Ch19_JohnsonR

= Lab: What is the relationship between electric field lines and equipotentials? = toc AP Physics

Prelaboratory Assignment

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

The equipotential lines will be the same distance from the charge. This is due a charges always spreading out equally from the original source. What will also happen is that higher voltages will be recorded around the positive charges and lower voltages will be recorded around the negative charges. This will happen because the particles that are traveling want to reach the negative charge and get away from the positive charge. This makes it harder to put the particles by the positive charge and easier to put it by the negative, thus giving the area around the negative less voltage and the area around the positive more voltage.


 * 1) 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.)

We know that charges move around equally from the source charge causing the equipotential lines to be the same distance from the charge. We also know that the particles coming from the charges want to reach the negative charge and leave the positive thus making it harder to place them close to the positive. That fact is why there will be higer voltage by the positive charge and lower voltage by the negative.


 * 1) How do you think you might test this hypothesis? (What might you measure and how?)

What we could do is place both a positive and negative charge onto a board with a grid on it. Then we could record the amount of charge on each point in order to see if our hypothesis was correct.


 * 1) Predict the electric field lines (and the equipotential surfaces) of the following situations:
 * 2) Two point sources (one negative and one positive)[[image:PHYSICS_PHOTO_1.jpg width="640" height="480"]]
 * 3) A circle (negatively charged) and a positive point charge in the very center of it.[[image:PHYSICS_PHOTO_2.jpg]]
 * 4) Two lines of charge (one negative and one positive)[[image:PHYSICS_PHOTO_3.jpg]]

In this experiment you will be constructing 3-dimensional plots of differently shaped electric fields. To do this you will measure and plot the electric potential between two charged points on a sheet of conductive paper. You will then view the plots from different perspectives.
 * Introduction ** :

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


 * Procedure ** :

Part A. Preparing the materials
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. 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:__** __**Two Positive Charges**__ crated by Chris Hallowell, Ryan Listro, Eric Soloman

Top View

Side View

The two peaks of the graph are where the two positive charges were located. They are peaks because the area around the positive charged logged the highest amount of volts. The area in between the two charges has levels that are very consistent. This is due to the same amount of charge being on both sides.



This graph shows the electric field lines going out of both positive charges. when the line came close they veered off in order to not cross. The lines are wavy, which should not theoretically happen, but they did veer off which should happen.

__**Dipole**__ created by Sam Fihma, Steve Thorwarth, Phil Litmanov

Top View

Side Views



The two peaks represent where the positive and negative charges were located. The peak on the left is the positve charge and the peak to the right is the negative charge. One can tell this because the positive charge would have the higher voltage around it and the negative will have the lower voltage due to electric potential being small aroung a negative charge and large around a positive when a positive test charge is being aministrated. The area in between the two charges is consitantently getting smaller due to its locations to the negative charge.



This graph shows the electric field lines travel from the positive charge into the negative charge. This is what should theoreticaly happen when there is a positive and negative charge. The lines between them are slightly wavy which they should not be.

__**Parallel "Plates"**__ created by Allison Irwin, Bret Pontillo, Richie Johnson

Top View Side View

This graph shows the positively charged line and the negatively charged line. The positive line is located on the top due to the high voltages, and the negative line is located on the bottom due to its low voltages. The area in between shows the charges gradually getting smaller. This is because they are approaching the negatively charged line.

This graph shows that the electric field lines go from the positive bottom to the negative bottom. These lines are very straight as well. The lines on the top are curving around in order to get closer to the negative. These lines are not straight.

__**Circle**__ created by Ross Dember, Erica Levine, Rebecca Rabin

Top View

Side View

This graph shows a positive test charge in the middle of a negatively charged circle. The middle is where the positive charge was located. This can be determined due to its high voltage. The circular area around the point is where a negative charge was because the values of the voltage around the point are very close to zero. The area in between the circle and the positive charge consistently decreases due to its location being further from the positive charge.

The electric field lines in this graph go out from the positive center and out toward the negative. These lines are very good also because they are straight.

**__CONCLUSION:__**

During this lab we constructed graphs for four different types of charge variations. They were the dipole, two positive charges, parallel plates, and circle. Each graph yielded different results in terms of equipotential lines and electric field lines. In the dipole there were two peaks in the graph located at each charge. The peak on the left was positive due to its high voltage and the peak to the right was negative due to its low voltage. The area in between the two graphs gradually decreased due to the further distance the test charge was from the positive charge. The next was the two positive charges. This graph had two positive peaks located on both sides of the graph. The peaks are known to be positive due to their high voltage. The area in between the two graphs is a constant high voltage, although not as high as the peak. This is due to the two positive charges on both sides. Due to the lack of a negative charge there was no place on the graph in which the values would drop to zero. Another graph was the circle. In this graph there was a positive charge in the center of a negatively charged circle. The graph showed a peak of high voltage located at the positive charge. Then the graph slopes down to zero where the negatively charged circle was located. The final graph was the parallel planes. In this graph there was two lines drawn, one positive and one negative. The positive line was located on the top of the graph and the negative line was located on the bottom of the graph. Again this is due to the high voltage at the top and the low voltage on the bottom. In between the lines the voltage gradually decreased due to its distance from the positive. In the end the graphs turned out pretty well. They all showed the electric field lines in their correct places. They all crossed the equipotential line perpendicularly and also did not cross each other. They also moved from the positive into the negative, which is what they are meant to do. Although they looked good they were not perfect because many of the lines did not flow well and some were not straight. The equipotential was also slightly off. Most had consistent levels around each charge but some had many different colors in the area around the charge, which should not happen. All in all, my hypothesis was slightly correct. All of the things that I had thought would happen did happen but they were not that pretty. For example the equipotential for most of the graphs varied, which should not happen they should have the same value. Another thing that worked but did not work perfectly were the electric force lines. They all went from the positive and to the negative as well as perpendicular to the equipotential lines, but they were not straight and did not flow well. What worked out well was that the area around the positive had higher voltages and the area around the negative has lower voltages. These minor problems were all caused from our error that occurred in this lab. The biggest source of error was the voltmeters that were used. When they were placed on the graph the numbers would fluctuate for a few seconds then stay on a number for a while but change again. This was a nightmare to deal with because you would never know if the reading was going to stay the same. Another problem with the meters was that they would vary their result based off of how hard you would push it on the grid. The harder you pushed the higher reading you would get. This made it so that you would have to apply the same amount of pressure each time. If we were to do this lab over again it would be helpful to use a different type of voltmeter in order to get more consistent results as well as less error.

=__Summary 1:__= Electric fields are similar to gravitational fields

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. On the other hand, work would not be required to move an object from a high potential energy location to a low potential energy location. 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.

the low energy location for a positive test charge is a location nearest a negative source charge and the high energy location is a location furthest away from a negative source charge.

electric potential energy is 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 potential is purely location dependent. Electric potential is the potential energy per charge.

electric potential is used to express the affect of an electric field of a source in terms of the location within the electric field. A test charge with twice the quantity of charge would possess twice the potential energy at a given location; yet its electric potential at that location would be the same as any other test charge.

battery powered electric circuit has locations of high and low potential. the positive terminal is described as the high potential terminal. The negative terminal is described as the low potential terminal. electric fields are based on the direction of movement of positive test charges.

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. And similarly, 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 **V** and is formally referred to as 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. 8 The standard metric unit on electric potential difference is the volt, abbreviated **V** One Volt is equivalent to one Joule per Coulomb. It is sometimes referred to as the **voltage**.

work must be done on a positive test charge to move it through the cells from the negative terminal to the positive terminal. This work would increase the potential energy of the charge and thus increase its electric potential. As the positive test charge moves through the //external circuit// from the positive terminal to the negative terminal, it decreases its electric potential energy and thus is at low potential by the time it returns to the negative terminal. 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.

simple circuit, two parts - an internal circuit and an external circuit. 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** 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. By doing so, the moving charge is losing its electric potential energy. The location just prior to entering any circuit element is a high electric potential location; and the location just after leaving 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. Two simple circuits and their corresponding electric potential diagrams are shown below.
 * Electric Potential Diagrams**

In Circuit A, there is a 1.5-volt D-cell and a single light bulb. In Circuit B, there is a 6-volt battery (four 1.5-volt D-cells) and two light bulbs. In each case, the negative terminal of the battery is the 0 volt location. The positive terminal of the battery has an electric potential that is equal to the voltage rating of the battery. The battery energizes the charge to //pump it// from the low voltage terminal to the high voltage terminal. By so doing the battery establishes an electric potential difference across the two ends of the external circuit. Being //under electric pressure//, the charge will now move through the external circuit. As its electric potential energy is transformed into light energy and heat energy at the light bulb locations, the charge decreases its electric potential. The total voltage drop across the external circuit equals the battery voltage as the charge moves from the positive terminal back to 0 volts at the negative terminal. In the case of Circuit B, there are two voltage drops in the external circuit, one for each light bulb. While the amount of voltage drop in an individual bulb depends upon various factors (to be discussed [|later]), the cumulative amount of drop must equal the 6 volts gained when moving through the battery.