Electric Forces & Fields PDF

Chapter 15 Electric Forces and Electric Fields 18.1 The Origin of Electricity The electrical nature of matter is inherent in atomic structure. m p 1.673 10  27 kg mn 1.675 10  27 kg me 9.1110  31 kg e 1.60 10  19 C...

Chapter 15 Electric Forces and Electric Fields 18.1 The Origin of Electricity The electrical nature of matter is inherent in atomic structure. m p 1.673 10  27 kg mn 1.675 10  27 kg me 9.1110  31 kg e 1.60 10  19 C coulombs Electrons have a charge of –e Protons have a charge of +e Two types of charges exist They are called positive and negative Like charges repel and unlike charges attract one another. Nature’s basic carrier of positive charge is the proton. Protons do not move from one material to another because they are held firmly in the nucleus. Nature’s basic carrier of negative charge is the electron. Gaining or losing electrons is how an object becomes charged. Electric charge is always conserved. Charge is not created, only exchanged Objects become charged because negative charge is transferred from 18.2 Charged Objects and the Electric Force It is possible to transfer electric charge from one object to another. The body that loses electrons has an excess of positive charge, while the body that gains electrons has an excess of negative charge. 18.2 Charged Objects and the Electric Force LAW OF CONSERVATION OF ELECTRIC CHARGE During any process, the net electric charge of an isolated system remains constant (is conserved). 18.2 Charged Objects and the Electric Force Like charges repel and unlike charges attract each other. 18.3 Conductors and Insulators Not only can electric charge exist on an object, but it can also move through an object. Substances that readily conduct electric charge are called electrical conductors. Conductors are materials in which the electric charges move freely in response to an electric force. Copper, aluminum and silver are good conductors. When a conductor is charged in a small region, the charge readily distributes itself over the entire surface of the material. Materials that conduct electric charge poorly are called electrical insulators. Insulators are materials in which electric charges do not move freely. Glass and rubber are examples of insulators.  Conductors Conductors are materials in which the electric charges move freely in response to an electric force. – Copper, aluminum and silver are good conductors. – When a conductor is charged in a small region, the charge readily distributes itself over the entire surface of the material. Section 15.2  Insulators Insulators are materials in which electric charges do not move freely. – Glass and rubber are examples of insulators. – When insulators are charged by rubbing, only the rubbed area becomes charged. There is no tendency for the charge to move into other regions of the material. Section 15.2  Semiconductors The characteristics of semiconductors are between those of insulators and conductors. Silicon and germanium are examples of semiconductors. Section 15.2 18.5 Coulomb’s Law F21 or F12 is force exerted by q2 on q1. F12 or F21 is force exerted by q1 on q2. 18.5 Coulomb’s Law COULOMB’S LAW The magnitude of the electrostatic force exerted by one point charge on another point charge is directly proportional to the magnitude of the charges and inversely proportional to the square of the distance between them. q1 q2   8.85 10  12 C 2 N m 2  F k r2 k 1 4 o  8.99 109 N m 2 C 2 18.5 Coulomb’s Law Example: Three Charges on a Line Determine the magnitude and direction of the net force on q1. 18.5 Coulomb’s Law q1 q2 8.99 109 N m 2 C 2 3.0 10 6 C 4.0 10 6 C  F12 k 2  2.7 N r 0.20m 2 F13 k q1 q3  8.99 10 9   N m 2 C 2 3.0 10  6 C 7.0 10 6 C  8.4 N r2 0.15m 2    F F12  F13  2.7 N  8.4 N 5.7N Activity Three point charges are aligned along the x-axis as shown in the figure. Draw free body (force) diagram for each charge. From each diagram write down the corresponding formula of the net force on each charge. Six 18.5 Coulomb’s Law Activity Determine the magnitude of the net force on the charge at the origin (5 nC).