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Physics Lab 212P-3 Template Electric Fields and Electric Potential NAME: ____________________________________ LAB PARTNERS: ____________________________________ ____________________________________ LAB SECTION: __________________________ LAB INSTRUCTOR: __________________________ DATE: __________________________ EMAIL ADDRESS: __________________________ Penn State University Physics 212P-3 Written by Redwing and Samarth 1 Physics Pre-lab 212P-3 Template Electric Fields and Electric Potential Name:__________________________ Section:_____ Date:__________ (Read this & answer the questions before coming to lab) Summary of relevant concepts: a) The electric potential energy U of a charge at any point in space is defined as the negative of the work done by the electric field E when the charge is moved from infinity to that point. Here, we define the electric potential at infinity to be 0. Note that this definition is equivalent to saying: "electric potential energy is the work YOU have to do in bringing a charge from infinity to that point." b) The electric potential V at any point in space is defined as the negative of the work done by the electric field when a charge of +1 C is brought from infinity to that point i.e. A V E ds c) We also talk about the potential difference between two points A & B: B V E d s A d) i.e. the negative of the work done by the electric field in moving a charge of +1C from A to B. e) Electric potential and potential differences are measured in VOLTS; electric potential energy is measured in joules. f) Equipotential surfaces are a convenient way of picturing the electric potential in any region. Most often, equipotential LINES are used to portray a cross-sectional view of equipotential surfaces. By definition, all points on an equipotential line/surface have the same potential. Penn State University Physics 212P-3 Written by Redwing and Samarth 2 Pre-lab Questions: Q1. What is a "conservative force?" (Recall: Physics 211.) ____________________________________________________________________________ ____________________________________________________________________________ Q2. When a mass that is free to move is released in the presence of a gravitational field, does it move from a region of high gravitational potential to a region of low gravitational potential or vice-versa? ____________________________________________________________________________ ____________________________________________________________________________ Q3. When a charge that is free to move is released in the presence of an electric field, does it move from a region of high electric potential to a region of low electric potential or vice-versa? Does your answer depend on whether the charge is positive or negative? Why/why not? ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________ Penn State University Physics 212P-3 Written by Redwing and Samarth 3 The figure below shows a region of space with a uniform electric field E. You can move charges from A to B along the three different paths shown. ACBD is a square of side L. Q4. Suppose a positive charge +q is moved from A to B. Calculate the work W done by the electric field on the charge, if the charge +q were taken: (a) first from A to C and then from C to B. (b) first from A to D and then from D to B. Penn State University Physics 212P-3 Written by Redwing and Samarth 4 Q5. Calculate the work done by the electric field on the charge if the charge were moved along the straight line AB. How does this compare with your answers to Q4? Q6. From your answers to Q4 & Q5, what is the potential difference V = VB - VA between points A and B ? Q7. An equipotential surface is defined as a surface on which there is no potential difference between any of the points. What are the equipotential surfaces for the problem above? Sketch a cross-sectional view of a few equipotential surfaces, showing surfaces with a constant potential difference between them. Penn State University Physics 212P-3 Written by Redwing and Samarth 5 Lab Activity 1: Visualizing the electric potential created by a dipole. Q1. Use the program to make a plot that shows equipotentials for the electric dipole with the following values: V = -3.2 V, -1.5 V, -0.7 V, -0.3 V, 0 V, +0.3 V, +0.7 V, +1.5 V, +3.2 V. (In case you cannot match these values exactly, don't worry about it. Get as close as you can.) In this plot, also show a few electric field lines. Print this figure and include it with your report. Penn State University Physics 212P-3 Written by Redwing and Samarth 6 Now, refer to your equipotential and electric field line plot, as well as to the exercises you carried out above, and answer the following questions. Q2. Do you notice any obvious geometrical relationship between equipotentials and electric field lines? Describe this relationship and why it makes sense. (Think about the direction of the electric field and the work done in moving a charge along an equipotential.) _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ Q3. If you were to bring a positive charge from infinity to any point on the perpendicular bisector of the electric dipole following an arbitrary path, how much work would you do? Justify your answer using your equipotential plot. _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ Q4. Is it true that wherever the electric potential V = 0, the electric field E is also 0? Justify/support your answer using information on your plot. _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ Penn State University Physics 212P-3 Written by Redwing and Samarth 7 Q5. Is it true (in general) that wherever the electric field E = 0, the electric potential V is also 0? Suggest an arrangement of charges that would help justify your answer. _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ Q6. Suppose you released a positive charge from a point located on the +0.3 V equipotential. Would it move to a point of higher or lower potential? _____________________________________________________________________________ Would it move to a point of higher or lower potential energy? _____________________________________________________________________________ Justify your answers based on the electric field lines in your plot. _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ Q7. Describe how your answers to Q6 would change if you used a negative charge instead. _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ Penn State University Physics 212P-3 Written by Redwing and Samarth 8 Lab Activity 2: A Real Experiment & A Return to (Messy) Reality. Q8. How does the potential vary when you probe different locations ON the silver ring? What we expected and why: ______________________________________________________________________________ ______________________________________________________________________________ What we observed: ______________________________________________________________________________ ______________________________________________________________________________ Q9. How does the potential vary at different positions INSIDE the silver ring? What we expected and why: ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ What we observed: ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ Penn State University Physics 212P-3 Written by Redwing and Samarth 9 Q10. How does the potential vary with position outside the silver ring? Use sketches to show the expected and observed equipotentials. Make sure you also indicate in the sketch below the positions of your pushpin connectors. expected Sketches of and equipotential patterns: observed Creative exercise: comment on discrepancies between your expectations and observations: ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ Penn State University Physics 212P-3 Written by Redwing and Samarth 10 Physics Post-lab 212P-3 Electric Fields and Electric Potential Name:__________________________ Section:_____ Q1. In the figure does the electric potential increase toward the right or the left? Toward the right Date:__________ Equipotential surface Field line Toward the left Q2. If the adjacent equipotential surfaces differ by 10 V and the right-most one is at an electric potential of –100 V, what is the electric potential of the left-most one? Q3. If we move an electron toward the right, is the work done on the electron by our force positive or negative? Positive Negative Q4. If we move an electron toward the right, is the work done on the electron by the electric field positive or negative? Positive Negative Q5. Is the electric potential energy of the electron increased or decreased as a result of the move to the right? Increased Decreased Penn State University Physics 212P-3 Written by Redwing and Samarth 11 Q6. If the surface of a charged conductor is an equipotential surface, does that mean that the charge is spread uniformly over the surface? Yes No Why or why not? ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ Penn State University Physics 212P-3 Written by Redwing and Samarth 12 Penn State University Physics 212P-3 Written by Redwing and Samarth 13