PHYSICS 125 LAB 2: ELECTRIC FIELDS AND EQUIPOTENTIALS
Goal:
This lab will investigate some of the properties of electric fields and their relation to the electric potential, by mapping out some equipotential lines for two or three configurations of conducting surfaces. You should learn to distinguish between the lines of force of an electric field and the equipotential lines corresponding to that field.
Background reading:
Review sections 15.4 (electric field) and 16.2 (equipotential surfaces) in your textbook.
Equipment needed:
Cork board (at least 9” x 12”)
Probe pins (T-shaped or “mallet head”, at least 3) and Push pins (at least 4)
Conducting sheets (prepared prior to class, two or three configurations of silver paint conductors)
Connecting wires (with banana plugs, on wall rack) and alligator clips (at least 5)
Digital multimeter (Fluke D810 or equivalent)
Blank sheets of paper (8.5” x 11”) and a ruler or straightedge
Battery (1.5 V)
Experimental Procedure:
Several sheets of conductive paper (usually black or dark-colored) have been prepared before the lab, each with a small number of “conductors” which are merely drawn on the surface of the paper with a conducting paint (this is a colloidal suspension of silver metal particles). Prepare a stack of several sheets of white paper (one for each member of your group) with a sheet of the conductive paper on top, with the silver-painted figures facing upwards. Use four pushpins to pin the paper onto the cork board. These silver-painted “conductors” will now be connected to a 1.5 V battery using the T-shaped pins. Push one pin through each of the “conductors” so that it is in contact with the paint (you have to push through the painted region). Then use an alligator clip, pushed over the banana plug on one end of the connecting wires, to connect one connecting wire to each of the two T-shaped pins. A wire from one of the conductors will go to the negative terminal of the battery (you might need another alligator clip on that end to make the connection) and the other wire should go to the positive terminal of the battery. With the battery connected like this, a small current will flow around the circuit, and the conductive paper will have a distribution of potential difference across its surface which is very similar to the electric potential that might appear in empty space between two conductors which have opposite electric charges on them.
To measure the electric potential we will use the digital multimeter, which measures the electric potential between two wires connected to the “COMMON” and the “V” inputs. Use a wire with banana plug ends to connect the “common” input of the multimeter to the negative terminal of the battery (it is customary to use a black wire for the negative or common connection). Then use another wire (customarily a red wire) to connect the “V” input of the multimeter to a T-shaped pin, which we will use to measure the potential. This pin will be used as a “probe” to test the potential at various points on the conductive sheet. For example, you might touch the pin to the silver-paint “conductor” that is connected to the negative battery terminal. You should see a voltage reading on the multimeter very close to zero (the push buttons on the multimeter should be set for DC, Voltage, and 2 V range). When you probe the voltage on the conductive sheet, you should avoid touching the sheet with your hand (especially if you have damp hands) since this will alter the electric potential, and you may need to push into the paper slightly to make a good electrical contact and get a stable multimeter reading.
Choose several potentials (also referred to as “voltages”) such as 0.3 V, 0.6 V, 0.9 V, and 1.2 V, and then find several points on the surface that result in this reading when the pin is used to probe those points. You will find that the points that have the same potential will be along a curve on the surface, which is called an “equipotential line”. (In three dimensions, this equipotential is actually a surface, not a line or curve). To record the positions of the points along an equipotential curve, simply push the pin through the paper, so that the positions of the holes allow you to draw the equipotential curve, after taking the stack off the cork board. Don’t forget to mark the position of the “conductors” by pushing the pins through the boundaries of the “conductors” so that you have a record of their shape and locations. After you find several of the equipotentials, dismantle your stack, give a white sheet to each lab partner, and use pens or pencils to draw the equipotential curves. Disconnect the battery while changing papers to conserve the battery charge. Repeat this procedure for each of the conductive sheets that you have. Typically, we will have one conductive sheet with two small circles, which gives equipotential curves similar to those of two (oppositely-charged) point charges: the dipole. We also have a sheet with two parallel line segments, which gives equipotential curves similar to those of a parallel-plate capacitor. Another, optional, sheet might have two parallel lines, with a small circle between to illustrate what happens when there is a spherical conductor in between two parallel plates. Or it might have a straight line and a circle.
Each member of the group should mark up their sheets to indicate the potential differences that were used to define the equipotential curves. Indicate the locations and polarity (positive or negative) of the “conductors” that were connected to the battery.
The electric field vector will point in the direction that a positive “test charge” would be forced by the electric field. Pick several points in between the “conductors” on your drawing and indicate the direction of the electric field at each point. This is a good opportunity to discuss these ideas with your lab partners or with the instructor.
When finished with this lab, turn in your sheets (with your name). No further report is required.
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