Ch19_ListroR

toc Chapter 19: Electrical Potential and Capacitance

= Guiding Questions =

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 what permits the ability to do work.

What is work? - It is the product of force times distance.

When is energy conserved? - Energy is always conserved due to the law of conservation of energy, which states it can never be created or destroyed; it can only be transferred.

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

What is electrostatic force? Is it conservative or nonconservative? - It's the force that results from the electrostatic interaction between electrically-charged particles. - It's 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=F*d E=Fe/q --> Fe =E*q

W= E*q*d

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)

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? = Classwork: 9/19 = = Electric Potential Difference Summary = = Classwork: 9/21 = = Lab: Electric Field Lines and Equipotentials = **Pre-Lab:** 1. The objective is stated in the title. What is your hypothesis? (Attempt to answer the question, to the best of your knowledge.) I think the equipotentials will always have the same strength and 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.) - Electric field lines show where the electric field of a certain charge is pointing (the direction). - Equipotentials represent the same electric potential at a certain point on electric field lines. Due to this knowledge, I can conclude 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 may test this by using a big board to chart specific points and measure the voltage at each point to 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)

**Purpose:** To determine electric field lines and equipotentials through the observation of 4 different situations involving positive and negative charges.

**Hypothesis/ Rationale:** I think the equipotentials will be of equal strength when they are the same distance from the source charge. Also, I believe the strength of the area closer to a positive charge will be stronger than those closer to a negative one because electric potential energy decreases as a particle moves closer. 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.

**Materials:** Volt meter, Alligator leads, Metal Push Pins, Cork Board, Power Supply, Silver Marker

**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. //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.

**Data:** 2+ Charges: Chris Hallowell, Ryan Listro, Eric Solomon Dipole: Sam Fihma, Steve Thorwarth, Phil Litmanov Parallel "Plates": Richie Johnson, Bret Pontillo, Allison Irwin Circle: Ross Dember, Erica Levine, Rebecca Rabin

**Graph:** __2+ Charges__ Top- Side- __ Parallel __ Top- Side-

__Dipole__ Top- Side-

__ Circle __ Top- Side- **Analysis:** 2+ Charges

At the two positive charges, there are "mountains," meaning that the charge is the greatest at each point. The equipotentials are concentric circles that surround each charge, and as the equipotentials from each pole meet, the circles become tighter and tighter. For the most part, this looks like it should in theory, although there is a slight discrepancy shown in the side view, which displays a difference between the two charges.

Dipole

This shows a "mountain" at the positive charge and a "valley" at the negative one. The equipotentials are concentric circles that surround each pole. There are slight discrepancies in the orange part of the graph (side view). In theory, there should not be a small "mountain" at about (8,12) in the side view.

Parallel Here, the two charge are parallel lines, thus creating equipotentials that follow the same pattern. This looks like it should in theory. Circle

The "mountain" in the center of the circle represents the positive charge, which has the greatest point of charge. The equipotentials are concentric circles that surround the center charge. This looks like it should in theory.

**Conclusion:** The four graphs plotted by the various groups were the Dipole, 2 + charges, Parallel Plates, and Circle. Each graph shows the various equipotential regions of the respective layout and the electric field lines corresponding to the equipotentials. The electric field lines move away from the positive and toward the negative in the Dipole, Circle, and Parallel Plate graphs. In the 2+ charges graph, the electric field lines move away from the other positive to a more negative area, which increases as the distance from the charges increases. In all the graphs the electric field lines run perpendicular to the equipotential boundary. All the results turned out pretty well. Because the graphs clearly show that by the positive charge the electric potential is high and by the negative charge the electric potential is low, my hypothesis was correct. The areas around the positive charge did have the highest voltage, which was shown in the "mountains", and the equipotential levels were all at points equidistant from the charge. However, the results were not perfect, which is due to the many factors of error experienced in this lab. The equipotential regions should all look even and when drawn, the electric field lines should be smooth without any imperfections. These inconsistencies are due to several sources of error, one source being that when pressing the device to the chart, the readings could be manipulated based on how much pressure was applied. More pressure gave higher readings and vice versa. It was very difficult to know if the device was being pressed with equal pressure every time, and thus resulted in human error and left inconsistencies in the graphs and equipotential regions. In addition, the actual meter where the readings showed up was not consistent, due to the variances in pressure. Because of this along with the fluctuating reading on the volt meter, we had to often times guess the number. In the future, I would improve the device we used to measure the voltage because there needs to be a way to keep the pressure applied consistent.

= **Classwork: 9/23** =