 # Chapter 16 Electric Charge and Electric Field 2002

• Slides: 15 Chapter 16 Electric Charge and Electric Field © 2002, B. J. Lieb Giancoli, PHYSICS, 5/E © 1998. Electronically reproduced by permission of Pearson Education, Inc. , Upper Saddle River, New Jersey Ch 16 Structure of the atom (protons, neutrons and electrons) • Nucleus contains positive charge • Protons have positive charge, neutrons are neutral • Mass of proton (and neutron) 1800 x mass of electron • Electrons have negative charge and are attracted to nucleus • Charge of electron is equal in magnitude to that of proton • Normal atom is neutral • Ion is atom that has gained or lost one or more electrons Ch 16 2 Conductors and Insulators Conductor: metal atoms in solids have one or two “free” electrons which move freely through the material Insulator: no free electrons so it will not conduct electricity Static Electricity: • Rubbing certain materials together can separate electrons from their atoms • Removing electrons from a material makes it positive • In solids, it is always the electrons that move because they are lighter • Induced charge: If you bring a + charge near a conductor, it will attract electrons to it leaving the other half of the metal positive. Ch 16 3 Coulomb’s Law • k 9. 0 109 N·m 2 / C 2 • Forces are equal in magnitude but opposite in direction • This equation gives the magnitude of the force -you have to figure the direction from the signs of the charges • For spherical charges, r is the center to center distance • C is the unit of charge-the Coulomb • Coulomb’s Law applies to charges at rest • e = 1. 60 x 10 -19 C electronic charge (positive) Ch 16 4 Example 1 Ch 16 5 Example 2 Ch 16 6 The Electric Field • Graphical representation of electrical forces • Electrical force “acts at a distance” like gravity • Electric field E surrounds every charge • We investigate the field with a small positive charge called a “test charge” q • The magnitude of the field is: • Units are N/C • E is a vector –direction of force experienced by positive test charge • The magnitude of q is so small that it does not disturb the charges that cause the field • To plot the field, move the test charge around the charges that cause the field • Since q is positive the field points away from a + charge and towards a - charge Ch 16 7 Field of a Point Charge q = Q 2 This is the field created by a point charge or a spherical charge distribution Q Ch 16 8 Electric Field is a Vector • Since test charge is positive, the direction of the electric field is the direction of the force felt by a positive charge • Field thus points toward a negative charge and away from a positive charge • If there are two or more charges creating the field then the field at any point is the vector sum of the fields created by each of the charges • The test charge does not contribute to the field and it is too weak to cause any of the charges creating the filed to move. Ch 16 9 Electric Field Lines • Graphical way of showing the electric field • You have seen graphical representations of the earth’s magnetic field-the electric field maps are similar. • We also refer to the gravitational field of the earth. • Sometimes called lines of force • Arrow on field line gives direction of force • The closer together the lines of force are, the stronger the electric field • Electric field lines start on positive charges and end on negative charges Ch 16 10 Electric Field of Point Charges Ch 16 11 Electric Field of Parallel Plates Ch 16 12 Electric Fields and Conductors • If a conductor is placed in an electric field, the electrons will rearrange themselves until the field inside the conductor is zero • This is called the static situation • In the static situation, the field outside the conductor is perpendicular to the surface of the conductor (if the field had a component parallel to the surface, it would cause the electrons to move. ) • The field inside a hollow conductor shell is zero (Fig 16 -33) • If a charge is placed within a spherical conductor shell, there is a field inside and outside the shell, but not in the conductor (Fig 16 -31) Ch 16 13 Electric Field Examples Study examples 16 -6, 16 -7 and 16 -8. Since the electric field is a vector, it is necessary to calculate the field created by each charge and then use vector addition to determine the net field. Ch 16 14 Example 3 Ch 16 15