Lecture 8 - Electricity & magnetism I Classical Physics - Continued Announcements Electricity & Magnetism • Today: v Start Electricity and Magnetism w a L s ’ March (Ch 6) m b o u l o C Homework 3 due ~ No magnetic charge • Mon., Sept 29: Review before Exam 1 Homework 4 due Ampere’s Law • Wed., Oct. 1: Exam 1 Covers material through the Review Franklin Chapters 1 – 5 of March; Ch. 1-2 of Lightman Electric Fields • Mon. Oct. 6: Solutions to Exam 1 Two signs of Electromagnetism and waves electric charge March (Ch 6) Magnetic Fields Faraday Timeline The state of science around 1700 Middle “Modern” Asia, Egypt • The Newtonian Paradigm (or World View): Greece, Rome Renaissance Ages Physics Mesopotamia • What is the world made of? Al-Khawarizmi Copernicus • Particles - objects that have mass. 0 • What Happens in the World? -1000 1000 2000 Plato • Forces cause particles to move in accordance with Ptolomy Newton’s Laws of Motion. Fibanacci Galileo Aristotle Erastosthenes Kepler Euclid Aristarchus • Determinism • Everything is determined ! Franklin Maxwell Ampere Newton • Universe acts like a giant machine, a giant clock with every part Coulomb Faraday obeying precise laws Volta • (The second law of thermodynamics was not yet 1600 1700 1800 1900 formulated) • See Timeline description of lives of various scientists on WWW pages. What forces do we know about? What was not understood around 1700? • Force of gravity: m 1 m 2 R • Force = G m 1 m 2 / R 2 • What is ordinary matter around us? • Force on mass m near surface of earth • Made up of particles? Atoms? F = G m m earth / R earth 2 = m g g = G m earth / R earth 2 x • What is light? • Force of due to a spring: • Newton viewed light as particles? F • Huygens, others said light acts as waves. • Force F = - K x • What is the source of energy for the sun? • Force of due to contact • What transmits the force? This was an open question in the • What is electricity, magnetism? 1700’s. What do you think? • Other? • Other? 1
Lecture 8 - Electricity & magnetism I Electrical Forces Metals and Insulators • Known long ago in Greece • Two types of materials • “Electret” (like rod rubbed with fur) causes forces known in ancient Greece • Metals conduct electricity • “Electron” is word invented in ancient Greece • Electric charge moves through material • Examples: Copper, Gold • Insulators do not conduct electricity • Electric charges stay can be induced by contact friction,but do not move through material unless there is “breakdown” that leads to sparks and lightening. • Examples: Glass rod, Rubber, paper, air • Known long ago in Greece • Studied by scientists with new approaches (scientific method; Newton’s laws; concept of fundamental forces) starting in the 1700’s Benjamin Franklin 1706 - 1790 Electrostatics • Regarded as the First Great • A kind of force. Does it fit in the Newtonian American Scientist picture? • 10th of 17 children, • Charge electroscope with rubber rod which has been rubbed with fur. Gold leaves separate. Left school at age of 10 Learned Craft of Publishing as apprentice • Bring same rubber rod close to top of electroscope. observe leaves separate further. • Became Businessman, Author, • Bring glass rod (rubbed with silk) close to top of Inventor, Scientist, Statesman electroscope. observe leaves approach each other. • Now repeat experiment, but charge with glass rod. Gold leaves • Discoveries on electricity were in still separate. advance of European Scientists. • Now rubber rod causes leaves to approach each other. • Famous kite experiments established electrical nature • Glass rod causes leaves to separate. of lightening • Similar Experiments with insulating “Pith Balls” • Invented “Lightening Rod” at time when Europe had • Explanation? medieval view of lightening - rang church bells to ward off lightening • There exist two kinds of charge. (Ben Franklin, 1751) • Unlike charges attract Like charges repel • Statesman representing the Colonies and the United States in Europe Forces between Charges - Demo Forces between Charges - Demo • Charge moves in a metal. Like charge repelled. Makes charge of the same sign on all both parts of the foil. • Charge can be transferred between insulators and Charged rod then stays in place. Causes electrical forces. + - Metal foils Charged rod Charged rod + + Charged Pith Ball Uncharged Charged - two Charged Pith Ball - same sign metal foils repel - opposite sign 2
Lecture 8 - Electricity & magnetism I Forces between Charges Forces between Charges • How do we describe these forces within Newtonian • Electrostatic forces MUCH stronger than gravity: system? • Electrostatic: F E = K q 1 q 2 / R 2 • Gravity: F G = G m 1 m 2 / R 2 • Need expression for forces in terms of the positions of charges • In meter- Kg - second system: K = 9.0 x 10 9 G = 6.67 x 10 -11 • Forces Law Coulomb (1785) Inverse square law: • Force between two protons at distance of 1 m: F = K q 1 q 2 / R 2 , q 1 , q 2 = charge (plus or minus) Charge = 1.6 x 10 -19 Coulomb Mass = 1.6 x 10 -27 Kg Like gravity, except electric force can be attractive F E = 2.3 x 10 -28 Newtons; F G = 1.8 x 10 -64 Newtons or repulsive Factor of 10 36 Magnetic Forces How can Gravitational Forces ever be important? • Yet another kind of force • Known since prehistory – lodestone compasses used in China • Electrostatic forces are zero between two neutral thousands of years ago objects (equal amounts of positive and negative) • Bar Magnets: Two poles (North & South) • Force description: Like poles repel; Unlike poles attract. • Explanation? • Gravitational forces always have the same sign (attractive) and never cancel out • Is there a magnetic charge (analogous to electric charge) ? • Try to isolate the charges: cut the magnet in half: • Force between sun and earth (both nearly neutral) S N S N S N is mainly gravitational • In fact, no experiment to date (and there have been many attempts) has shown evidence for the existence of magnetic charge. We believe the source of the magnetic force is not a new kind of charge, but is due to motion of electric charge. Magnetic Forces due to Electric Current Magnetic Forces due to Electric Current Loop • Current is charges in motion • Causes force on magnet • Current in a loop causes magnetic forces just like a magnet • Example: Compass near wire with current current current S N wire Top View Side View Current Loop Bar Magnet 3
Lecture 8 - Electricity & magnetism I The Field Concept The Field Concept (continued) • Michael Faraday (1791 - 1867) had the idea that • Electric and Magnetic Fields at a given point in forces between bodies were cause by Fields that fill space determine the force on a “test” electric all space and act on the bodies charge if it were placed at that point in space. • Electric Field E • Electric field E causes force F = qE (q = charge) due to positive charge • Magnetic field B causes force F = qvB on charge moving with speed v perpendicular to B + This “harmless” equation has the seeds of trouble for the Newtonian picture! • Magnetic force depends on the speed of the particle. • Faraday (building upon Ampere’s work) discovered • Force is meant to be an absolute quantity. The laws of physics the interdependence of Electric & Magnetic Fields: are supposed to be the same for two different observers (people or instruments) even if they are moving at constant v • A moving or changing electric field generates a magnetic field with respect to one another --- Remember Galileo, Newton ! and a moving or changing magnetic field generates an electric • But speed is not the same to the different observers! What is field. going on? Will this lead to a breakdown of the ideas of Galileo and Newton? Demo Summary E-M Cannon • New Forces and ideas in the Newtonian World! v • Electric Charge: • Connect solenoid to a source • Property of particles... Determines new of alternating voltage. The flux Coulomb Force: F = K q 1 q 2 / R 2 through the area ⊥ to axis of ~ solenoid therefore changes in • Fields – new idea in Newtonian physics: time. A conducting ring placed on side view • Extend through space top of the solenoid will have a • Electric Fields: created by electric charges B current induced in it opposing this F • change. There will then be a force • Magnetic Fields: created by electric charges on the ring since it contains a in motion • current which is circulating in the B • Principles used in electric generators, …. presence of a magnetic field. • • Electric Fields and Magnetic Fields are not independent of F each other. A changing magnetic field generates an • Fields are real! B electric field and a changing electric field generates a magnetic field. Maxwell realized the full significance of this top view interrelationship --- next time. 4
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