Exp. #2-3 : Measurement of Equipotential Lines by Using Conducting Plates

Transcription

1 PAGE 1/11 Exp. #2-3 : Measurement of Equipotential Lines by Using Conducting Plates Student ID Major Name Team # Experiment Lecturer Student's Mentioned Items Experiment Class Date Submission Time Submission Place Introductory Physics Office Report Box # Students should write down Student s Mentioned Items at the cover page of Experiment Reports, and then complete Experiment Reports by adding contents to the attached papers (if needed) in terms of the following sections. Contents of the reports should be written by hand, not by a word processor. Instead, it is allowed that figures and tables are copied and attached to papers. Completed Experiment Reports should be submitted to the place due to the time specified by Experiment Lecturers. The Experiment Report score per each Experiment Class is evaluated by max. 50 points (basically 15 points). Solutions of Problems in Experiment Reports are not announced to the public according to the General Physics Laboratory - Administration Rule. If a student permits other students to pirate one s Experiment Reports or a student pirates Experiment Reports of other students regardless of permission of original creators, the corresponding Experiment Report score and Active Participation score will be zero in case of exposure of such situation. Unless Experiment Reports are submitted to the place due to the time specified by Experiment Lecturers, the corresponding Experiment Report score will be zero. If the submission rate of Experiment Reports is less than or equal to two thirds, the grade of General Physics Laboratory will be F level. In order to decide grades of General Physics Laboratory at the end of current semester, the detailed scores of General Physics Laboratory will be announced at Introductory Physics Office homepage. Based on the announcement, students can raise opposition of score error. Since the public evidence is needed for the confirmation of opposition, students should keep one s Experiment Reports completed evaluation by Experiment Lecturers until the Experiment Report score decision If a student is absent from the Experiment Class because of proper causes, the corresponding student should submit documents related to absence causes to Introductory Physics Office regardless of cause occurrence time until the grade decision of General Physics Laboratory. If a student moves the Experiment Class arbitrarily without permission of Introductory Physics Office, it is noted that the total Experiment Scores will be zero. Lecturer's Mentioned Items Submission Time/Place Check Experiment Report Points Evaluation Completion Sign 50

2 PAGE 2/11 1. Objective Student ID Name Equipotential lines are measured on conducting plates, and the properties of electric field lines and equipotential lines are understood. 2. Theory (1) Electric field and electric field lines An electric field always exists between source electrodes that have a potential difference. If an object with a charge is located at a certain point, the electric field will exert an electric force. The electric field at this point is defined by the electric force per unit charge as follows: (Eq. 1) then the relation between the electric field and the electric potential will be given by or. (Eq. 3) Here, is the unit vector normal to equipotential lines and it implies that electric field lines are perpendicular to equipotential lines. The electric field and the electric force have the same or opposite directions depending on the charge so that the electric field is parallel to the electric force. If the directions of the electric field at sufficiently many points are represented by arrows, the arrows can be connected to form electric field lines. Then the magnitude of the electric field will be indicated by the density of the electric field lines. The electric field is stronger where the electric field lines are close together and weaker where they are far apart. (2) Electric potential and equipotential lines If the electric potential energy of an object with a charge at a certain point is given by, then the electric potential at this point is defined by the electric potential energy per unit charge as follows: (Eq. 2) In an electric field, there are points that have the same electric potential. If these points are connected, equipotential lines in two-dimensional plane (or equipotential surfaces in three-dimensional space) can be obtained. Fig. 1. Rough forms of electric field lines (dashed lines) and equipotential lines (solid lines) in two-dimensional plane. (3) Relation between electric field lines and equipotential lines In an electric field, there are infinite numbers of electric field lines and equipotential lines, but we can only draw a representative sample of them. If a few electric field lines and equipotential lines are drawn for an isolated point charge, the electric field lines will be in a radial direction and equipotential lines (or equipotential surfaces) will be in the form of circles (or spherical surfaces) as shown in Fig. 1(a). When a positive point charge and a negative point charge are placed together, the electric field lines and equipotential lines can be roughly drawn as shown in Fig. 1(b). Since the electric potential energy at all the points in one equipotential line is constant, the work needed to move charges along the equipotential lines is zero. This means that the equipotential lines are perpendicular to electric field lines. Since work done by the electric field is positive when a positive charge moves from a high electric potential position to a low electric potential position, the electric field lines are defined to point to the lower electric potential. Therefore, the direction of the electric field will be the direction resulting in the most rapid decrease of the electric potential. If the differential displacement in the direction resulting in the most rapid decrease of the electric potential is,

3 PAGE 3/11 3. Experimental Instruments Items Quantity Usage Clean up method Conducting plate 3 ea. They are used to measure equipotential lines for various source electrode configurations. They should be fixed on the cork plate. Fixed test electrode 1 ea. It is used as a fixed test electrode. Mobile test electrode 1 ea. It is used as a mobile test electrode. Galvanometer 1 ea. It is used to search points with the same electric potential. It should be placed inside the basket of the experiment table. It should be placed inside the basket of the experiment table. It should be placed inside the basket of the experiment table. Power supply -to-wall power connection cable 1 ea. It is used to connect the power supply to the wall power. It should be placed inside the basket of the experiment table. Power supply 1 ea. It is used to provide an electric potential on the conducting plate. It should be placed at the center of the experiment table.

4 PAGE 4/11 < How to Use the Power Supply > [7] In order to prevent an electricity accident, note that one's body must not be in contact with connection parts of the experimental instrument-to-power supply connection cables during the measurement. If the electric circuit becomes open during the measurement, the current will suddenly become zero and the current control indication lamp will turn off. In this case, after rotating all voltage and current adjust knobs to the minimum, turn off the power supply and correct the electric circuit in the same manner of [3]. [1] After confirming that the power supply is off, use the power supply-to-power connection cable to connect the power supply to the wall power and keep the power supply off. Use the experimental instrument-to-power supply connection cables to connect the experimental instrument to and terminals of the power supply. Without special condition, do not connect the ground terminal (GND or COM) of the power supply. [8] After the experiment is finished, turn off the power supply in the way opposite to turning on the power supply. That is, after rotating the current adjust knob to the minimum, rotate the voltage adjust knob to the minimum and turn off the power supply. Note that you must turn off the power supply when all voltage and current adjust knobs are at the minimum, or else a severe electricity accident may be happened. After turning off the power supply, clean up the experimental instruments according to the suggested method. [2] After confirming that all voltage and current adjust knobs of the power supply are at the minimum, turn on the power supply. In some models of the power supply, the power lamp or the voltage control indication lamp (CV) will turn on when turning the power supply on. [3] Rotate the voltage adjust knob (VOLTAGE - COARSE) slowly to increase the voltage. When approaching to a sufficiently high voltage for a normal electric circuit, the current control indication lamp (CC) will turn on. But for an open circuit, no matter how high voltage is, the current will remain zero and the current control indication lamp will be off. In this case, after rotating all voltage and current adjust knobs to the minimum, turn off the power supply and correct the electric circuit. The inspection of the electric circuit must be done only after the power supply is off. [4] In specific experiments that need a current supply, rotate the current adjust knob (CURRENT) slowly to set the desired current while checking if the current control indication lamp is on. [5] Even in properly functional electric circuits, when rotating the current adjust knob to increase the current, it can be observed that the current control indication lamp will suddenly turn off and the nonzero current does not increase any longer. This is caused by an insufficient voltage, which can be solved by rotating the voltage adjust knob more to increase the voltage. After the current control indication lamp turns back on, the current can be increased by rotating the current adjust knob. [6] Note that all voltage and current adjust knobs should be rotated slowly, or else the abrupt change of the voltage or the current may cause an electricity accident. Do not rotate the voltage fine adjust knob (VOLTAGE FINE) unless it is absolutely needed, and keep it at the minimum.

5 PAGE 5/11 4. Experimental Procedures (0) Setting before the experiment 1) After confirming that the power supply is off, use a power supply-to-wall power connection cable to connect the power supply to the wall power and keep the power supply off. Connect source electrodes to the power supply. The fixed test electrode should be connected to terminal of the galvanometer and the mobile test electrode should be connected to terminal of the galvanometer. 5) By using a proper program, draw the graph representing the equipotential lines and source electrodes. (1) Source electrode configuration consisting of small circular electrodes Measure the equipotential lines for the source electrode configuration consisting of small circular electrodes. Note that the location and size of source electrodes should be recorded. 2) After confirming that all the voltage and current adjust knobs of the power supply are set to the minimum, turn the power supply on. Rotate the voltage adjust knob slowly to set the voltage to the proper value, and keep the current adjust knob in the minimum. 3) Place the fixed test electrode at the center O of the conducting plate. After placing the mobile test electrode in the upper region of the fixed test electrode, search for 4 points where the scale of galvanometer becomes zero and write down the coordinates of points. In the same manner, after placing the mobile test electrode in the lower region of the fixed test electrode, search for 4 points where the scale of galvanometer becomes zero and write down the coordinates of points. An equipotential line can be measured by performing these procedures. (2) Source electrode configuration consisting of bar electrodes Measure the equipotential lines for the source electrode configuration consisting of bar electrodes. Note that the location and size of source electrodes should be recorded. 4) Place the fixed test electrode at points R1, R2, R3, L1, L2, L3 and repeat the procedure done in step 3). By doing so, the equipotential lines passing through each point respectively will be measured. Answer the following questions. 1. Bar electrodes with infinite length can be treated as the part of planar electrodes with infinite area. Show that the uniform electric field is produced between planar electrodes with infinite area.

6 PAGE 6/11 (3) Source electrode configuration consisting of small circular electrodes and a ring-shaped conductor 1) Measure the equipotential lines for the source electrode configuration consisting of small circular electrodes and a ring-shaped conductor. Note that the location and size of source electrodes should be recorded. 5) If all the measurements are finished, confirm that the all the voltage and current adjust knobs of the power supply are set to the minimum and turn off the power supply. Finally, clean up the experimental instruments according to the suggested method. 2) Place the fixed test electrode in the center O of the conducting plate and investigate various points inside the ring-shaped conductor to confirm that the interior of the ring-shaped conductor is equipotential. Then find and record the coordinates of points that have the same potential as the interior of the ring-shaped conductor. Answer the following questions. 2. Show that the interior of the ring-shaped conductor is equipotential in terms of Gauss' law, charge distribution, etc. By using this phenomena, explain why the wireless phone does not work inside the elevator. 3) Place the fixed test electrode at points R1 and L1 which are above the ring-shaped conductor and scan for points near the ring-shaped conductor. 4) Place the fixed test electrode at points R2, R3, L2, L3 and measure the equipotential lines which are between small circular electrodes and the ring-shaped conductor.

7 PAGE 7/11 5. Experimental Values (1) Source electrode configuration consisting of small circular electrodes Coordinates of the center Radius Location of source electrodes electrode electrode Coordinates of the points on equipotential lines L3 L2 L1 O R1 R2 R3 Upper #4 Upper #3 Upper #2 Upper #1 Fixed test electrode Lower #1 Lower #2 Lower #3 Lower #4 Perform the measurement in the way that points are located farer away from the fixed test electrode as the upper or lower # increases.

8 PAGE 8/11 (2) Source electrode configuration consisting of bar electrodes Coordinates of the left corner at the top Coordinates of the right corner at the bottom Location of source electrodes electrode electrode Coordinates of the points on equipotential lines L3 L2 L1 O R1 R2 R3 Upper #4 Upper #3 Upper #2 Upper #1 Fixed test electrode Lower #1 Lower #2 Lower #3 Lower #4 Perform the measurement in the way that points are located farer away from the fixed test electrode as the upper or lower # increases.

9 PAGE 9/11 (3) Source electrode configuration consisting of small circular electrodes and a ring-shaped conductor Coordinates of the center Radius Location of source electrodes electrode electrode Ring-shaped conductor Coordinates of the points on equipotential lines L3 L2 L1 O R1 R2 R3 Upper #4 Upper #3 Upper #2 Upper #1 Fixed test electrode Lower #1 Lower #2 Lower #3 Lower #4 Perform the measurement in the way that points are located farer away from the fixed test electrode as the upper or lower # increases.

10 PAGE 10/11 6. Results and Discussions (This page should be used as the first page of the corresponding section. If the contents exceed this page, additional contents should be written by attaching papers. Contents should be written by hand, and not by a word processor. Attaching copied figures and tables to the report is allowed.) Write down contents in terms of the following key points. 1. Explain the form of equipotential lines and electric field lines for the source electrode configuration consisting of small circular electrodes. 2. Explain the form of equipotential lines and electric field lines for the source electrode configuration consisting of bar electrodes. 3. Explain the form of equipotential lines and electric field lines for the source electrode configuration consisting of small circular electrodes and a ring-shaped conductor.

11 PAGE 11/11 7. Solution of Problems (This page should be used as the first page of the corresponding section. If the contents exceed this page, additional contents should be written by attaching papers. Contents should be written by hand, and not by a word processor. Attaching copied figures and tables to the report is allowed.) 8. Reference