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. © 2010 Pearson Education, Inc.

15.1 Electric Charge Like charges repel; unlike charges attract. © 2010 Pearson Education, Inc.

15.1 Electric Charge integer multiples of the charge on the electron SI unit of charge: the coulomb, C. All charges are integer multiples of the charge on the electron (typically an easy AP test question): © 2010 Pearson Education, Inc.

15.1 Electric Charge Charge is conserved: The net charge of an isolated system remains constant. © 2010 Pearson Education, Inc.

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

15.2 Electrostatic Charging An electroscope may be used to determine if an object is electrically charged. © 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. © 2010 Pearson Education, Inc.

15.3 Electric Force The force exerted by one charged particle on another is given by (Coulomb’s Law): © 2010 Pearson Education, Inc.

15.3 Electric Force If there are multiple point charges, the force vectors must be added to get the net force. © 2010 Pearson Education, Inc.

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 For multiple charges, the total electric field is found using the superposition principle: For a configuration of charges, the total, or net, electric field at any point is the vector sum of the electric fields due to the individual charges. © 2010 Pearson Education, Inc.

15.4 Electric Field It is convenient to represent the electric field using electric field lines, or lines of force. These lines are drawn so the field is tangent to the line at every point – radial lines that are perpendicular to the circular lines. © 2010 Pearson Education, Inc.

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 Electric field lines of a dipole: © 2010 Pearson Education, Inc.

15.4 Electric Field Electric field lines due to like charges: (a) equal charges; (b) unequal charges. © 2010 Pearson Education, Inc.

15.5 Conductors and Electric Fields Excess charges on a conductor will repel each other, and will wind up being as far apart as possible – poles. Any excess charge on an isolated conductor resides entirely on the surface of the conductor – Faraday’s Cage, current in a wire. © 2010 Pearson Education, Inc.

15.5 Conductors and Electric Fields The force from neighboring charges is less when the curvature of the surface is large: 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.

15.6 Gauss’s Law for Electric Fields: A Qualitative Approach If a greater amount of charge is enclosed, more field lines cross the surface. © 2010 Pearson Education, Inc.

15.6 Gauss’s Law for Electric Fields: A Qualitative Approach Here, surface 1 surrounds the positive charge and has lines exiting it. Surface 2 surrounds the negative charge and has lines entering it. Surface 3 does not enclose any charge, and the same number of lines exit as enter. Surface 4 encloses both charges; as they are equal in magnitude, the same number of lines exit the surface as enter it. © 2010 Pearson Education, Inc.

Summary of Chapter 15 Like charges repel; unlike charges attract. Charge is conserved. Electrons move freely inside conductors, but not inside insulators. Objects may be charged electrostatically by friction, conduction, or induction. Polarization is the separation of positive and negative charge within an object. © 2010 Pearson Education, Inc.

Summary of Chapter 15 Coulomb’s law: The electric field, a vector, is the force per unit charge. Electric fields from multiple charges add by superposition. Electric field lines are used to represent the electric field. A conductor has zero electric field inside and has all excess charge on its surface. © 2010 Pearson Education, Inc.

Summary of Chapter 15 The electric field is always perpendicular to the surface of a conductor. The charge density and electric field are greatest on a conductor where the curvature is largest. © 2010 Pearson Education, Inc.