Chapter 16 Electric Charge and Electric Field 2008

Chapter 16 Electric Charge and Electric Field © 2008 Giancoli, PHYSICS, 6/E © 2004. 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 protons and neutrons • Protons have positive charge, neutrons are neutral • Mass of proton ≈ mass of neutron • 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 more “free” electrons per atom which move freely through the material Insulator: no free electrons so it will not conduct electricity Ch 16 3

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 free electrons that move • Electrical charge on the plastic rod induces a separation of charge in scraps of paper and thus attracts them. Ch 16 4

Induced Charge 1. (a) If you bring a + charge near a conductor, it will attract electrons to it leaving the other half of the metal positive. 2. (b) If they touch, then electrons move to the positively charged object, leaving the conductor positively charged. Ch 16 5

Coulomb’s Law • Forces are equal in magnitude but opposite in direction • For spherical charges, r is the center to center distance • This equation gives the magnitude of the force--you have to figure the direction from the signs of the charges Ch 16 6

Coulomb’s Law • k 9. 0 109 N·m 2 / C 2 • C is Coulomb -- the unit of charge • e = 1. 60 x 10 -19 C electronic charge (positive) Ch 16 7

Coulomb’s Law Ch 16 8

Example 1. Particles of charge Q 1 = +5. 00 C, Q 2 = -6. 0 C and Q 3 = +8. 0 C are placed in a line separated by 0. 40 m between each pair. Calculate the force on Q 2. Q 1 + Q 2 _ Q 3 + This is the magnitude, we get direction from charges. Ch 16 Force is directed to the right. 9

Example 2. Particles of charge Q 1 = +5. 00 C, Q 2 = -6. 00 C and Q 3 = +8. 00 C are placed on the corners of a square of side 0. 400 m as shown below. Calculate the force on Q 2 (Magnitude and direction). Q 1 Note that the charges and distances are + the same as in Example 1, so we do not need to use Coulombs Law again. Q 2 Ch 16 _ + Q 3 10

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 field is given by: Ch 16 11

The Electric Field • 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 12

Field of a Point Charge Q This is the field created by a point charge or a spherical charge distribution Q Ch 16 13

Electric Field is a Vector • Field thus points toward a negative charge and away from a positive charge • Since test charge is positive, the direction of the electric field is the direction of the force felt by 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 field to move. Ch 16 14

Example 3 A. A +100 C point charge is separated from a -50 C charge by a distance of 0. 50 m as shown below. (A) First calculate the electric field at midway between the two charges. (B) Find the force on an electron that is placed at this point and then calculate the acceleration when it is released. Q 1 Q 2 E 1 E 2 _ + Ch 16 15

Example 3 B. A +100 C point charge is separated from a -50 C charge by a distance of 0. 50 m as shown below. (A) First calculate the electric field at midway between the two charges. (B) Find the force on an electron that is placed at this point and then calculate the acceleration when it is released. Q 1 Q 2 + _ In part A we found that E = 2. 1 x 107 N/C and is directed to the right. ( to left ) Ch 16 16

Example 4. A +100 C point charge is separated from a -50 C charge by a distance of 0. 50 m as shown below. Sketch the electric field at the point x as shown. Q 1 + Ch 16 Q 2 _ 17

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. • 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 are directed out from positive charges (a) and in toward negative charges (b). Ch 16 18

Electric Field of Point Charges Ch 16 19

Electric Field of Point Charges Ch 16 20

Electric Field of Parallel Plates Ch 16 21

Electric Fields and Conductors • 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 in the conductor to move until there was only a perpendicular component. Ch 16 22

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 • The field inside a hollow conductor shell is zero (Fig 16 -33) • This makes a metal car a relatively safe place in an electrical storm. Ch 16 23