Group: LAB 03 Electric Fields and Potentials Names: (Principle Coordinator) (Lab Partner) (Lab Partner) Motto: Say map! Say map! Dora the Explorer Goals: Developing an intuitive picture of the electric field and equipotential lines produced by various distributions of charge Understanding the relationship between the electric field and potential Experiment: In this lab unit you will map planar electric fields produced by charged conductors (electrodes) painted with metallic ink on conductive paper The conductors are charged by applying a potential difference across them to produce a small current in the paper The field and equipotential lines between the conductors can be traced using probes connected to an voltmeter: as explained in the pre-lab reading, the potential difference is expected to be maximum along field lines and zero along equipotential lines Equipment Ancillary Equipment DC power supply, electrometer, cables Main Apparatus A Field Mapper Kit includes conductive paper with cm grid, pushpins, conductive ink pen and circular template, corkboard Three electrode patterns with various geometries will be already drawn with conductive ink on the conductive paper Setup For each electrode pattern you will: Place the pattern under study on the corkboard Connect the respective electrodes to the power supply using the pushpins To make sure that the electrodes are uniformly conductive, connect the voltmeter across the pushpin of each electrode and various places on the respective ink pattern to check out that the reading is very close to zero Measurements To measure equipotential lines: 1. Touch the ground lead of the voltmeter to an electrode 2. With the other lead, touch a point on the paper (keep the lead vertical and avoid grid points). The voltage indicated will be the potential difference relative to the electrode touched by the ground lead 3. To map the equipotential line corresponding to the respective potential move the lead on the paper trying to keep a constant reading on the voltmeter. Trace the line with a white pencil To measure electric field lines: 1. Bring the two voltmeter leads about 1 cm apart and tape them together. 2. Touch the ground lead to the paper and rotate the other about looking for the maximum potential difference. Mark the point 3. Draw an arrow with white pencil from the ground lead to the mark 4. Repeat the procedure by placing the ground lead on the mark. This way, the line can be sketched by segments Field Mapper Kit Electrodes Voltmeter probes Power Supply Conductive graphite paper 1
PATTERN 1: Parallel Plate Capacitor Use the frame below to sketch tentatively how you expect the electric field and equipotential lines to look like between the plates of the capacitor. Account for the signs of the charges on the plates. Also, do not forget to indicate the direction of the field using on the field lines 1. Take the capacitor through the steps listed on the front page to map some equipotential and field lines to confirm your inference. Use the 30-volt output of the power supply You do not have to plot the entire field. It suffices to plot 4-5 equipotential and field lines in one quadrant about the center of the capacitor and then extrapolate to the rest of the space by symmetry Indicate on the map the voltage for each equipotential line that you represent After you are done draw a 2-cm radius circle in the middle of the capacitor. Allow it to dry. This is your PATTERN 3 to be studied later. How does the field you mapped match your prediction? How can the fringing effect at the edges of the plates be reduced? Based on Example 1 in Pre-Lab 3, estimate the value of electric field in the center of the capacitor. Then use Example 2 in Pre-Lab 2 to estimate the density of electric charge on the plates. E = σ = 2
PATTERN 2: Electric Dipole Again, use the frame below to sketch your expectation about the electric field and equipotential lines between the two shown electrodes emulating a dipole + Take the dipole through the steps listed on the front page to map some equipotential and field lines to confirm your inference. Use the 30-volt output of the power supply Plot 5-6 equipotential and field lines in one quadrant and then extrapolate by symmetry Indicate on the map the voltage for each equipotential line that you represent How does the field you mapped match your prediction? How does the finite shape of the paper affect the shape of the field? Explain your reasoning. Based on your map and Example 2 in Pre-Lab 3, estimate the amount of charge on each circle 3
PATTERN 3: Conductor inside the Parallel Plate Capacitor Sketch the expected electric field and equipotential lines for this new arrangement. Note that the circle is not connected to the power source 2. Take the circle-line combination through the mapping process Represent only the 3-4 lines using a different color than on PATTERN 1 How does the field you mapped match your prediction? How does the metallic circle distort the original field inside the capacitor? Can you extrapolate the field lines inside the circle? What would happen with the field if the circle moved perpendicular on the plates? Comment on the potential of the circle. 4
CONCLUSIONS In the space below, summarize the experiment and what you have learned. Discuss sources of errors. Suggest other experiments that could be performed using the same apparatus. When complete, submit this form and your field maps neatly finished and labeled. 5