Lecture Outline Chapter 15 College Physics, 7 th Edition Wilson / Buffa / Lou © 2010 Pearson Education, Inc.

Units of Chapter 15 Electric Charge Electrostatic Charging Electric Force Electric Field Conductors and Electric Fields Gauss’s Law for Electric Fields: A Qualitative Approach © 2010 Pearson Education, Inc.

15.1 Electric Charge Electric charge is a fundamental property of matter; electric charges may be positive or negative. The atom consists of a small positive nucleus surrounded by a negative electron cloud. It’s easier to use a solar system model. © 2010 Pearson Education, Inc.

15.1 Electric Charge The Law of Charges or the Charge-Force Law: Like charges repel; unlike charges attract. © 2010 Pearson Education, Inc.

15.1 Electric Charge SI unit of charge: the coulomb, C. All charges are integer multiples of the charge on the electron: © 2010 Pearson Education, Inc.

15.1 Electric Charge The Law of Conservation of Charge: The net charge of an isolated system remains constant. Aka…charge must be conserved! It’s easy to think of conservation of charge if using ionic compounds as the example. © 2010 Pearson Education, Inc.

15.1 Electric Charge When you shuffle across a carpeted floor on a dry day, the carpet acquires a net positive charge. A.) Will you have a deficiency or excess of electrons? Why? B.) If the charge the carpet acquired has a magnitude of 2.15 nC, how many electrons were transferred?

15.2 Electrostatic Charging Conductors transmit charges readily. Semiconductors are intermediate; their conductivity can depend on impurities and can be manipulated by external voltages. Insulators do not transmit charge at all. © 2010 Pearson Education, Inc.

15.2 Electrostatic Charging An electroscope may be used to determine if an object is electrically charged. Let’s talk about how an electroscope works. © 2010 Pearson Education, Inc.

15.2 Electrostatic Charging 4 methods of Electrostatic Charging: –Charging by Friction –Charging by Conduction (Contact) –Charging by Induction –Charge Separation by Polarization

15.2 Electrostatic Charging Charging by friction: Certain insulator materials are rubbed (typically with cloth or fur) and they become electrically charged by a transfer of charge. The transfer of charge is due to the frictional contact between the materials. Walking across the carpet in the winter. “Static Cling” Rubbing a Balloon on your Hair © 2010 Pearson Education, Inc.

15.2 Electrostatic Charging An electroscope can be given a net charge by conduction—when it is touched with a charged object, the excess charges flow freely onto the electroscope. “Conduction” refers to the flow of charge during a short period of time due to contact” © 2010 Pearson Education, Inc.

15.2 Electrostatic Charging An electroscope may also be charged by induction, if there is a way of grounding it while charge is being induced. Induction is a method used to charge without actually touching the object. © 2010 Pearson Education, Inc.

15.2 Electrostatic Charging Charge may also be moved within an object—without changing its net charge—through a process called polarization. Polarization is a separation of positive and negative charge. © 2010 Pearson Education, Inc.

15.3 Electric Force The force exerted by one charged particle on another is given by: [This is the magnitude of force between 2 point charges.] Called Coulomb’s Law. © 2010 Pearson Education, Inc.

15.3 Electric Force A rubber comb combed through dry hair can acquire a net negative charge. That comb will then attract small pieces of uncharged paper. Since the paper has no net charge, you might expect there to be no electric force on it. Which charging mechanism explains this phenomenon, and how does it explain it? –Conduction –Friction –Polarization

15.3 Electric Force Two point charges of -1.0 nC and +2.0 nC are separated by a distance of 0.30m. What is the electric force on each particle?

15.3 Electric Force What is the magnitude of the repulsive electrostatic force between two protons in a nucleus? Take the distance from the center of these protons to be 3.00 x m. If the protons were released from rest, what would the magnitude of the acceleration be? How does this compare with acceleration due to gravity?

15.3 Electric Force A configuration of three charges is shown in the figure below. What is the net electric force on q 3.

15.4 Electric Field Definition of the electric field: The direction of the field is the direction the force would be on a positive charge. © 2010 Pearson Education, Inc.

15.4 Electric Field Charges create electric fields, and these fields in turn exert electric forces on other charges. Electric field of a point charge: © 2010 Pearson Education, Inc.

15.4 Electric Field In the figure below, what quadrant is the electric field at the origin? The first? Second? Third? Explain. Calculate the magnitude and direction of the electric field at the origin due to these charges.

15.4 Electric Field A convenient way of graphically representing the electric field is by use of electric lines of force (electric field lines). There are rules that dictate field lines.

15.4 Electric Field Rules for drawing electric field lines: 1. Closer lines mean a stronger field. 2. The field is tangent to the lines at every point. 3. Field lines start on positive charges and end on negative charges. 4. The number of lines entering or leaving a charge is proportional to the magnitude of the charge. 5. Field lines never cross. © 2010 Pearson Education, Inc.

15.4 Electric Field

Use the electric field line rules to construct a typical electric field line due to an electric dipole. © 2010 Pearson Education, Inc.

15.4 Electric Field

15.5 Conductors and Electric Fields Electric charges are free to move within a conductor; however, in a static electric field, electrons are at rest which means there is no electric field: The electric field is zero inside a charged conductor. Excess charges on a conductor will repel each other, and will wind up being as far apart as possible. Any excess charge on an isolated conductor resides entirely on the surface of the conductor. © 2010 Pearson Education, Inc.

15.5 Conductors and Electric Fields The electric field at the surface of a charged conductor is perpendicular to the surface. © 2010 Pearson Education, Inc.

15.5 Conductors and Electric Fields Excess charge tends to accumulate at sharp points, or locations of highest curvature, on charged conductors. As a result, the electric field is greatest at such locations. © 2010 Pearson Education, Inc.