2
ãTheodore Gotis
Oakton Community College
I. Introduction and Objectives
The goal of this lab is to qualitatively determine the electric field produced by a parallel plate capacitor and other configurations.
II. Equipment Needed
Conductive Ink Pen Cork Board
Carbon Conducting Paper Power Supply
2 Wires Connecting Power Supply to Electrodes Push Pins
White Paper Copy of Conducting Paper
Digital MultiMeter with test leads
III. Theory
If you carry a bucket of water from the ground to the top of a cliff you are increasing the potential energy of the water. What does this mean? Because we live within the gravitational pull of the earth there is the potential for you to drop the bucket and the water to fall down the side of the cliff. The farther the water falls, the faster it will be traveling when it lands and therefore the more energy it will have. You “build up” this energy lifting the bucket further and further from the ground.
There is an analogous situation when you have a build up of electric charge. It is a bit more complicated because in electrostatics we have charges that repel as well as attract. If a proton (positive charge) and an electron (negative charge) pulled farther and farther apart, there is an increase in potential energy. This increase results from the attraction between the electron and proton, much like the attraction between you and the earth.
IV. Experimental Procedure: “Parallel Plate Capacitor”
Set Up
1) Place the points of the push pins through the circles at the end of your wires leads, then insert one push pin at the end of each electrode (the silver lines) on your conducting paper. If you do not have a new conducting sheet, be sure to push the pin into the same hole used by previous students.
2) Connect the other ends of the leads to a power supply, then connect a DMM (Digital Multimeter) to the power supply to read the voltage.
3) Turn on the power supply and set the voltage of the power supply to 10V dc current.
Figure 1: Connecting Wire to Electrode
(Image taken from Pasco Manual)
4) Connect the test leads to a second DMM and set the meter to dc volts. Now you are ready to measure the potential difference between points on the conducting paper.
Check Electrode Conductivity
5) Place one voltmeter test lead near the push pin of an electrode. Place the voltmeter’s second test lead to another point along the same electrode. Record this potential difference.
6) Now place the second test lead near the push pin of the second electrode. Record this potential difference.
7) If the electrode is conducting properly, the maximum potential between any two points on the same electrode will not exceed 1% of the potential applied between the two electrodes. (Divide the number you recorded in step #7 by the number in step #8, if the result is less than 1% you are ready to go.)
8) If the voltage across the same electrode is greater than 1% of the voltage applied between the two electrodes, then remove the paper from the corkboard, return it to your instructor, and take another conducting sheet.
Figure 2: Measuring Equipotential Lines
Taking Measurements
9) You have been provided with a white sheet of paper with a grid identical to the grid on your conducting paper. All your measurements are to be recorded on this white paper, NOT on the conducting paper.
10) Equipotentials are plotted by connecting one test lead of the voltmeter (the ground) to one of the electrode push pins. This electrode becomes the reference lead. Try this now. Notice that as you move the test lead away from the reference lead the potential difference indicated on your voltmeter becomes larger. Is this what you would expect?
11) To map an equipotential, move the test lead until the desired potential is indicated on the voltmeter. For example, with the reference lead on one electrode, move the test lead until the voltmeter reads 6V. Mark this position on your white paper grid. Now move the test lead, but only in the direction that keeps the voltmeter at a reading of 6V. Continue to mark these points every centimeter or so.
Note: DO NOT attempt to make measurements by placing the leads on the grid marks on the conducting paper. Touch the voltmeter leads only on the solid black areas of the paper.
12) When you are done marking all the points along an equipotential, use a pencil to connect the points.
13) Repeat steps #10 to 11 for at least 3 more equipotential lines (four lines total when you are done).
V. Experimental Procedure: “Unknown Figure”
Set Up
1) Place the points of the push pins through the circles at the end of your wires leads, then insert one push pin at the end of each electrode (the silver lines) on your conducting paper. If you do not have a new conducting sheet, be sure to push the pin into the same hole used by previous students.
2) Connect the other ends of the leads to a power supply, then connect a DMM (Digital Multimeter) to the power supply to read the voltage.
3) Turn on the power supply and set the voltage of the power supply to 10V dc current.
4) Connect the test leads to a second DMM and set the meter to dc volts. Now you are ready to measure the potential difference between points on the conducting paper.
Check Electrode Conductivity
5) Place one voltmeter test lead near the push pin of an electrode. Place the voltmeter’s second test lead to another point along the same electrode. Record this potential difference.
6) Now place the second test lead near the push pin of the second electrode. Record this potential difference.
7) If the electrode is conducting properly, the maximum potential between any two points on the same electrode will not exceed 1% of the potential applied between the two electrodes. (Divide the number you recorded in step #7 by the number in step #8, if the result is less than 1% you are ready to go.)
8) If the voltage across the same electrode is greater than 1% of the voltage applied between the two electrodes, then remove the paper from the corkboard, return it to your instructor, and take another conducting sheet.
Unknown Figure Electric Field Lines
A diagram of the unknown figure you will be using in the lab is drawn below. Draw a prediction for the shape and direction of the electric field lines on the diagram below. Then proceed to measure the equipotential lines and see if you measurerements correspond to your prediction.
Taking Measurements
9) You have been provided with a white sheet of paper with a grid identical to the grid on your conducting paper. All your measurements are to be recorded on this white paper, NOT on the conducting paper.
10) Equipotentials are plotted by connecting one test lead of the voltmeter (the ground) to one of the electrode push pins. This electrode becomes the reference lead. Try this now. Notice that as you move the test lead away from the reference lead the potential difference indicated on your voltmeter becomes larger. Is this what you would expect?
11) To map an equipotential, move the test lead until the desired potential is indicated on the voltmeter. For example, with the reference lead on one electrode, move the test lead until the voltmeter reads 6V. Mark this position on your white paper grid. Now move the test lead, but only in the direction that keeps the voltmeter at a reading of 6V. Continue to mark these points every centimeter or so.
Note: DO NOT attempt to make measurements by placing the leads on the grid marks on the conducting paper. Touch the voltmeter leads only on the solid black areas of the paper.
12) When you are done marking all the points along an equipotential, use a pencil to connect the points.
13) Repeat steps #10 to 11 for at least 3 more equipotential lines.
VI. Analysis of Data
Now you can sketch the electric field from the equipotential lines you found for the “parallel plate capacitor” and the “unknown figure.” Remember the electric field lines run perpendicular to the equipotential lines. Draw a set of lines (5 or more) that run perpendicular to the equipotential lines. Be sure to use a different colored pen or pencil than the one you used for the equipotential lines.
VII. Questions
1) Are the electric field lines you drew what you would expect for a parallel plate capacitor and for the unknown figure? Explain.
2) What errors might have created field lines that were distorted or innacurate?