Mapping of Equipotential Lines and Electric FieldsObject: To map the equipotential lines and the lines of force in an electric field produced by two oppositely charged point charges and by two oppositely charged lines of charge.

Reference: HRW Ch. 22 & Ch. 24

Apparatus: 1. Conducting paper and mounting board. 2. 1.5 volt battery. 3. Four Electrodes. 4. 1 galvanometer. 5. Graph paper.

Introduction:

In this experiment, you are going to map the electrical territory between two opposite electrical charges. First, connect the battery to the terminals on the board. Use the conducting paper with the two point charges. Connect the two probes to the galvanometer. Put the fixed probe (the one with a stand) somewhere between the charges. Touch the end of the hand-held probe to the paper at a few places (figure 1). Notice that the needle of the galvanometer usually deflects one way or the other. The galvanometer detects the electrical current flowing between the points touched by the probes. When there is a current flow between the points, the points- are said to be at different electrical potentials. Move the hand-held probe around until there is no deflection of the needle. In this case, no current flows between the points and they are said to be at the same electrical potential.

Fixed Probe

Hand-heldProbe

We have not yet defined what we mean by this term "electrical potential". For the moment, we will just consider it to be a name for some condition that determines if an electrical current will flow between the points. Think of it as something like the gravitational potential associated with height: water

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will flow from a higher level to a lower level but will not flow on its own between two points on the same level.

Move the hand-held probe to another point where there is no deflection. This point would also be at the same potential as the fixed point. In fact, there are many points that you could find that are all at the same potential as the fixed point. If all these points were found and connected with a line, this would be called an equipotential line. In this experiment,, you are going to find and plot seven to ten equipotential lines in the space between the charges.

Procedure:

1. Leaving the fixed probe in its original position, find about 8 more points that are at the same potential. Keep a record of these points by plotting them directly on a piece of graph paper. [Notice that there is a grid and coordinates on the black conducting paper. This is to aid you in transferring the points to the graph paper]. Connect these points with a smooth line. This is your first equipotential line.

2. Move the fixed probe to another point a centimeter or two from the first point. Use the hand-held probe to again find equipotential points and an equipotential line. Repeat this procedure to find a total of seven to ten equipotential lines spaced between the point charges: You must first develop a plan for systematically moving the probes. Develop the plan, collect the data, and draw your map.

3. Repeat the procedure of 1 and 2 using the conducting paper with the two lines of charge and draw another map.

4. Once you have these maps of the equipotential fines, you are going to use them to find the lines of force or electric field lines. These lines of force are everywhere perpendicular to the equipotential lines. Before you try to draw them, read sections 20-4 in Walker.

The Laboratory Report:

A report should be turned in for this experiment. At the end of the report include:

1. ABSTRACT: One paragraph summary of the experiment including goals, procedure, results. The purpose of this is to capture your audience.

2. INTRODUCTION: An more detailed introduction explaining the purpose of the exercise and including brief definitions of

a. equipotential lines. b. electric lines of force. c. how a and b are related.

3. EXPERIMENTAL: An explanation of how the equipotential lines and lines of force were determined in this experiment.

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4. RESULTS & DISCUSSION: Your plots and any discussion of the results you feel is needed to adequately explain what the plots represent and what you find from them.

5. CONCLUSIONS: Wrap it up! Explain what you learned from this lab and what you got out of it.

Note: The lab report needs to be turned in at the beginning of lab next week. There are not any calculations, so what I will primarily be looking for, and grading, in this report is your presentation. Make sure you turn in something you are proud of...your good name is attached to it! Make it worth it!!

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