Electric Potential and Equipotential Surfaces
Pre-Lab
1. What are the 2 electrode configurations investigated in part 2?
2. What does the electrical potential difference between the 2 terminals of the battery represent?
3. What is the electric potential assigned to the negative battery terminal of a 1.5V battery? ______V.
4. What is an equipotential surface?
5. How will equipotential surfaces be depicted in this lab?
Electric Potential and Equipotential Surfaces
Part 1 – The Potential Difference of a Battery
Equipment: Digital Multimeter (use as a voltmeter - red lead in VΩ jack; black lead in COM jack, 200V scale), 2 D batteries, 2 battery holders, alligator clip leads.
Introduction: A battery consists of electrolyte chemicals, sandwiched in between 2 electrodes, or terminals, made of different metals. Separate chemical reactions do work (i.e transfer energy) to separate charge. As a result, negative ions gather at the negative terminal and the positive ions gather at the positive one. A physical separator keeps the opposite charge apart. In summary, a battery transforms chemical energy into the electrical potential energy of separated charges.
The voltmeter measures the electrical potential energy per coulomb of separated charge. This quantity is called the potential difference (∆V). In equation form:
∆U/Q = ∆V
An equivalent term for potential difference is voltage differential, or, for short, voltage. The units of potential difference are Joules per Coulombs, which we call the volt (V). As an example, if ∆V for the battery is measured as 1.5 V, this means that one coulomb of positive charge (“electron holes”) will “lose” 1.5 Joules of energy going from positive terminal to negative terminal. Since charge can’t go through the battery, the two terminals must be connected by a conductor (e.g. a wire) for that to occur. The electrical potential energy is transformed into thermal energy, light energy, or kinetic energy.
To calculate ∆V, the voltmeter uses the black probe to read the “initial” potential and the red probe to read the “final” potential. Then it subtracts initial value from the final value. The actual values are unimportant; it is the difference that is measured. By convention, the negative terminal is given a value of 0 V of electric potential, and the positive terminal is given the ∆V value for its electric potential.
1. Set up the voltmeter as described above and use it to find the potential difference between the positive and negative terminals of a D battery. This is the difference in electrical potential energy per coulomb separated charge. Use the red and black probes so you get a positive number: ∆V = ______V.
2. To get a positive number, on which terminal (positive/negative) did you use the red lead?______The black lead?______
3. Put the battery in the battery holder (spring goes on negative terminal). The holder makes a conductive contact with the battery terminals. Repeat the measurement, using the probes on the battery holder. Does the presence of the battery holder change the potential difference between the battery terminals? ______
4. Attach a red alligator lead to the positive side of the battery holder and a black one to the negative side. Repeat the measurement. Does the addition of the conductive wires change the potential difference between the battery terminals? ______
5. Connect the 2 D batteries using the battery holders. If there are no 2-battery holders available, attach two single battery holders end to end (in series). They should snap together. Make sure the positive terminal of one battery is attached to the negative terminal of the other. Measure the potential difference of the 2 batteries joined in series, or in the 2-battery holder:______V
Part 2 - Equipotential Map
Equipment: Digital Multimeter (use as a voltmeter), with leads, red and black leads with alligator clips, 2 D-batteries, battery holder, conductive paper sets (parallel plate capacitor, electric dipole), white graph paper, cork board, metal push pins, ruler, colored pencils.
Introduction: The apparatus consists of a cork board, on which is placed a sheet of black carbonized paper imprinted with a grid. The grid has electrode configurations painted on with metal conductive paint reinforced by aluminum foil. The painted lines act as extensions of the positive and negative terminals of the battery when connected. The black paper acts as a resistor. The electrode configurations provided are:
1. Two parallel linear electrodes representing a 2-dimensional cross section of a parallel plate capacitor
2. Two dots representing the point charges of an electric dipole.
These configurations produce a 2-dimensional potential difference distribution which is representative of a 3-dimensional charged objects in space.
Parallel Plate Capacitor
1. Take the black paper with the plate capacitor drawn on it and lay it on top of the corkboard. Push a metal push pin as far as it goes through the center of each plate. These will act as electrodes.
2 D Battery Pack
2. Connect the circuit as shown, using the 2 batteries together. Connect the negative terminal of the battery to the one of the push pins and also to the “com” port (black) on the multimeter. Connect the positive battery terminal to the push pin on the other side. Use the red probe lead and connect it to the red volt/ohm.terminal on the multimeter. Select the 200 DCV scale. Ask your lab instructors for help if needed.
a. Measure the potential difference between the “plates”: ______V.
b. Label the plates with potential values, using the same convention as for the battery terminals
c. Determine the magnitude and sign of the work done by the capacitor’s electric field if one coulomb of positive charge were to move from the negative to the positive plate. Recall that the units of volts are Joules per Coulomb.
______Joules
c. Will the positive charge gain or lose potential energy? How much?
______Joules Gained/ Lost (Circle one)
3. Use the multimeter and red probe lead to determine the potential at many different places both inside and outside of the capacitor. Record the values and positions on the white graph paper.
4. An equipotential surface is a three-dimensional depiction of constant values of electric potential. In two dimensions, this can be drawn as a line or curve, connecting points of equal potential. Use the illustration above as a guide (note values are not the same!), and draw 5 equally-spaced equipotentials between the capacitor plates on the WHITE graph paper (first you’ll have to draw the plates in the appropriate position). Do not draw on the black paper! If you are using 2 D batteries, equipotentials should be drawn at 0.5, 1.0, 1.5, 2.0, and 2.5 V. If you are using another power source, ask the lab instructors which potentials to draw. Use a colored pencil in the green/blue/violet range and label each line.
5. Determine the magnitude of the E field (in V/m) and draw 3 representative electric field lines on your graph. Use another color (red/orange/pink). Note the graph is marked in centimeters, not meters. Attach the completed graph to the end of the lab.
E = ______Volts/meter
Part 2 Electric Dipole
1. Replace the parallel plate capacitor configuration with that of the electric dipole and repeat the above procedure with the modifications as listed below. Use drawing as a guide.
2. Find the value of the potential at the midpoint. This should approximate a vertical line on the paper. Draw this equipotential. What is its value ? ______Volts
3. Draw the same equipotentials, using the same color as for part 1.
4. Draw 5 electric field lines on your graph. Recall the electric field lines are perpendicular to the equipotentials. Use the color you used for field lines in part 1. There is a diagram in your text that may be helpful.
5. How does your diagram compare to that illustration?
Attach graph to end of the lab.
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