Electric Charge and Electric Field 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

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)  1800x 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 Ch16

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 Ch16

Static 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 free electrons that move Electrical charge on the plastic rod induces a separation of charge in scraps of paper and thus attracts them. Ch16

Induced Charge (a) If you bring a + charge near a conductor, it will attract electrons to it leaving the other half of the metal positive. (b) If they touch, then electrons move to the positively charged object, leaving the conductor positively charged. Ch16

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 Ch16

Coulomb’s Law C is Coulomb -- the unit of charge k  9.0  109 N·m2 / C2 C is Coulomb -- the unit of charge e = 1.60x10-19 C electronic charge (positive) Ch16

Coulomb’s Law Ch16

Example 1. Particles of charge Q1 = +5. 00 C, Q 2 = -6 Example 1. Particles of charge Q1 = +5.00 C, Q 2 = -6.0 C and Q3 = +8.0 C are placed in a line separated by 0.40 m between each pair. Calculate the force on Q2. _ Q2 + Q1 + Q3 This is the magnitude, we get direction from charges. Force is directed to the right. Ch16

Example 2. Particles of charge Q1 = +5. 00 C, Q 2 = -6 Example 2. Particles of charge Q1 = +5.00 C, Q 2 = -6.00 C and Q3 = +8.00 C are placed on the corners of a square of side 0.400 m as shown below. Calculate the force on Q2 (Magnitude and direction). + Q1 Note that the charges and distances are the same as in Example 1, so we do not need to use Coulombs Law again.  _ Q2 + Q3 Ch16

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: Ch16

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 Ch16

Field of a Point Charge Q This is the field created by a point charge or a spherical charge distribution Q Ch16

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. Ch16

E1 E2 Example 3A. 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. + Q1 _ Q2 E1 E2 Ch16

Example 3B. 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. + Q1 _ Q2 In part A we found that E = 2.1x107 N/C and is directed to the right. ( to left ) ( to left ) Ch16

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. + Q1 _ Q2 Ch16

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). Ch16

Electric Field of Point Charges

Electric Field of Point Charges

Electric Field of Parallel Plates Ch16

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. Ch16

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. Ch16