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Experiment 12 Date/ Time: Tues 2:00 p.m. Stephanie Ferrera Partner: Jasmine Fung Introduction/ Goals: In Experiment 12, students were asked to investigate the shapes of equipotential surfaces that surround charged conductors. By doing so, students were able to interpret their measurements of electric fields and electric potential lines. There are four main goals to keep in mind throughout the lab, which students should be able to interpret by the end of the experiment. Firstly, students should be able to explain the difference between the concepts of electric fields and electric potentials. Secondly, students should be able to correctly draw and identify electric field lines and electric equipotential surfaces and curves on the same diagram. Students should also be able to explain the relationship between both. Thirdly, students should be able to determine how electric fields and electric potentials behave near conductors. Lastly, students should be able to explain the experiment and the concepts within. Overall, the experiment should allow students to see the meaning of electric fields and equipotential lines, and how they relate. Equipment: Probe Experiment 12 (lab book) Experiment 12 mapping board Voltmeter Power Source Conducting Plates Banana plug cables (4x) Setup / Procedures 1. Using banana cables, attach the probe to the Voltmeter 2. Next, connect the power source to the mapping board using the banana cables. Make sure one cable connects to the probe and the other to the board. Experiment 12 Date/ Time: Tues 2:00 p.m. Stephanie Ferrera Partner: Jasmine Fung 3. With the brown side facing up, tighten the conducting plate to the bottom side of the mapping board. 4. Align the Experiment 12 mapping sheets to the topside of the mapping board. Make sure to align the board with the X & Y axis provided. This part is crucial so be as accurate as possible. 5. Turn on the power source. 6. Adjust the power source to precisely 6V 7. By rubbing the probe across the surface of the mapping board, one can see different values. Search for 5 different values of 1V, 2V, 3V, 4V, and 5V. Make sure to indicate the points by drawing dots and labeling. 8. Connect all of the dots for 1V together to form the electric potential lines. Repeat for 2V, 3V, 4V, and 5V. 9. Draw lines from the positive to the negative point charges. Make sure each field line is perpendicular to each equipotential surface line. Lastly, show the direction of the field lines by adding arrowheads. 10. Turn off the power source 11. Remove conducting plate, and replace with insulator/conductor plate (Refer back to steps 3 and 4 for help) 12. Repeat steps 5-10 for the insulator / conductor plate. 13. Predict how the third map (point charge and line charge) will appear. Ask TA for approval or further explanation. 14. Remove conducting plate, and replace with new conducting plate (Refer back to steps 3 and 4 for help) 15. Repeat steps 5-10 for the new conducting board. Theory: Experiment 12 uses four main equations to help explain the theory of the electric fields and electric potential. Firstly, the electric field (E) is actually defined in terms of a force on a positive test charge q’. Thus, E= F/q’. An electric field can also be placed at a point, with distance r, from a point charge q. This point would be located at the origin of the coordinate system. Thus, E = q/(4*pi*r^2* ϵ0). The electric potential, on the other hand, is defined in terms of the electrical potential energy of a point charge at that position. It’s important to note, that the electrical potential of any point in space can be defined in these terms. Thus, V= q/(4*pi*r* ϵ0). The electric field produces a vector quantity where as the electric potential will produce a scalar Experiment 12 Date/ Time: Tues 2:00 p.m. Stephanie Ferrera Partner: Jasmine Fung quantity. The electric field can be represented graphically with two different methods. The first method is by drawing a set of lines, which give the direction of the electric field at any point in space. The next way is to draw a set of surfaces such that the electric potential has the same value at every point on the surface. Lastly, we need to look at the physical result of the electric field; the electric field is equal to the negative gradient of the electrical potential. This direction is perpendicular to the equipotential and has a magnitude given by E= - ΔV / Δs. Observation/ Discussion: 1. 2 Point Charges: There was a positive and negative point charge within the board. Surrounding both the positive and negative points, there are circular equipotential lines. As you moved away from the positive charge to the negative charge, the points decreased in voltage. As the equipotential lines moved away from the positive point, the lines became more and more vertical. However, once it passed a certain point, in this case where the 3V points were located, the lines became curved again. Finally, circling around the negative conductor. It’s also important to note that the electric field lines went from positive to negative. 2. Conductor and Insulator: This board had a conductor and insulator plate and two point charges. The equipotential lines were curved towards the positive point charge. As you moved towards the negative point charge, however, the lines started curving towards it. The potential lines also decreased in voltage as you moved away from the positive charge towards the negative charge. The electric filed lines flowed from positive to negative. As the electric field lines approached the insulator, the lines became parallel and went around the insulator. As the field lines approached the Experiment 12 Date/ Time: Tues 2:00 p.m. Stephanie Ferrera Partner: Jasmine Fung conductor, however, the lines became normal to the conducting surface and seemed to go into rather than around the surface. 3. Point and Line Charge Prediction: We predicted that the equipotential lines would start out curved towards the positive charge. As it would move closer to the negative line charge, the equipotential lines would become more and more vertical and parallel to the negative line charge. It would also decrease in voltage as it approached the negative line charge. 4. Point and Line Charge: Our predictions were correct. The equipotential lines started curved towards the positive charge. As it approached the negative line charge, the lines became more and more vertical and parallel to the negative line charge. The electric field lines flowed from the positive point charge to the negative line charge, and the equipotential lines decreased in voltage.