Ch19_Demberro

toc =Chapter 19 =

Guiding Questions 1-6

 * 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.
 * 2) What is energy? **What objects need to perform work.**
 * 3) What is work? **Amount of force done over a certain distance.**
 * 4) When is energy conserved? **All the time, only has different forms.**
 * 5) What is the difference between conservative and non-conservative types of forces and energies? **Conservative types of forces and energies are traditional, and often mechanical. They follow Newton's laws where force one way is equal and opposite to the force the opposite way. Non-conservative types exist in outer space and some instances involving friction.**
 * 6) What is electrostatic force? Is it conservative or nonconservative? **Interaction between charged objects. Conservative.**
 * 7) Combine the equations for work and for electric field strength to get a new expression for work. **W=Eqd**
 * 8) In a uniform electric field, a charge moves from one place to another. What are the only types of energy present in this situation? **Kinetic and Electric Potential Energy.**
 * 9) Use this to find an expression for the change in potential energy. [**Delta]PE=EQd**
 * 10) 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. []
 * 11) 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 voltage of an object when it moves from one distance to another in respect to another charge. Its symbol is V for volts. The equation is [[image:Screen_shot_2011-10-14_at_10.44.06_PM.png width="163" height="32"]]. It is a relative value since it is determined based on a chosen spot that can be placed anywhere.**

Lesson 1 Summary
Part 1 The flow of charge through wires allows us to perform everyday activities. One of the fundamental principles that must be understood in order to grasp electric circuits pertains to the concept of how an electric field can influence charge within a circuit as it moves from one location to another. Electric force was described as a non-contact force. Electric force is an action-at-a-distance force. Action-at-a-distance forces are sometimes referred to as field forces. The space surrounding a charged object is affected by the presence of the charge; an electric field is established in that space. Other charges in that field feel the unusual alteration of the space. Whether a charged object enters that space or not, the electric field exists. Space is altered. 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. Work must be done by an external force to move an object against nature - from low potential energy to high potential energy. On the other hand, objects naturally move from high potential energy to low potential energy under the influence of the field force. In a similar manner, to move a charge in an electric field against its natural direction of motion would require work, which would in turn add potential energy to the object. Work must be done to move the object //against// //nature//. Part 2 If gravitational potential is a means of rating various locations within a gravitational field in terms of the amount of potential energy per unit of mass, then the concept of electric potential must have a similar meaning. 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. Thus, the electric potential energy is dependent upon the amount of charge on the object experiencing the field and upon the location within the field. Dependent upon at least two types of quantities: 1) Electric charge - a property of the object experiencing the electrical field, and 2) Distance from source - the location within the electric field
 * Electric Field and the Movement of Charge **
 * Electric Field, Work, and Potential Energy **
 * Electric Potential **
 * The Gravitational Analogy Revisited **

Electric potential is the potential energy per charge. Electric potential becomes simply a property of the location within an electric field.
 * Electric Potential in Circuits **

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. The positive terminal is described as the high potential terminal. Similar reasoning would lead one to conclude that the movement of positive charge through the wires from the positive terminal to the negative terminal would occur naturally. 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. Part 3 If a Coulomb of charge (or any given amount of charge) possesses a relatively small quantity of potential energy at a given location, then that location is said to be a location of low electric potential. 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 Because electric potential difference is expressed in units of volts, it is sometimes referred to as the ** voltage **.
 * Electric Potential Difference **

If a 12 volt battery is used in the circuit, then every coulomb of charge is gaining 12 joules of potential energy as it moves through the battery. And similarly, every coulomb of charge loses 12 joules of electric potential energy as it passes through the external circuit. 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. 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. In a battery-powered electric circuit, the cells serve the role of the charge pump to supply energy to the charge to lift it from the low potential position through the cell to the high potential position. The ** internal circuit ** is the part of the circuit where energy is being supplied to the charge. 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 an //uphill// movement 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. When at the positive terminal of an electrochemical cell, a positive test charge is at a high ** electric pressure ** in the same manner that water at a water park is at a high water pressure after being pumped to the top of a water slide. Being under high electric pressure, a positive test charge spontaneously and naturally moves through the external circuit to the low pressure, low potential location. As a positive test charge moves through the external circuit, it encounters a variety of types of circuit elements. Each circuit element serves as an energy-transforming device. Light bulbs, motors, and heating elements (such as in toasters and hair dryers) are examples of energy-transforming devices. In each of these devices, the electrical potential energy of the charge is transformed into other useful (and non-useful) forms. The moving charge is doing work upon the light bulb to produce two different forms of energy. By doing so, the moving charge is losing its electric potential energy. The loss in electric potential while passing through a circuit element is often referred to as a ** voltage drop **. By the time that the positive test charge has returned to the negative terminal, it is at 0 volts and is ready to be re-energized and //pumped back up to// the high voltage, positive terminal.
 * Electric Potential Difference and Simple Circuits **

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Potential Difference- Voltage (Scalar); Depends on total amount of potential energy and the amount of charge carrying it. V=(EPE/q)=-W/q=J/C Voltage is electric potential difference, electric potential, electric "pressure". NOT electric potential energy. Equipotential Surface- Outlining where spots have same charge.

Practice problem 7
A uniform electric field of magnitude 250 V/m is directed in the positive x direction. A +12-μC charge moves from the origin to the point (20-cm, 50-cm). What was the change in the potential energy of this charge? Through what potential difference did the charge move?

Equipotential Lab
Purpose: Demonstrate the relationship between equipotential and electric field lines.

Hypothesis: The areas surrounding the main charge(s) will all have the same or similar charge in a circular way. This is due to the definition of equipotential, which states the areas around a charge will be make a circumference of equal electric potential.

Prelab: 1. The objective is stated in the title. What is your hypothesis? (Attempt to answer the question, to the best of your knowledge.) The areas surrounding the main charge(s) will all have the same or similar charge in a circular way. This is due to the definition of equipotential, which states the areas around a charge will be make a circumference of equal electric potential. 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.) The electric field always goes from positive to negative. Always are/should be perpendicular to equipotential lines. All charges on same equipotential lines have same electric field strength. 3. How do you think you might test this hypothesis? (What might you measure and how?) Map equipotential of different 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) They will go from positive to negative. -A circle (negatively charged) and a positive point charge in the very center of it. Emit from the center away. -Two lines of charge (one negative and one positive) Positive to negative.

Procedure: 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+ (Chris Hallowell, Ryan Listro, and Eric Solomon) Dipole (Sam Fihma, Steven Thorwarth, and Eric Solomon) Parallel Plates (Richie Johnson, Allison Irwin, and Bret Pontillo) Circle (A previous year's class)

Graphs: Two positive (From Chris Hallowell, Ryan Listro, and Eric Solomon): Dipole (one negative, one positive)- (From Sam Fihma, Steven Thorwarth, and Eric Solomon) Left-Side view; Right- Top view Parallel (From Richie Johnson, Allison Irwin, and Bret Pontillo): Side Top Circle (From a previous year's class): Left- Side view; Right- Top view Analysis: Two Positives: Dipole: Parallel Circle: All four graphs give a fairly accurate view of how the field lines should look.

Conclusion: All four graphs individually represent the electric field lines and the equipotentials they perpendicularly intersect. They each have the same characteristics such as the lines starting from the source charge, lines never intersecting each other, lines heading towards negative charges from positive, and and lines bisecting the equipotentials they cross. The graph with two positives gives the expected results as there is an asymptote-like invisible line that prevents field lines from bisecting since the electron charges will repel each other. For the dipole graph, the opposite occurs. Since one side is positive and the other is negative, the lines will travel towards the negative charge from the positive and converge at the charge. Outside field lines will head the opposite direction as not all field lines should go to negative charge. The parallel plate graph looks the most different from the others, yet the lines are still traveling from negative to positive, only the position of the positive charge differs. Finally, the circle gives a typical representation, as the positive charge in the circle has concentric circles all representing equipotentials with positive charge in the center. My hypothesis was mostly correct since the circular regions of the graph prove the equality of the equipotentials. I also failed to take into full account of more nontraditional-looking graphs such as the parallel plates one. These results were not perfect since the method relied heavily on human error. For example, getting the results involved pulling away the charge at the exact same time, which is nearly impossible. Also, my group was getting extremely large values which were obviously examples of faulty equipment. Therefore, these results could be fixed with more accurate equipment and some mechanism that would place the charge for the exact time it should have been place.