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Part 4 of Our Book: Electricity & Magnetism The “Transrapid Maglev” Train, Shanghai, China. “Maglev” Magnetic Levitation. It makes no contact with the rails! It’s weight is 100% supported by electromagnetic forces!! Section 23.2 Copyright © 2009 Pearson Education, Inc. Chapter 23: Electric Fields The comb & the pieces of paper have opposite static electric charge, so they attract each other. Section 23.2 Copyright © 2009 Pearson Education, Inc. Some Fun with Static Electricity! Mother & daughter are both charged with static electricity. Each hair on their heads is charged & exerts a repulsive force on all other hairs. Section 23.2 Copyright © 2009 Pearson Education, Inc. More Fun with Static Electricity! This woman is electrically charging her body. Each hair becomes charged & exerts a repulsive force on the other hairs, resulting in this “stand-up” hairdo. Copyright © 2009 Pearson Education, Inc. p690 Some Topics in Chapter 23 • Static Electricity; Electric Charge & Its Conservation • Electric Charge in the Atom; Insulators & Conductors • Induced Charge; The Electroscope Coulomb’s Force Law The Electric Field • Electric Field Calculations for Continuous Charge Distributions • • • • • • Electric Field Lines Electric Fields & Conductors Motion of a Charged Particle in an Electric Field Electric Dipoles Electric Forces in Molecular Biology: DNA Some Applications: Photocopy Machines & Computer Printers Use Electrostatics Section 23.2 Copyright © 2009 Pearson Education, Inc. Electricity and Magnetism The Laws of Electricity & Magnetism are Very Important: • In everyday life, they play a central role in the operation of many modern electronic devices. • In basic materials physics, the interatomic and intermolecular forces responsible for the formation of solids and liquids are electric in nature. Section 23.2 Copyright © 2009 Pearson Education, Inc. Brief History of Electricity & Magnetism Ancient Chinese •Some documents suggest that magnetism was observed as early as 2000 BC in China. Ancient Greeks • Electrical and magnetic phenomena were known as early as 700 BC. Experiments with amber & magnetite Section 23.2 Copyright © 2009 Pearson Education, Inc. 1600: William Gilbert • Gilbert showed electrification effects were not confined to just amber. The electrification effects were a general phenomena. 1785: Charles Coulomb Coulomb confirmed the inverse square law form for electric forces. Section 23.2 Copyright © 2009 Pearson Education, Inc. 1819: Hans Oersted • Oersted found that a compass needle deflected when near a wire carrying an electric current. 1831: Michael Faraday & Joseph Henry • Faraday & Henry showed that when a wire is moved near a magnet, an electric current is produced in the wire. Section 23.2 Copyright © 2009 Pearson Education, Inc. 1873: James Clerk Maxwell • Maxwell used observations & other experimental facts as a basis for formulating the laws of electromagnetism. He achieved the Unification of Electricity & Magnetism!!!!! (Maxwell’s Equations!!!) (The “Theme” of Physics 2401!) Section 23.2 Copyright © 2009 Pearson Education, Inc. Electricity & Magnetism: Forces • The concept of Force came originally from Isaac Newton. It connects the study of electromagnetism to one of the main topics of Physics I: Newton’s Laws of Motion! •As we said earlier, The Electromagnetic Force between charged particles is one of the Fundamental Forces of Nature. Section 23.2 Copyright © 2009 Pearson Education, Inc. From Physics I: Newton’s Laws of Motion Newton’s 2nd Law: Based on experiment! Not derivable 1 mathematically!! ∑F = ma A VECTOR equation!! Holds component by component. ∑Fx = max, ∑Fy = may, ∑Fz = maz This is one of the Most Fundamental & Important Laws of Classical Physics!!! Section 23.1: Properties of Electric Charge • Experiments (first done by Coulomb!) show that there are Two kinds of electric charges. They are called positive & negative • Negative Charges Are the type possessed by electrons. • Positive Charges Are the type possessed by protons. • Experiments (Coulomb) also show that: Charges of the same sign repel one another & charges with opposite signs attract one another. Section 23.2 Copyright © 2009 Pearson Education, Inc. Static Electricity Conservation of Electric Charge Experimental Fact Objects can be charged by rubbing. Section 23.2 Copyright © 2009 Pearson Education, Inc. Experimental Facts As we already said Charge comes in 2 types: Positive (+) & Negative (-). Like charges repel, opposite charges attract. Also Electric Charge is Conserved: The arithmetic sum of the total charge cannot change in any interaction Section 23.2 Copyright © 2009 Pearson Education, Inc. •In the figure, the rubber rod is negatively charged. •The glass rod is positively charged. •So, The two rods will attract each other. Section 23.2 Copyright © 2009 Pearson Education, Inc. More About Electric Charges Experimental Fact: Electric charge is always conserved in an isolated system. • For example, charge is not created in the process of rubbing two objects together. • The electrification is due to a Transfer of charge from one object to another. Section 23.2 Copyright © 2009 Pearson Education, Inc. Conservation of Electric Charge Example • A glass rod is rubbed with silk. • Electrons are transferred from the glass to the silk. • Each electron adds a negative charge to the silk. • An equal positive charge is left on the rod. Section 23.2 Copyright © 2009 Pearson Education, Inc. More About Electric Charges Experimental Fact: Electric charge is Quantized • That is, an electric charge q is ALWAYS an integer multiple of the charge on an electron e. • Or, electric charge q exists only as discrete packets: q Ne N Huge integer! e Fundamental Unit of Charge -19 |e| 1.6 10 C Electron: q = -e, Proton: q = +e Section 23.2 Copyright © 2009 Pearson Education, Inc. Insulators and Conductors Conductor A Conductor is a material in which charge flows freely. The most common types of conductors are metals. Section 23.2 Copyright © 2009 Pearson Education, Inc. Insulators and Conductors Insulator An Insulator is a material in which almost no charge flows. Most non metallic materials are insulators. Section 23.2 Copyright © 2009 Pearson Education, Inc. Insulators and Conductors Semiconductor A Semiconductor is a material with special properties, somewhere in between conductors & insulators. Without semiconductors (especially silicon, Si), much of our technology would not exist! Section 23.2 Copyright © 2009 Pearson Education, Inc. More on Conductors • Electrical Conductors are materials in which some of the electrons are “free electrons”. “Free electrons” are not bound to the atoms. “Free electrons” can move relatively freely. through the material. • Examples of good conductors include copper, aluminum and silver. Experimental Fact • When a good conductor is charged in a small region, the charge readily distributes itself over the entire surface of the material. Section 23.2 Copyright © 2009 Pearson Education, Inc. More on Insulators • Electrical Insulators are materials in which all of the electrons are bound to atoms. • These electrons cannot move relatively freely through the material. • Examples of good insulators include glass, rubber and wood. Experimental Fact • When a good insulator is charged in a small region, the charge is unable to move to other regions of the material. Section 23.2 Copyright © 2009 Pearson Education, Inc. More on Semiconductors • The electrical properties of Semiconductors are somewhere between those of insulators & conductors. • Examples of semiconductor materials include silicon & germanium. • These materials are commonly used in making electronic chips. Experimental Fact • The electrical properties of semiconductors can be changed by the addition of controlled amounts of certain atoms to the material. Section 23.2 Copyright © 2009 Pearson Education, Inc. Section 23.2 Charging Objects by Induction Experimental Fact • When a charged object is brought near enough to an uncharged object, the uncharged object can become charged. This process is called Charging by Induction • It is important to note that Charging by Induction requires contact with the object inducing the charge! Section 23.2 Copyright © 2009 Pearson Education, Inc. Example •Assume that we start with a neutral metallic sphere. See Figure a. • Since it is neutral, the sphere has the same number of positive & negative charges. Section 23.2 Copyright © 2009 Pearson Education, Inc. A Similar Example Experimental Fact: • As we’ve just said, metal objects can be charged by induction. Section 23.2 Copyright © 2009 Pearson Education, Inc. • Experiment I: Now, place a charged rubber rod near the sphere. See Figure b. It does not touch the sphere. • The electrons in the neutral sphere are redistributed due to interaction with the rod. See Figure b. Section 23.2 Copyright © 2009 Pearson Education, Inc. Definition Grounding a Conductor The process of placing a conducting wire between the conductor & the earth such that the wire touches both the conductor & the earth. Section 23.2 Copyright © 2009 Pearson Education, Inc. • Experiment II: Ground the charged sphere, while leaving the charged rubber rod near it. See Figure c. • This allows some electrons to leave the sphere through the ground wire, as is shown in Figure c. Section 23.2 Copyright © 2009 Pearson Education, Inc. A Similar Example Experimental Fact As we’ve just said, metal objects can be charged by induction, either while connected to ground or not: Section 23.2 Copyright © 2009 Pearson Education, Inc. • Experiment III: Now, remove the ground wire, as is shown in Figure d. There will now be more positive charges than negative charges on the sphere. • So, obviously, the charges will no longer be uniformly distributed on the sphere. • That is, A positive charge will be induced on the sphere. Section 23.2 Copyright © 2009 Pearson Education, Inc. • Experiment IV: Now, remove the rod, as is shown in Figure e. The electrons remaining on the sphere will redistribute themselves. • There will still be a net positive charge on the sphere. • The charge on the sphere will again be uniformly distributed. Note: The rod will have lost none of its negative charge during this process. Section 23.2 Copyright © 2009 Pearson Education, Inc. Charge Rearrangement in Insulators A process similar to induction can happen in insulators. • The charges within the molecules of the material are rearranged. • The proximity of the positive charges on the surface of the object and the negative charges on the surface of the insulator results in an attractive force between the object and the insulator. See the figure. Section 23.2 Copyright © 2009 Pearson Education, Inc. Experimental Fact As we just said, nonconductors won’t become charged by conduction or induction, but will experience charge separation: Section 23.2 Copyright © 2009 Pearson Education, Inc. An Electroscope is an instrument used for detecting charge. Section 23.2 Copyright © 2009 Pearson Education, Inc. An Electroscope can be charged either by conduction or by induction. Section 23.2 Copyright © 2009 Pearson Education, Inc. A charged Electroscope can be used to determine the sign of an unknown charge. Section 23.2 Copyright © 2009 Pearson Education, Inc.