CH19_PontilloB

Chapter 19 toc



Questions
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 capacity of something to perform work What is work? Work is performed when a force acts upon an object to cause displacement When is energy conserved? Energy is always conserved when it changes forms. What is the difference between conservative and non-conservative types of forces and energies? Conservative – the energy of the work performed is stored. Non-conservative – the energy of the work performed is not stored. What is electrostatic force? Is it conservative or nonconservative? Electrostatic force is the attraction or repulsion between two objects Conservative Combine the equations for work and for electric field strength to get a new expression for work. W = E*q*d 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 Electrical Potential Energy Use this to find an expression for the change in potential energy. U = q(V2 -V1) 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? a. When work is done on a charge, its potential energy is changed to a higher value. Now there is a difference of electrical potential energy between the two locations that the charge was at. This change is potential difference.  b. Voltage. It can be positive or negative.

**Notes 9/19/11**
Conductor For any shape the inside, the electric field is zero (Tesla) All the excess charge stays on the surface of conductor The pointier the surface the more excess charge at more pointy areas

Milkan Oil drop experiment 1.6 x 10^-19

Elastic Potential Energy Notes
Electric Potential energy Do work on charge and move it and gains electric potential energy (epe)

W=Fed Fe=Eq W=Eqd

Work to put into system- it is negative Sign does not matter in terms of magnitude

Potential Difference If a small charge or large charge is carrying the energy

V = (change in) PE/q =-w/q

Volts- capital V stands for volts- units not voltage but the amount of energy it carries

Names of Voltage Electric potential difference Electric potential Electric Pressure

These are not the same as electric potential energy

Voltage is scalar and no absolute value Signs matter

Equipotential Surface

Summary 9/20 Lesson 1
Electric Field and Movement of the 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. 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. 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.

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. Dependent upon at least two types of quantities:Electric charge - a property of the object experiencing the electrical field, and Distance from source - the location within the electric field.Electric potential is the potential energy per charge. Electric potential becomes simply a property of the location within an electric field. Charge moving through the wires of the circuit will encounter changes in electric potential as it traverses the circuit. One can 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.

Electric Potential Difference Moving the charge against the electric field from location A to location B, work will have to be done on the charge by an external force. The work done on the charge changes its potential energy to a higher value; and the amount of work that is done is equal to the change in the potential energy. As a result of this change in potential energy, there is also a difference in electric potential between locations A and B. 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. Electric potential difference is expressed in units of volts, and is sometimes referred to as the voltage.Electric circuits are all about the movement of charge between varying locations and the corresponding loss and gain of energy that accompanies this movement.

Prelab 9/21
1. The objective is stated in the title. What is your hypothesis? (Attempt to answer the question, to the best of your knowledge.)

The equipotentials for the electric field lines will be equal at points which are of equal distance from the charge. The areas surrounding the positive charge will have a higher voltage than those surrounding the negative charge.

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 definition of equipotential is the area where the electric potentials of points on electric field lines are equal to each other. When a positive test charge gets closer to a positive charge, its electric potential goes up and vice versa. Less work is required to bring a particle towards a negative charge, which means the voltage will be lower.

3. How do you think you might test this hypothesis? (What might you measure and how?)

We would use a board to chart points and measure the voltage at various distances from a charge.

Predict the electric field lines (and the equipotential surfaces) of the following situations: > >
 * Two point sources (one negative and one positive) - top of photo
 * A circle (negatively charged) and a positive point charge in the very center of it. - middle of photo
 * Two lines of charge (one negative and one positive) – last in the photo

Lab Equipotential Surfaces 9/22/11:
Purpose: What is the relationship between electric field lines and equipotentials?

Hypothesis: The equipotentials for the electric field lines will be equal at points which are of equal distance from the charge. The areas surrounding the positive charge will have a higher voltage than those surrounding the negative charge. The definition of equipotential is the area where the electric potentials of points on electric field lines are equal to each other. When a positive test charge gets closer to a positive charge, its electric potential goes up and vice versa. Less work is required to bring a particle towards a negative charge, which means the voltage will be lower.

Procedure: Data: Parallel "Plates": Bret Pontillo, Allison Irwin, Richie Johnson 2+ Charges: Chris Hallowell, Ryan Listro, Eric Solomon Dipole: Sam Fihma, Steve Thorwarth, Phil Litmanov Circle: Ross Dember, Erica Levine, Rebecca Rabin Graph: **2 Positive Charges: Chris Hallowell, Ryan Listro, Eric Solomon** **Parallel Plates: Bret Pontillo, Richie Johnson, Allison Irwin**  **Circle:** **Dipole:** Analysis: Parallel Plates - This shows the theoretical lines near perfection. The lines are supposed to be going from the positive plate to the negative plate and follow a smooth and straight shape. Dipole - This shows the theoretical lines near perfection. The lines are supposed to be directed away from the positive charge and towards the negative charge. The equopotential lines should be more circular than they are. 2+ Charges - This shows the theoretical lines near perfection. The lines are supposed to be directed away from the center of each charge. There are some irregular points that are lower than they should be in between the two source charges. It is also noticeable that the electric potential lines are less circular than what was expected Circle - This shows the theoretical lines near perfection. The lines are supposed to come out perpendicular to the positive charges in all direction. The equipotential lines are not as circular around the center charge as what is expected. Conclusion: The four graphs our class plotted was the Dipole, Two Positive Charges, Parallel Plates, and a Circle. In each graph, there are equipotential levels and electric field lines moving away from the positive regions and into the negative regions. These graph show that equipotential surfaces are areas equidistant from the negative and positive charges. Each of the electric field lines are moving perpendicularly through each equipotential level. The 2+ charges graph shows the electric field lines moving out of the positive charges and due to the fact that there are no negative charges, the field lines move away from each other. The dipole graph shows the field lines leaving the positive charge and moving towards the negative charge. The parallel plates graph shows the field lines moving out from the positive charge. The field lines that are on the side of the negative charge, move toward the negative charge. The circle graph is very similar to the 2+ charges graph because the field lines are moving away from the positive charge and and eventually, away from each other. My hypothesis, "The equipotentials for the electric field lines will be equal at points which are of equal distance from the charge. The areas surrounding the positive charge will have a higher voltage than those surrounding the negative charge," was correct. The areas around the positive charge did have the highest voltage, and the equipotential levels were all at points equidistant from the charge. The electric field lines did move from regions of positive to negative. What I did not mention previously (alluding to my hypothesis) was that the lines have to be perpendicular with the equipotential levels. There was one small problem. The equipotential regions should all look similar and even and when drawn, the electric field lines should be smooth. A few sources of error came up throughout our process. 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. Looking at our graphs, this error had minimal effect on them. 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.
 * 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.
 * Ross Dember, Erica Levine, Rebecca Rabin**
 * Sam Fihma, Steve Thorwarth, Phil Litmanov**