Ch19_RabinR

= Chapter 19: Electric Potential and Capacitance = toc http://www.physicsclassroom.com/Class/circuits/

Guiding Questions (#1-6)
9/19/2011 1. Review what you know about energy from last year’s notes! Also look in the Cutnell and Johnson text and on The Physics Classroom. -What is energy? Energy is the ability to do work. Stored energy is called potential energy, and moving energy is called kinetic energy. (Measured in joules) -What is work? Force acts upon an object to cause a displacement of the object. Work (Joules) = Force (Newtons) X Distance (meters). Force points in the same direction as the resulting displacement. If the distance s is zero, the work is zero, even if a force is applied. If the object does not move, the force acting on the object does no work. If the force and the displacement do not point in the same direction, the force is directed at an angle relative to the displacement. To find work use the component Work = force X displacement X cosine(theta). -When is energy conserved? Energy is conserved when kinetic and potential energy are added together. The two forms may be interconverted or transformed into one another. -What is the difference between conservative and non-conservative types of forces and energies? Conservative Version 1: A force is conservative when the work it does on a moving object is independent of the path between the object's initial and final positions. Conservative Version 2: A force is conservative when it does no net work on an object moving around a closed path, starting and finishing at the same point. (Examples: Gravitational force, elastic spring force, electric force) Nonconservative: A force is nonconservative if the work it does on an object moving between two points depends on the path of the emotion between the points. (Examples: Static and kinetic frictional forces, Air resistance, tension, Normal force, Propulsion force of a rocket) -What is electrostatic force? Is it conservative or nonconservative? The electrostatic force can alter the motion of an object by contributing to the net external force that acts on the object. An electrostatic force is a conservative force. 2. Combine the equations for work and for electric field strength to get a new expression for work. W = Eqd 3. In a uniform electric field, a charge moves from one place to another. What are the only types of energy present in this situation? Electrostatic and kinetic energy 4. Use this to find an expression for the change in potential energy. EPE = KeF 5. Check this out! Real footage of So Cal Edison opening a switch on a 500kV line while its under load to make repairs. Turn it up, the sound is cool. [] 6. What is the definition of potential difference? What is the equation, symbol and unit of potential difference? Why is potential difference a relative value, not an absolute value? Potential difference is the difference in electric potential. Two players involved is the total amount of energy and the amount of charge carrying it. The equation for this is. The symbol ** V .** The unit is the volt. Potential difference is a relative value and not an absolute value because it is not displacement. It is the actual distance away. Therefore is if one object is at -3 and another at 4, the distance is 7. If you were to take the difference in absolute values it would only be 1 which is very off.

Class Notes
'

Lesson 1 Summary
9/20/2011 A charged object creates an electric field - an alteration of the space or field in the region that surrounds it. Electric field is a vector quantity whose direction is defined as the direction that a positive test charge would be pushed when placed in the field. It is simply natural for objects to move from high energy to low energy; but work is required to move an object from low energy to high energy. The chemical energy is used to do work on a positive test charge to move it from the low potential terminal to the high potential terminal. Chemical energy is transformed into electric potential energy within the //internal circuit// (i.e., the battery). The internal circuit is the part of the circuit where energy is being supplied to the charge. For the simple battery-powered circuit that we have been referring to, the portion of the circuit containing the electrochemical cells is the internal circuit. The external circuit is the part of the circuit where charge is moving outside the cells through the wires on its path from the high potential terminal to the low potential terminal. When at the positive terminal of an electrochemical cell, a positive test charge is at a high electric pressure. The loss in electric potential while passing through a circuit element is often referred to as a voltage drop.

Pre-laboratory Assignment
9/21/2011 1. The objective is stated in the title. What is your hypothesis? (Attempt to answer the question, to the best of your knowledge.) While the equipotentials will be the same distance from the charge, there will be high voltage around the positive areas and low voltage around the negative areas. 2. What is the rationale for your hypothesis? (Provide detailed reasoning here. This may take the form of a list of what you already know about the topics, with a summary at the end.) This is because as an object gets closer to the negative charge the electric potential will decrease. 3. How do you think you might test this hypothesis? (What might you measure and how?) By mapping the equipotential of different strength electric field lines. 4. Predict the electric field lines (and the equipotential surfaces) of the following situations: -Two point sources (one negative and one positive) Away from the positive, towards the negative -A circle (negatively charged) and a positive point charge in the very center of it. Away from circle -Two lines of charge (one negative and one positive) Away from positive, towards negative

Lab: Equipotential Surfaces
9/22/2011 PURPOSE: What is the relationship between electric field lines and equipotentials? HYPOTHESIS W/ RATIONALE: While the equipotentials will be the same distance from the charge, there will be high voltage around the positive areas and low voltage around the negative areas. This is because as an object gets closer to the negative charge the electric potential will decrease. PROCEDURE Materials: Volt meter (VOM), Alligator leads (2), Metal push pins (2), Cork board, Power supply, Silver marker Part A //Preparing the materials// 1. Select a sheets with silver conductive lines drawn on it. Use a conductive ink pen to draw one of the given shapes. 2. Place the sheet on the cork pad. Place one metal pin through each of the two painted silver points on the conducting paper. 3. Insert black probe in to COM socket of the voltmeter (VOM) and insert red probe into other Voltmeter socket. Then, set selector to 20V. 4. Set power supply to 20V. Test power supply with VOM to make sure that it is working. 5. Attach one lead wire from the power supply to one metal pin, then attach another wire from the other clip of the power supply to the second metal pin on the corkboard. 6. Attach the black COM wire from the voltmeter to one of the pins. Part B //Recording data// 7. Create a numbered grid in Excel using the conducting sheet as a reference. 8. You will only do points 5 to 15 on the vertical axis, and 5 to 20 on the horizontal axis. 9. Touch the red wire from the voltmeter gently to point (5,5). Use the first number that appears on the voltmeter. Enter your data directly into Excel. Move to the next point (5,6). Repeat for all points until you reach (15, 20). 10. Repeat for the other designs. Part C //Graphing data// 11. Highlight entire table 12. Graph a SURFACE 13. Create two views: Side and Top 14. Adjust scale to “2”. (It does “5” as a default.) 15. If graph is not relatively smooth, go back and remeasure. 16. Put your name(s), lab title, and date on the header/footer. 17. Email me a copy of your Excel document and I will compile all of them into one document and email them to everyone. DATA 2+ Charges - shared from Chris Hallowell, Ryan Listro, Eric Solomon Dipole - shared from Sam Fihma, Steven Thorwarth, Phil Litmanov Parallel Plates - shared from Richie Johnson, Bret Pontillo, Allison Irwin Circle - shared from previous years GRAPH 2+ Charges - shared from Chris Hallowell, Ryan Listro, Eric Solomon Dipole - shared from Sam Fihma, Steven Thorwarth, Phil Litmanov Side View Top View Parallel Plates - shared from Richie Johnson, Bret Pontillo, Allison Irwin Top View Side View Circle Side View Top View ANALYSIS 2+ Charges - This picture portrays the theoretical lines almost perfectly. The lines are supposed to be directed away from the center of each charge however, they should be smooth and straight with no bumps or curves. Dipole - This picture portrays the theoretical lines almost perfectly. The lines are supposed to be directed away from the positive charge and towards the negative charge however, they should be smooth and straight with no bumps or curves. Parallel Plates - This picture portrays the theoretical lines almost perfectly. The lines are supposed to be going from the positive plate to the negative plate and maintains smooth and straight shape. Circle - This picture portrays the theoretical lines almost perfectly. The lines are supposed to come out perpendicular to the positive charges in all direction however, they should be smooth and straight with no bumps or curves. CONCLUSION The four graphs, 2+ Charges, Dipole, Parallel Plates, and Circle, each show the equipotential surface with their electric field lines. They all maintain the characteristics of electric field lines: 1. no intersecting lines, 3. away from positive, towards negative, 3. Most dense around objects with greater charge, more lines the stronger the field is 4. Lines are perpendicular to surface. The 2+ Charges show electric field lines all moving away from each positive charge. Even though the lines are moving away from each charge, they never meet each other due to the great repulsion between like charges. The Dipole shows electric field lines moving away from the positive charges towards the negative charge. The Parallel Plate shows electric field lines moving into the negative source. The Circle shows electric field lines all moving away from the center positive charge. In the analysis section, it is clearly shown the accuracy of our results. The graphs are not perfect, however. The lines are very squiggly but do show a good representation of the theoretical graphs. My hypothesis was partially correct and partially incorrect. I knew that the electric field lines must run away from positives and towards negatives however they do not seem to be the same distance from each charge. I also did not include the shapes of the graphs, which will vary based on the charges. Although this lab was successful there was much error in the procedure. It was impossible for the experimenter to produce identical readings based on the pressure the experimenter added to the board, the timing in which the experimenter read the voltage, and the fact that the readings constantly jumped from number to number. In my groups experiment specifically we reached voltages way over 20 and also never reached a 0 voltage which is vital in a circle graph. We then had extremely inaccurate results and relied on previous experimenters for data. To fix these errors, a more efficient device would be necessary to produce the readings of voltage at each coordinate.
 * Used because original data never reached 0 and exceeded 20V
 * Original data never reached 0 and exceeded 20V