Mapping the Electric Field and Equipotential Lines. Multimeter Pushpins Connecting wires

Transcription

1 Circle Your Lab Day: M T W Th F Name: Lab Partner: Lab Partner: Mapping the Electric Field and Equipotential Lines. Equipment: Cork board Conductive paper DC Power supply Multimeter Pushpins Connecting wires Grease pencil Copper electrodes Theory: Coulomb s Law gives us the electric force between two point charges. There are times when it is more convenient to be able to describe this influence in terms of the source charge only. In these cases we talk about the force per unit charge produced at a point in space by the source charge. This is called the electric field of the source charges. In this experiment we wish to map out the electric fields for a two source charge configurations. However, direct measurement of the electric field would be quite difficult. Instead we exploit the fact that the electric force is a conservative force, so we can define an electric potential energy. We actually use the electric potential energy per unit charge, or just called the electric potential which is more directly related the electric field, to help map out the electric field. Components of the electric field vector are given by the rate of change of the electric potential in a given direction, One consequence of the above equations is that if one can identify a line (or surface) along which the potential has a constant value, namely an equipotential line (equipotential surface), than the electric field is necessarily perpendicular to that line at all points, see Figure 1. Therefore, determining a sufficient number of the equipotential lines for a charge configuration allows one to determine the shape of the electric field for that configuration also. Figure 1: Electric field lines and equpotentials for a positive point charge. There is a technical difficulty, however, with setting up and controlling static charge distributions. That is, it is difficult to place charges of desired magnitudes at precise desired locations. For this reason we will simulate static charge distributions. We will use a small direct current flowing through electrodes stuck on conducting paper. The electrodes are cut out from copper tape to look like our desired static charge distributions. The electric field shapes, potential and equipotential lines should be identical to the static charge configurations. UNCG P&A PHY212 L Page 1

2 Experiment Setup: The equipment used and the experimental setup is shown in Figure 2. Notice that in the figure the multimeter probes are shown lying on the conducting paper. During measurement taking you will of course be holding them so only the metal point part of the probes will touch the paper. Figure 2: Setup for the Mapping the Electric Potential and Electric Field experiment showing the multimeter, D.C. power supply, conducting paper for the dipole configuration (left) and the "Parallel Plate" configuration (right). 1. In order to keep the conductive paper stationary, mount the conductive paper on the cork-board by placing push pins at the corners. 2. After peeling off the backing, stick the copper electrodes (two circular ones for Project 1; and two 10 cm long strips for Project 2) on the conductive paper Figure 3 10 cms apart. 3. Connect the electrodes to the DC power supply using the wires with push pin. Make sure that the wire makes good contact with the electrode. See Figure Connect the other end of the wires to the power supply. 5. Switch on the power supply only after all the connections are done. Adjust the voltage knob on the power supply so that the input voltage is 10 V. 6. Check the electrodes for proper conductivity (a damaged electrode could skew your results). Connect one voltmeter lead near a push pin on an electrode. Touch the multimeter s second lead to other points on the same electrode. The maximum potential between any two points on the same electrode should not exceed 1% of the potential applied between the two electrodes. That is, if potential difference between the terminals of the power supply is 10V than the reading on the volt meter should be between -0.1V and +0.1V. Should the difference in voltage between any two points on the same electrode exceed 1% consult with your lab instructor. UNCG P&A PHY212 L Page 2

3 Procedure: Project 1: Electric Dipole opposite charges. 1. Place the reference probe (black lead of multimeter) midway between the two electrodes, along an imaginary line that connects the two electrodes. This should be the point where the electrostatic potential is 0 V. 2. Hold the black lead at 0 V position. With the red lead, locate five or more points where potential is 1 V. As you locate each point, press the tip of the lead into the sheet to leave an indentation. After finding the five points, "connect the dots" using the grease pencil. Label the equipotential line as 1 V. 3. Repeat procedure 2 for at least five other equipotential lines. Q. Does your trace of equipotential line and field lines make sense? Q. Looking at the equipotential lines can you deduce how the electric field due to a dipole would look like? Project 2: Parallel Plates. 1. Draw a thin line from one plate to the other, connecting the midpoints of the two plates. 2. Place the black multimeter lead on the negative electrode. This is your reference probe. 3. Measure the potential difference between your reference probe and the red multimeter probe every 1 cm along your line, starting at 1 cm. Tabulate your measurements. x (cm) V (volts) UNCG P&A PHY212 L Page 3

4 4. Make a graph of the potential along your line as a function of distance from one plate (the reference probe is at x = 0.0 cm). You can use MS Excel, LoggerPro or any other graphing software you wish to use. Every member of the group should do this independently. Turn in your graph with the lab report next week. Q1. Describe in words how the potential varies with distance on your graph. Q2. Determine a formula that describes how potential varies between your plates as a function of position. Hint, Φ = kx. What is k for your plates? What is its physical significance? 5. Using the procedure described in Project 1 2, draw three equipotential lines for the parallel plate configuration. UNCG P&A PHY212 L Page 4

5 HOMEWORK H1. How do your sketches compare to the corresponding figures in your textbook? A. Point charge (dipole) configuration B. Parallel-plate configuration H2. N/C and V/m can both be used as units for electric field. Show that they are equivalent units. UNCG P&A PHY212 L Page 5