LABORATORY 4: ELECTRIC POTENTIAL Objectives: determine the variables that affect electric potential for a static arrangement of charges. . relate the electric potential to charges and the distance from the charge visualize the equipotential lines for a collection of positive and negative charges. Materials Required: Computer with Excel and access to Charges and Fields simulation: https://phet.colorado.edu/en/simulation/charges-and-fields a Software Requirements: PRET. Windows Macintosh Chromebook Linux iPad Mobile Phone Chrome, Edge Chrome, Safari Chrome Not recommended Safari Not recommended Introduction: An electric field (E-field) is a vector field surrounding an electric charge that exerts force on other charges, attracting or repelling them. Electric field is a vector quantity and vectors were cumbersome and difficult to work with. Electric potential was introduced as a way to explain a "difficult" vector field in terms of an "easy" scalar field. Electric field lines "flow" from positive charges to negative charges. A positive charge is like an open faucet and a negative charge is like an open drain. The flowing can be described by a vector field (a quantity that has magnitude and direction at any location) and the flow is due to a difference in a scalar field (a quantity that has magnitude only at any location). For example, the flow of a water in a river is caused by a difference in altitude, the flow of air (wind) is caused by a difference in atmospheric pressure. We defined the quantity that serves the same role for the electric field as height does for rivers or the pressure does for the wind, as electric potential. Electric potential is defined as the amount of work needed to move a unit charge from a reference point to a specific point against an electric field. In the International System of Units (SI), electric potential is expressed in units of joules per coulomb (i.e., volts), and differences in potential energy are measured with a voltmeter. Equipotential lines are connected lines of the same potential. A collection of such points form a surface that is called an equipotential surface. These often appear on field line diagrams. Equipotential lines are always perpendicular to field lines, and therefore perpendicular to the force experienced by a charge in the field. If acharge moves along an equipotential line, no work is done; if a charge moves between equipotential lines, work is done. Activity 1: The Electric Potential due to a Charge 1. Start the Charges and F ields PM simulation and explore it. Make sure that the Voltage, Values, and Grid boxes are checked. VIEW the electric potential MEASURE the electric field at a precise location CHANGE the background color of the sim to white for _ projection Options... PhET Websme PLOT equipotehtial lines Repun a Problem Check ch Updales Screenshol Full Screen Aboui. . Charges And Fields 2. Place a single positive charge at the center of the screen. Place the cross-hair tool at the position in space you want to measure the electric potential. Where are the values the same around the positive charge? Where are they different? 3. Make a prediction on how do equipotential lines (lines of same electric potential value) look for a single positive charge. 4. Use the simulation to test your prediction. Were you correct? Explain Why or why not. 5. Hit reset and place a single negative charge into the center of the screen. Use the cross-hair tool dot sensor to measure the electric potential. Where are the values the same around the negative charge? Where are they different? 6. How does the electric potential compare with the electric potential due to a single positive charge at the same distance from the charge? 7. How do equipotential lines (lines of same electric potential value) look like for a single positive charge? 2 2,-"5'23 8. Use the simulation to visualize the equipotential lines for a collection of positive and negative charges. Move the charges around to Visualize the changes in the equipotential lines. Activity 2: The Dependence of the Electric Potential on the Distance from the Charge 9. Take at least 6 measurements to investigate the relationship between the distance from the charge and the measured value on the sensor. Use the tape measure. List your results in Table 1: Table 1: Trial Distance 1' l/r Electric Potential V (\"0 (M1) (V) GUIAUJNtt 10. Use Excel to plot a graph of V 178.?\" (see LUO_Excel le for help). Make sure that your graph is a scatter pl_0t. Customize the graph - graph title and label the axes (using the Chart Tools menu). Add the best-t (it may or may not be a linear t) line passing through your data points (use the T rendline menu) and check the Display Equation on Chart option near the bottom. Insert a copy (screenshot) of your graph in the space below. 11. Does a linear t describe the dependence of the magnitude of the electric potential V on the distance from the charge 1'? 12. Use Excel to plot a graph of V v5.1/r (see LUO_Excel le for help). Make sure that your graph is a scatter plot. Add the best-t (it may or may not be a linear t) line passing through your data points (use the T rendline menu) and check the Display Equation on Chart option near the bottom. Insert a copy (screenshot) of your graph in the space below. 13. Does a linear t describe the dependence of the magnitude of the electric potential V on the inverse of the r? 14. Based on your results, what do you think the relationship is between distance and electric potential? Activity 3: The Dependence of the Electric Potential on the Charge 15. Place one positive charge at the center of the screen. Use the cross-hair tool sensor to read the magnitude of the electric potential at a certain distance r away from the charge. Record your reading in Table 2. Table 2 T 2 Trial Charge q Electric Potential V gnC) (V) 1 2 3 4 5 16. Place a second positive charge on top of the rst one so they are stacked on top of one another. Use the cross-hair tool sensor to read the electric potential at the same distance r away from the charge as in the previous step. Record your reading in Table 2. 17. Repeat the procedure by adding one positive charge at the time until there are ve stacked charges on top of one another. For each step read the electric potential at the same distance r away from the charge as in the previous steps and record your readings in Table 2. 18. Use Excel to plot a graph of V 195. q. Make sure that your graph is a scatter plot. Add the best-t line passing through your data points and check the Display Equation on Chart option near the bottom. Insert a copy (screenshot) of your graph in the space below. 19. Based on your results, what do you think the relationship is between the electric potential and the charge creating it is? Activity 4: The Electric Potential due to TWO Charges 20. Are there going to be places around the two positive charges where the electric potential is 0? Where would they be if so? 21. Place two positive charges close to each other. Use the simulation to test your prediction. Were you correct? Explain why or why not. 22. Are there going to be places around the two negative charges where the electric potential is 0? Where would they be if so? 23. Place two negative charges close to each other. Use the simulation to test your prediction. Were you correct? Explain why or why not. 24. Are there going to be places around the two charges one positive, and one negative where the electric potential is 0? Where would they be if so? 25. Place one positive charge and one negative charge close to each other. Use the simulation to test your prediction. Were you correct? Explain why or why not. References: CCBY license, 1M Interactive Simulations, University of Colorado Boulder, http://phehcoloradoedu jmrl) v