Ch19_IrwinA

__GQ Chapter 19 #1-6: 9/19/11__


toc

**__Physics Classroom Summary: Current Electricity Lesson 1__**
A. Electric Field & Movement of Charge An electric field can influence charge within a circuit as it moves from one location to another. 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. Thus, the electric field direction about a positive source charge is always directed away from the positive source. And the electric field direction about a negative source charge is always directed toward the negative source. When gravity does work upon an object to move it in the direction of the gravitational field, then the object loses potential energy. To move a charge in an electric field against its natural direction of motion would require work. The exertion of work by an external force would add potential energy to the object. The natural direction of motion of an object is from high energy to low energy; but work must be done to move the object //against// //nature//. The high energy location for a positive test charge is a location nearest the positive source charge; and the low energy location is furthest away. B. Electric Potential The amount of force involved in doing the work is dependent upon the amount of charge being moved (according to Coulomb's law of electric force). The greater the charge on the test charge, the greater the repulsive force and the more work that would have to be done on it to move it the same distance. The electric potential energy is dependent upon the amount of charge on the object experiencing the field and upon the location within the field. Electric potential is the potential energy per charge. (Electric Potential =PE/Q) A battery powered electric circuit has locations of high and low potential. Charge moving through the wires of the circuit will encounter changes in electric potential as it traverses the circuit. Within the electrochemical cells of the battery, there is an electric field established between the two terminals, directed from the positive terminal towards the negative terminal. As such, the movement of a positive test charge through the cells from the negative terminal to the positive terminal would require work, thus increasing the potential energy of every Coulomb of charge that moves along this path. This corresponds to a movement of positive charge against the electric field. The positive terminal is described as the high potential terminal. The movement of positive charge through the wires from the positive terminal to the negative terminal would occur naturally. Such a movement of a positive test charge would be in the direction of the electric field and would not require work. The charge would lose potential energy as moves through the //external circuit// from the positive terminal to the negative terminal. The negative terminal is described as the low potential terminal. This assignment of high and low potential to the terminals of an electrochemical cell presumes the traditional convention that electric fields are based on the direction of movement of positive test charges. In a certain sense, an electric circuit is nothing more than an energy conversion system. In the electrochemical cells of a battery-powered electric circuit, 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). Once at the high potential terminal, a positive test charge will then move through the external circuit and do work upon the light bulb or the motor or the heater coils, transforming its electric potential energy into useful forms for which the circuit was designed. The positive test charge returns to the negative terminal at a low energy and low potential, ready to repeat the cycle (or should we say //circuit//) all over again. C. Electric Potential Difference Electric potential is a location-dependent quantity that expresses the amount of potential energy per unit of charge at a specified location. This difference in electric potential is represented by the symbol delta V and is formally referred to as the electric potential difference. By definition, the electric potential difference is the difference in electric potential (V) between the final and the initial location when work is done upon a charge to change its potential energy. Delta V = Vb –Va =Work/Charge = delta PE / Charge (in volts) Because electric potential difference is expressed in units of volts, it is sometimes referred to as the voltage. The loss of this electric potential energy in the external circuit results in a gain in light energy, thermal energy and other forms of non-electrical energy. The cells simply supply the energy to do work upon the charge to move it from the negative terminal to the positive terminal. By providing energy to the charge, the cell is capable of maintaining an electric potential difference across the two ends of the external circuit. Once the charge has reached the high potential terminal, it will naturally flow through the wires to the low potential terminal. Role of the Cell: 1. Supplies the energy, 2. Pumps the charge from – to + terminal, 3. Maintains change in V across the external circuit. 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. The movement of charge through the internal circuit requires energy since it is in a direction that is //against the electric field//. The movement of charge through the external circuit is natural since it is a movement in the direction of the electric field. Being under high electric pressure, a positive test charge spontaneously and naturally moves through the external circuit to the low pressure, low potential location. The location just prior to entering the light bulb (or any circuit element) is a high electric potential location; and the location just after leaving the light bulb (or any circuit element) is a low electric potential location. ] The loss in electric potential while passing through a circuit element is often referred to as a voltage drop. An electric potential diagram is a convenient tool for representing the electric potential differences between various locations in an electric circuit.

__Class Notes 9.21.11 -__
GQ 19 #7

__Equipotential Surfaces Lab: 9.22.11-__
__Pre-Lab Questions:__ __Purpose:__ What is the relationship between electric field lines and equipotentials? __Hypothesis:__ See Questions 1&2 of Pre Lab Assignment ^ __Procedure:__ __Data:__ ***Data was comprised collectively by the students of Period 8 AP Physics Class 2011-2012*** __Graphs:__ __Analysis:__ ***Images for theoretical electric field lines were taken from:** http://share.ehs.uen.org/node/9411 & http://equalrightsforall.net/electromagnetism/electrostatics/uniform_E.htm
 * 1)  The objective is stated in the title. What is your hypothesis? - If the electric field is strong, there will be more electric field lines. If there are more electric field lines, the equipotential surface will be greater.
 * 2) What is the rationale for your hypothesis? -We already know that the number of electric field lines is proportional to the strength of the electric field. We also know that electric field lines are perpendicular to equipotential lines (lines on which electric potential is constant). So if there are more equipotential lines, the equipotential surface will be larger.
 * 3) How do you think you might test this hypothesis? By using a digital voltmeter, we will be able to map the electric potential around charge configurations to find the relationship between electric field lines and equipotentials.
 * 4) Predict the electric field lines (and the equipotential surfaces) of the following situations:
 * Two point sources (one negative and one positive) -
 * Two point sources (one negative and one positive) -
 * A circle (negatively charged) and a positive point charge in the very center of it. -
 * Two lines of charge (one negative and one positive) –
 * 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.
 * 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.
 * 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.

__Conclusion:__ These graph show that equipotential surfaces are areas equidistant from the negative and positive charges. They also show that electric potential energy is the greatest when closest to the positive charge (in our case 20V). The EPE decreases that further away from the positive charge and the closer you get to the negative charge. The data and graphs show this really well. In the parallel plates graph, the rows of charge clearly decrease as you move from the positive charge to the negative charge. I realize now that my hypothesis did not make sense. I now understand how the equipotential surfaces represent the electric potential energy, which is greatest closest to the positive charge. One possible source of error is our use of the voltmeter. Depending on how we held the probe, the reading came up differently each time. Another source of human error was our placement on the probe on the grid. There was not way to ensure that we were putting it in the exact location each and every time. Hopefully this human error was minimal though, and judging from our graphs, it should not have affected our results too much. If we were to do this lab over though, the use of a mechanically operated probe would have eliminated this minimal error. It would be able to get the exact reading in the perfect location.

**__Class Notes 9/23/11:__**
