting plate

The conducting plate has nearly zero resistance. When the plate is connected to the power supply by using electrodes, the plate then conducts electricity and has electric field. It provides a surface for us to test the potential difference within the plate by using flying probe. So the equipotential line could be found easily.

2. Comparing the equipotential line patterns with the electric field patterns in Part A of Experiment C3

Experiment C3 is sprinkling semolina powder onto the non-conducting oil in an electric field. The powders then form lines, which are electric field lines.

The direction shown on an electric field line is the direction of the force acting on a free positive test charge. Therefore, electric field lines are directed from the positive charge to the negative charge.

In this experiment, the lines potted are equipotential lines for two oppositely electrode. No work is done when a charge is moved from one point to another point of an equipotential line. And equipotential lines are always perpendicular to electric field lines.

3. Decreasing the applied voltage by half or increasing it by double

If the applied voltage is decreased by half or increased by double the value, the patterns of the equipotential line patterns drawn would not change. It is because the equipotential line pattern is independent of the magnitude of the potential difference but only dependent of the shapes of electrodes, so the equipotential line patterns drawn would not change.

4. The precautions of the experiment

The conducting plate is fragile. Moving on the plate, the flying probe may damage the conducting plate. And if the three fixed electrodes locked too tight on the plate, they may wear the plate out. If the plate is damaged, its function may be affected and the point marked may be inaccurate and hence the experimental results. Therefore the conducting plate must be well protected to prevent wearing out. Don’t lock the electrodes too tight. Don’t scrape the conducting plate with the flying probe or anything else.

5. The sources of error and improvement

In the experiment, there were some sources of error, which made the equipotential lines not correct actually

1) Poor apparatus

The pen is too bold. The points marked are pale and easy to be removed. The line drawn through all the points were then inaccurate.

The linear electrodes are too short and not straight. So the equipotential lines plotted may not be equal to we expected.

For result set C – two parallel line, the length of two linear electrodes were different. So it is different from the set up in lab menu.

The conducting plate had been already worn out. The results become inaccurate.

To fix the problems, apparatus with better quality should be used instead. And the conducting plate should be well protected to prevent wearing out.

2) Poor set up

It is difficult to set up the apparatus as same as those in lab menu. For example, in set C, the two linear electrodes were not parallel. As a result, the lines drawn contain error.

The only way to solve the problem is setting up the apparatus more carefully.

3) Human Error

The scales on the galvanometers were observed by the human naked eyes. The records were inaccurate. Using a hand lens when reading, Reading the scale when the pointer is directly on top of its image in the mirror behind was important to lower the error. Apart from these, the scale was read by several people repeatedly could help the accuracy too.


Through the experiment, The equipotential line patterns of electric fields set up by electrodes of (A) two point, (B) point and line, (C) Two parallel lines were found. And the equipotential lines are always perpendicular to electric field lines.

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