Mapping Electric Field and Equipotential Lines
Introduction:
An electric field is a region in which forces of electrical origin are exerted on any electric charges that be present. If a force, F, acts on a charge, q, at some particular point in the field, the electric field strength, E, at that point is defined as
E = F / q.
Since E is a vector quantity, it also has direction, and we define the direction of the electric field as the direction of the force on a positive test charge placed at the point in the field.
Faraday introduced the concept of lines of force as an aid in visualizing the magnitude and direction of an electric field. A line of force is defined as the path traversed by a free test charge as it moves from one point to another in the field. The relative magnitude of the field intensity is indicated by the spacing of the lines of force.
Procedure:
1. Mount the electrode sheet to the corkboard with tacks or push pins. Place a long pin into each of the two electrodes (painted with silver conducting paint). Turn on the multimeter and set it to measure on the 20 volt scale for DC voltage. Measure the voltage across the lantern battery by touching the negative probe to the negative terminal and the positive probe to the positive terminal. Record this value below:
Voltage across battery = ______V
2. Connect the negative terminal of the lantern battery to one of the long pins. Connect the positive terminal of the battery to a switch and then the switch to the other electrode. Close the switch and touch the positive probe from the digital multimeter to one electrode and the negative probe to the other electrode. Record the value below. If this value is not within 1% of the value measure in number 1 check your connections again.
Voltage across electrodes = ______V
3. Take the negative probe from the digital multimeter and place it into the electrode paper between the two electrodes and near the edge of the paper so it is out of the way.
4. Take out the white paper copy of the electrode paper. This is where you will be taking data. Do NOT write or mark on the black electrode paper.
5. Use the positive probe from the digital multimeter and touch it to the black electrode paper. Find a convient value for the potential (such as 1.0 V or 1.5 V). Mark this location on your white paper with a +. Record the value of the voltage at this point above the +. Move the probe around the sheet and locate a series of points which provide the same voltmeter reading. Mark these points and then connect them with a smooth line. This line defines an equipotential line.
6. Repeat step 5 for another equipotential line. Mark the white paper with a different colored pencil. The new starting point should be between 1 and 2 cm from the previous line. Continue this process until the entire conducting sheet is mapped. This may take a while.
7. Remembering that the lines of force are perpendicular to the equipotential lines, draw in the electric field lines with arrows. Use a blue pen to draw the electric field.
Observations / Questions:
Use PhotoBooth to take pictures of your work – both collecting data and the final result.
1. Are some lines of force closer together? What about equipotential lines? What does this mean for each?
2. Place both the positive and negative probe at different places along the same line of force where the electric field lines are widely spaced apart. Move the positive probe away from the negative probe slowly. Move the positive probe away in the other direction. Do the same thing where the electric field lines are narrowly spaced. Record your observations.
3. Write a paragraph or two describing the method you used to collect data. Why were you able to use this method? Include pictures to describe your work.