ch33_kmlv

Chapter 33: Electric Fields and Potential

33.1 Electric Fields [] "The satellite and the electron both experience forces; they are both in force fields." Source: Hewitt, Paul G., __Conceptual Physics.__ Menlo Park, California: Addison Wesley Longman, Inc., 1999.
 * The force field that surrounds a mass is a gravitational field.
 * “Action at a distance” is the idea that things not in contact could exert forces on one another.
 * An electric field is a force field that fills the space around every electric charge or group of charges. Measured by force per charge (N/C).

**33.2 Electric Field Lines** [] "Imaginary lines with arrow heads show direction along which hypothetical positive charges would move." Source: Hewitt, Paul G., __Conceptual Physics.__ Menlo Park, California: Addison Wesley Longman, Inc., 1999.
 * There are 3 rules to drawing electric field lines:
 * 1) Field lines must begin on positive charges or at infinity and must terminate on negative charges or at infinity.
 * 2) The number of lines drawn leaving a positive charge or approaching a negative charge is proportional to the magnitude of the charge.
 * 3) No two field lines from the same field can cross each other.



[] (left) "Equal and opposite charges" (middle) "Oppositely charged plates"(right) "Oppositely charged cylinder and plate" Source: Hewitt, Paul G., __Conceptual Physics.__ Menlo Park, California: Addison Wesley Longman, Inc., 1999.

[] "Equal like charges" Source: Hewitt, Paul G., __Conceptual Physics.__ Menlo Park, California: Addison Wesley Longman, Inc., 1999.

33.3 Electric Shielding []
 * Static charges on a conductor try to spread out in order to be as far away from each other as possible.
 * No unbalanced charges will be in the center of a conductor
 * Charges on a conductor will gather on the outside surface
 * <span style="color: #93ebeb; font-family: 'Times New Roman', Times, serif;">Therefore, the largest charge will be where the radius of curvature is the smallest.
 * <span style="color: #93ebeb; font-family: 'Times New Roman', Times, serif;">There is no electric field on the inside of a conductor.

[] "(left) Work is done to lift the ram of the pile driver against the gravitational field of the earth. In an elevated position, the ram has gravitational potential energy. When released, this energy is transferred to the pile below. (right) Similar energy transfer occurs for electric charges." <span style="font-size: 11pt; color: #d451fb; font-family: Arial, Helvetica, sans-serif;"> Source: Hewitt, Paul G., __Conceptual Physics.__ Menlo Park, California: Addison Wesley Longman, Inc., 1999. [] "The small positive charge has more potential energy when it is closer to the positively charged sphere because work is required to move it to the closer location" <span style="font-size: 3pt; color: #000000; font-family: Arial, Helvetica, sans-serif;">Source: Hewitt, Paul G., __Conceptual Physics.__ Menlo Park, California: Addison Wesley Longman, Inc., 1999.
 * 33.4 Electric Potential Energy**
 * <span style="color: #d451fb; font-family: 'Times New Roman', Times, serif;">Potential energy exists anywhere work has been done to put an object into a new position.
 * <span style="color: #d451fb; font-family: 'Times New Roman', Times, serif;">The work required to move an object in an electric field will be stored as electric potential energy in the object.
 * <span style="color: #d451fb; font-family: 'Times New Roman', Times, serif;">The electric potential energy in an object can be restored by returning the object to its initial position.

**33.5 Electric Potential** [] (left) "An object of greater charge has more electric potential energy in the field of the charged dome than an object of less charge, but the electric potential of any amount of charge at the same location is the same." <span style="font-size: 3pt; color: #000000; font-family: Arial, Helvetica, sans-serif;">Source: Hewitt, Paul G., __Conceptual Physics.__ Menlo Park, California: Addison Wesley Longman, Inc., 1999.
 * <span style="color: #008080; font-family: 'Times New Roman', Times, serif;">The Electric potential energy per unit of charge for some location in an electrical field is electric potential.
 * <span style="color: #008080; font-family: 'Times New Roman', Times, serif;">Electric Potential = Electric Potential Energy / Charge
 * <span style="color: #008080; font-family: 'Times New Roman', Times, serif;">Electric potential is measured joules of energy per coulomb of charge or also known as a volt or voltage.

(right) "As one million electrons are added to a neutral balloon, its potential rises from zero to 5000 volts. This is like the 5000 C spark from a sparkler. Temperature is the measure of kinetic energy per molecule, and voltage a measure of the potential energy per coulomb. If there aren't many molecules at the high temperature, or coulombs at high voltage, there is too little energy to do any harm." []

<span style="display: block; font-size: 120%; color: #5353ea; font-family: 'Times New Roman', Times, serif; text-align: center;">** 33.6 Electric Energy Storage ** [] "Practical Capacitors."
 * <span style="color: #5353ea; font-family: 'Times New Roman', Times, serif;">Electrical energy can be stored in capacitors
 * <span style="color: #5353ea; font-family: 'Times New Roman', Times, serif;">Capacitor has two plates or strips, one of which will take a positive charge and the other a negative.
 * <span style="color: #5353ea; font-family: 'Times New Roman', Times, serif;">The opposite charges in a capacitor can be held in place by placing the two plates close together with an insulator between them, until the charges can equalize (usually by connecting an electric circuit between the plates).

** 33.7 Van de Graaff Generator** []
 * <span style="color: #00ff00; font-family: 'Times New Roman', Times, serif;">A device for building a very high voltage charge on a metal sphere which can then be used for various purposes.
 * <span style="color: #00ff00; font-family: 'Times New Roman', Times, serif;">Works by causing a small static charge to be deposited on a moving belt in the base, the belt carries the charge to the top sphere where it is removed from the belt on the inside of the sphere. The charge then travels to the surface of the sphere leaving the generator ready to move more charge off the belt. (Will continue until a charge large enough to cause a long spark to some other object or to ionize a way into the air is created).

**Review:** Terms: <span style="font-family: Symbol; mso-fareast-font-family: Symbol; mso-bidi-font-family: Symbol; msofareastfontfamily: Symbol; msobidifontfamily: Symbol; msolist: Ignore;">· __**Electric Field**__- A force field that fills the space around every electric charge or group of charges. Measured by force per charge (N/C). <span style="font-family: Symbol; mso-fareast-font-family: Symbol; mso-bidi-font-family: Symbol; msofareastfontfamily: Symbol; msobidifontfamily: Symbol; msolist: Ignore;">· **__Electric Potential Energy__**- Energy a charge has due to its location in an electric field. <span style="font-family: Symbol; mso-fareast-font-family: Symbol; mso-bidi-font-family: Symbol; msofareastfontfamily: Symbol; msobidifontfamily: Symbol; msolist: Ignore;">· **__Electric Potential__**- Electric potential energy per coulomb (J/C) at a location in an electric field; measured in volts often called voltage. <span style="font-family: Symbol; mso-fareast-font-family: Symbol; mso-bidi-font-family: Symbol; msofareastfontfamily: Symbol; msobidifontfamily: Symbol; msolist: Ignore;">· **__Van de Graaff Generator-__** An electrostatic machine which uses a moving belt to accumulate very high electro statically stable voltages on a hollow metal globe. <span style="font-family: Symbol; mso-fareast-font-family: Symbol; mso-bidi-font-family: Symbol; msofareastfontfamily: Symbol; msobidifontfamily: Symbol; msolist: Ignore;">· **__Volt-__** The SI unit of electric potential. One volt (symbol V) is the electric potential difference across which on coulomb of charge gains or loses on joule of energy. <span style="font-family: Symbol; mso-fareast-font-family: Symbol; mso-bidi-font-family: Symbol; msofareastfontfamily: Symbol; msobidifontfamily: Symbol; msolist: Ignore;">· **__Voltage-__** Electric potential; measured in volts.

Questions:

1. What is meant by the expression action at a distance?

2. How are gravitational and electric fields similar?

3. Why is an electric field considered a vector quantity?

4. How do the directions of field lines compare with the direction of the force that acts on a positive test charge in the same region?

5. How is the strength of an electric field indicated with field lines?

6. How do the electric field lines appear when the field has the same strength at all points in a region?

7. Why are occupants in a car struck by lightening safe?

8. What is the relationship between the amount of work you do on an object and its potential energy?

9. How can the electric potential energy of a charged particle in an electric field be increased?

10. What will happen to the electric potential energy of a charge particle in an electric field when the particle is released and free to move?

11. How is an electric field different from a gravitational field?

12. Suppose that the strength of the electric field about an isolated point charge has a certain value at a distance of 1m. How will the electric field strength compare at a distance of 2m from the point charge?

13. a) If you do 12 J of work to push 0.001 C of charge from point A to point B in on electric field, what is the voltage difference between points A and B? b) When the charge is released, what will be its kinetic energy as it flies back past its starting point A? What law guides your answer?

14. a) Suppose that you start with a charge of 0.002 C, twice the charge of the previous example, and find that it takes 24 J of work to move it from point A to point B. Now what is the voltage difference between points A and B? b) If this charge is released, what will be its kinetic energy as it flies back past point A?

Answers:

1. Action at a distance refers to the interaction between two (or more) objects that are not in contact with each other.

2. Gravitational and electric fields are similar in that they both are a means of exerting a force between to objects not directly in contact with each other.

3. An electric field is considered a vector quantity because it has both a magnitude and a direction.

4. The electric field lines will point in the direction of the force a positive charge would feel if it were placed in the electric field at the point.

5. When drawing electric field lines the strength of the field is indicated by how close together the field lines are drawn.

6. If the electric field has the same strength at all points in a region (such as between two charge parallel plates) then the electric field lines will be parallel and equally spaced.

7. Occupants of a car are safe when struck by lightening because the charge will travel along the conductive outside of the car, the inside of the car will be shielded from the effect.

8. The amount of work done in moving an object is equal to the object’s change in potential energy (plus the change in other forms of energy if they change as well).

9. The electric potential energy of a charged particle in an electric field can be increase by doing work on the particle in a direction opposite that of the electric field.

10. When a charged particle in an electric field is released and is free to move the object’s potential energy will be converted to kinetic energy.

11. An electric field is different from a gravitational field in that an electric field acts on charge rather than mass, it can be directed outward in addition to inward, and it can be shielded by the presence of a conductor.

12. By the inverse square law if the distance between two charges changes from 1 m to 2 n (doubles) then the force between them will decrease to ¼ of the original value.

13. a) Electric potential (voltage) is the ratio of the charge in electric potential energy to the charge involved, and work is the change in potential energy so: math \displaystyle V = {(\frac {\Delta PE}{q})} ={(\frac {W}{q})}= {(\frac {12 J}{0.001 C})} = 12,000 V math

b) The principle of conservation of energy says that the energy release will be equal to the energy pout into the system when t he object was moved, therefore the energy release will be 12 J.

14. a) math \displaystyle V = {(\frac {\Delta PE}{q})} ={(\frac {W}{q})}= {(\frac {24 J}{0.002 C})} = 12,000 V math b) The same as it took to move the object, 24 J.

Source: Hewitt, Paul G., __Conceptual Physics.__ Menlo Park, California: Addison Wesley Longman, Inc., 1999.