Ch19_HallowellC

toc =9/19/11= HW: Guide 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. What is energy? - Energy is the ability to do work (such as causing motion or interacting molecules).

What is work? - It is the application of force over a distance. (W= Fd)

When is energy conserved? - Energy is always conserved due to the law of conservation of energy. Energy can never be created or destroyed. it can be transformed from one form to another.

What is the difference between conservative and non-conservative types of forces and energies? - Conservative forces are forces that store energy. These forces include gravity, elastic forces, and electric forces. - Non-conservative forces are forces that do not store energy and lose the energy. Friction and air resistance are non-conservative forces.

What is electrostatic force? Is it conservative or nonconservative? - It is the force that is the result of electrostatic interaction between electrically-charged particles. - It is a conservative force due to the law of electric charge, which states that charges must be conserved.

2. Combine the equations for work and for electric field strength to get a new expression for work. W=Fd E=Fe/q ... = ... Fe =Eq

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? Kinetic Energy and Electric Potential Energy

4. Use this to find an expression for the change in potential energy. U = q(V2 -V1)

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? - 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. - Voltage (V) - Potential difference is a relative value because it depends on which way work is done. It matters whether work is done against the electric field or with the electric field.

=9/20/11= Summary: Lesson 1, Method 1 Electric Field and the Movement of Charge **Electric Field, Work, and Potential Energy** On the other hand, energy would be required to move a massive object against its gravitational field. A stationary object would not naturally move against the field and gain potential energy. Energy in the form of work would have to be imparted to the object by an external force in order for it to gain this height and the corresponding potential energy. The important point to be made by this gravitational analogy is that work must be done by an external force to move an object against nature - from low potential energy to high potential energy.

In a similar manner, 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 in turn 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//. Consider the diagram above in which a positive source charge is creating an electric field and a positive test charge being moved against and with the field. In Diagram A, the positive test charge is being moved against the field from location A to location B. Moving the charge in this direction would be like going against nature. Thus, work would be required to move the object from location A to location B and the positive test charge would be gaining potential energy in the process. This would be analogous to moving a mass in the uphill direction; work would be required to cause such an increase in gravitational potential energy. In Diagram B, the positive test charge is being moved with the field from location B to location A. This motion would be natural and not require work from an external force. The positive test charge would be losing energy in moving from location B to location A. This would be analogous to a mass falling downward; it would occur naturally and be accompanied by a loss of gravitational potential energy. One can conclude from this discussion that 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.  Just as we reasoned here, moving a positive test charge against the electric field will require work and result in a gain in potential energy. On the other hand, a positive test charge will naturally move in the direction of the field without the need for work being done on it; this movement will result in the loss of potential energy.

Electric Potential Work would in turn increase the potential energy of the object. Potential energy is the stored energy of position of an object and it is related to the location of the object within a field.

**The Gravitational Analogy Revisited**

Consider the electric field created by a positively charged Van de Graaff generator. The direction of the electric field is in the direction that a positive test charge would be pushed; in this case, the direction is outward away from the Van de Graaff sphere. Work would be required to move a positive test charge towards the sphere against the electric field. 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. If two objects of different charge - with one being twice the charge of the other - are moved the same distance into the electric field, then the object with twice the charge would require twice the force and thus twice the amount of work. This work would change the potential energy by an amount that is equal to the amount of work done. > 1) Electric charge - a property of the object experiencing the electrical field, and2) Distance from source - the location within the electric field While electric potential energy has a dependency upon the charge of the object experiencing the electric field, electric potential is purely location dependent. Electric potential is the potential energy per charge. A positive test charge would be at a high electric potential when held close to a positive source charge and at a lower electric potential when held further away. In this sense, electric potential becomes simply a property of the location within an electric field.

 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. When a Coulomb of charge (or any given amount of charge) possesses a relatively large quantity of potential energy at a given location, then that location is said to be a location of high electric potential. This part will be devoted to an understanding of electric potential difference and its application to the movement of charge in electric circuits. Consider the task of moving a positive test charge within a uniform electric field from location A to location B as shown in the diagram at the right. In 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. This difference in electric potential is represented by the symbol **V** and is formally referred to as the **electric potential difference**. 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. In equation form, the electric potential difference is The standard metric unit on electric potential difference is the volt, abbreviated **V.** One Volt is equivalent to one Joule per Coulomb.Because electric potential difference is expressed in units of volts, it is sometimes referred to as the **voltage**.

=9/21/11= Equation for EPE (electric potential energy)

W=Fd, Fe=Eq so W=Eqd and W=EPE so EPE = Eqd

also

-W=EPE

Electric Potential Difference: V=voltage

V= change in potential energy/q = -W/q

Other names for voltage: - electric potential difference - electric potential - electric pressure

Guide Question #7

Pre-Lab Activity 1. The objective is stated in the title. What is your hypothesis? (Attempt to answer the question, to the best of your knowledge.) I believe that no matter how many electric field lines there are, the equipotentials will always be the same distance from the charge for every field line.

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.) - I already know that electric field lines show where the electric field of a certain charge is pointing (the direction). - I also know that equipotentials represent the same electric potential at a certain point on electric field lines. Overall, due to this knowledge, I feel that no matter how many lines there are, or what the charges are, the equipotentials will always be the same distance from the charges for every electric field line.

3. How do you think you might test this hypothesis? (What might you measure and how?) We might test this by using a big board to chart specific points and determine what the electric potential is from each charge.

4. Predict the electric field lines (and the equipotential surfaces) of the following situations: a. Two point sources (one negative and one positive)

b. A circle (negatively charged) and a positive point charge in the very center of it.

c. Two lines of charge (one negative and one positive)

=9/22/11=
 * Lab: What is the relationship between electric field lines and equipotentials?**
 * Due Date: 9/26/11**

To determine the relationship between electric field lines and equipotentials through the use of 4 different situations using positive and negative charges.
 * PURPOSE:**

I believe that the equipotentials for the field lines will be the same distance from the charge. The areas around the positive charges will have a higher voltage, whereas the areas around the negative charges will have a lower voltage. This will occur because when a particle gets closer to a negative charge, the amount of electric potential goes down because it is closer to the final destination, the negative charge.
 * HYPOTHESIS:**

|| Volt meter (VOM)
 * MATERIALS:**
 * Alligator leads (2) || Metal push pins (2) ||
 * Cork board || Power supply || Silver marker ||

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.
 * PROCEDURE:**

//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.

//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.

2+ Charges: Chris Hallowell, Ryan Listro, Eric Solomon
 * DATA:**

Dipole: Sam Fihma, Steve Thorwarth, Phil Litmanov

Parallel "Plates": Richie Johnson, Bret Pontillo, Allison Irwin

Circle: Ross Dember, Erica Levine, Rebecca Rabin

2+ Charges: Top View Side View
 * GRAPHS:**

Dipole: Top View Parallel "Plates": Top View Side View

Circle: Top View

Side View

2+ Charges:
 * ANALYSIS**:

Dipole:

Parallel "Plates":

Circle: I feel that these lines look like they should in theory for the most part. If they were perfect, the lines would not wiggle at all, but due to the possible inaccuracies when obtaining the data, the lines were not completely straight.

Each graph in the analysis section shows the electric field lines moving away from the positive charges and toward the negative 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. I believe that our results turned out pretty well. Overall, the shapes of the electric field lines were relatively correct. The only possible issue was the fact that the equipotential levels were not perfect, therefore the electric field lines wiggled in some locations on the graphs. My hypothesis was correct in the sense that the areas around the positive charges will have a higher voltage and the areas around the negative charges will have lower voltages. However, the beginning of my hypothesis was not correct in all the graphs. The equipotential levels varied slightly in their distances from the charges. We were able to see this in the different colors of the graphs. Very often, one color would be seen in multiple places on the graph and the shapes of the circles would not be consistent. My hypothesis was incorrect because there were some possible sources of error in the experiment. The first source of error could have come from the device that we were obtaining the voltages from. When we touched a certain point, the instrument would flash many numbers across the screen and it was very hard to figure out which number was exactly correct. Another source of error could have come from the atmosphere of our experiment. The day in which we conducted our experiment was very warm and humid. This could have altered the correct measurements and as a result, affected our data and graphs. In the future, this lab could be altered by using a device that gave a definite voltage in a certain location. The device we used was useful, however many errors could have resulted from its performance.
 * CONCLUSION**:

=9/23/11=

Example Problem

Set-up

Work