Electricity & Magnetism

Electricity & Magnetism The “Transrapid Maglev” Train, Shanghai, China. “Maglev” Magnetic Levitation. It makes no contact with the rails! It’s weight is 100% supportedby electromagnetic forces!! Section 23.2

Chapter 17: Electric Forces & Fields The comb & the pieces of paper have opposite static electric charge, so they attract each other. Section 23.2

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

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!! p690

Some Topics in Ch. 17 • Static Electricity; Electric Charge & Its Conservation • Electric Charge in the Atom; Insulators & Conductors • Coulomb’s Force Law • Electric Forces & Fields • Electric Field Calculations: • For point charges. • For continuous Charge Distributions • Electric Field Lines • Electric Fields & Conductors • Motion of Charged Particles in an Electric Field

Topics in Ch. 17 • Coulomb’s Force Law • The Electric Field • Electric Dipoles • Electric Forces in Molecular Biology: DNA • Some Applications: • Photocopyers & Computer Printers Use Electrostatics • Electric Flux & Gauss’s Law • Equivalence of • Gauss’s Law & Coulomb’s Law • Experimental Basis of • Gauss’s & Coulomb’s Laws

Electricity and Magnetism • The Laws of Electricity & Magnetismare 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

Physics is an Experimental Science! Thousands of experimentsover hundreds of yearshave shown: • Electric & magnetic forces act onelectric charges & currents. • Electric charges & currents also act as sources of electric & magnetic fields. • Electricity & magnetism are really a single unified phenomena, called Electromagnetism.

Electromagnetism is described by Maxwell’s Equations, (4 of them!) which are the theme of this course! • This is discussed in detail in Ch. 23. From now to then, we introduce & discuss various aspects of Electromagnetism: Electric Charge, Electric Fields & Forces, Electric Current,Magnetic Fields& Forces,Electromagnetic Waves,Optics

Electromagnetism is described by Maxwell’s Equations, (4 of them!) which are the theme of this course! • This leads to a theory of electromagnetic radiation. • Light is an example • Electromagnetism is the basis for the study of optics at the end of the course.

Brief History of Electricity & Magnetism • Ancient Chinese • Some documents suggest that • magnetism was observed as • early as 2000 BC in China. • Ancient Greeks • Electrical & magnetic phenomena were known as early as 700 BC. • Experiments with amber & magnetite Section 23.2

1600: William Gilbert • Gilbert showed that electrification effects were not confined to just amber & that electrification effects were a general phenomena. • 1785: Charles Coulomb experimentally confirmed the • inverse square law form for electric forces. Section 23.2

1819: Hans Oersted • Found that a compass needle deflected when it was near a wire carrying an electric current. • 1831: Michael Faraday & Joseph Henry • Showed that when a wire is moved near a magnet, an electric current is produced in the wire. Section 23.2

1873:James Clerk Maxwell • Maxwellused observations & experimental facts as a basis for formulating the laws of electromagnetism. He achieved the • Unification of Electricity & Magnetism!!!!!(+ Optics!) • (Maxwell’s Equations!!!) • (The “Theme” of Physics 1404!) Section 23.2

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

Based on experiment! Not derivable mathematically!! Review of Physics I: Newton’s Laws of Motion Newton’s 2nd Law: ∑F = ma AVECTOR equation!! Holds component by component. A ∑Fx = max, ∑Fy = may, ∑Fz = maz One of the Most Fundamental, ImportantLaws of Classical Physics!!!

Section 17.1: Observational Facts • As we’ve said, the discovery of electricity is usually credited to the Greeks • About 2700 years ago • They observed electric charges & the forces between them in many situations • They used amber • A type of dried tree sap • After amber is rubbed with a piece of animal fur, it can attract small pieces of dust

The Greek word for amber was “elektron” from which we get the words electron and electricity. • Modern experiments use plastic and paper. • The force occurs even when the plastic & paper are not in contact. (Action at a distance!)

Electric Charge • Experiments(first done by • Coulomb!) show that there are • Two kinds of electric charges. • They are called positive & • negative Section 23.2

Two kinds of electric charges. • positive & negative • Negative Charges: • The type possessed by electrons. • Positive Charges: • 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

More Observational Facts Like charges repel each other,unlike charges attract. • Like charges: both positive or both negative • Unlike charges: one positive, one negative • The “like” & “unlike” apply to signs of the charges, not their magnitudes

Static Electricity - Conservation of Electric Charge Experimental Fact Objects can be charged by rubbing. Section 23.2

Experimental Facts • As we already said, charge comes in 2 types: • Positive(+) & Negative(-). • Like charges repel, unlike charges attract. Also • Electric Charge • is Conserved: • The arithmetic sum of • the total charge cannot • change in any interaction Section 23.2

In the figure, the • rubber rod is • negatively charged. • The glass rod is • positively charged. • So, The 2 rods will attract each other. Section 23.2

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

Charge is Conserved • The total charge on an object is the sum of all the individual charges carried by the object • Charge can move from place to place, and from one object to another, but the total charge of the universe does not change

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

More About Electric Charges • Experimental Fact: • Electric charge is Quantized • That is, an electric charge q is ALWAYSan • 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 • |e|  1.6  10-19 C • Electron: q = -e, Proton: q = +e Section 23.2

So, What is Electric Charge? • Charge is a fundamental property of matter • In that respect, charge is analogous to mass. • The amount of charge on a particle determines how it reacts to electric & magnetic fields • An actual definition is not possible • The SI unit of charge is the Coulomb • In honor of Charles de Coulomb Electron charge = -e = -1.6 x 10-19 C Proton charge = +e = +1.6 x 10-19 C • The symbol e is used to denote the magnitude of the charge on an electron or proton The symbols q & Q are used to denote charge in general

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

Insulators and Conductors Insulator An Insulator is a material in which almost no charge flows.Most non metallic materials are insulators. Section 23.2

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

More on Conductors • Electrical Conductorsare materials in which some 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

More on Insulators • Electrical Insulatorsare 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

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

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

Example • Assume that we start with a neutral metallic sphere. See Figure a. • Since it is neutral, it • has the same number of positive & negative charges. Section 23.2

A Similar Example • Experimental Fact: • As we’ve just said, metal objects can be • charged by induction. Section 23.2

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

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

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

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

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 beinduced on the sphere. Section 23.2

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

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

Experimental Fact As we just said, nonconductors won’t become charged by conduction or induction, but will experience charge separation: Section 23.2

An Electroscope is an instrument used for detecting charge. Section 23.2

An Electroscope can be charged either by conduction or by induction. Section 23.2

A charged Electroscope can be used to determine the sign of an unknown charge. Section 23.2

Electricity & Magnetism