Lecture 26 Review of Course The Great Ideas of Physics General - PDF document

Lecture 26 Review of Course The Great Ideas of Physics General Comments A Review • What have we done this semester? t 1 • We have studied the most important developments in physics, t 2 t 4 stressing conceptual understanding. A 34 A 12 • Science since antiquity (mostly Western) Quantum • Revolutions in thought: Mechanics t 3 • Scientific Method, Classical Physics ~ 1300 – 1900 • Space - T ime, Quantum Behavior Key advances 1900 - 1930 Classical Physics • This review does NOT cover the entire semester in detail • We have had two previous summaries. • Today: The overall picture Relativity More detail on the last part of the course: Quantum mechanics and the last 2 lectures. Scientific Knowledge Timeline • Framework for Understanding: Middle “Modern” Asia, Egypt Greece, Rome Renaissance Ages Physics Mesopotamia • “Logical Approach” Al - K h awarizmi Copernicus • Induction vs. Deduction (Bacon vs Descartes) • The Problem of Induction: How to go beyond a 0 collection of facts to new concepts? - 1 000 1000 2000 Plato Ptolomy • The problem of Deduction: How to demonstrate that an Fibanacci abstract idea applies to nature? Galileo Aristotle Erastosthenes Gutenberg Kepler • “Historical Approach” Euclid Aristarchus Printing Press • Normal science → crisis → extraordinary science (Kuhn) Maxwell Franklin Ampere Newton • Paradigm Coulomb Faraday • Anomalies exist only in the context of a paradigm • Revolution leads to a new paradigm Volta 1600 1700 1800 1900 • We have followed historical approach • “Classical Physics” was complete around 1880 • Documented “Revolutions” • See Timeline description of lives of various • Set stage for understanding the way science worked and scientists on WWW pages. works in practice Astronomy The Copernican Revolution • The Renaissance was a “rebirth” of knowledge in Initial Paradigm: The Two - S p here Universe • many ways • Large sphere containing the stars on its surface rotates about a small sphere, the Earth, with a • Science and especially physics was at the center period = 1 day. Anomaly: The Problem of the Planets • Re-examination of the “ultimate questions” of cosmology • and the “practical questions” of understanding what we • Five planets exhibit anomalous (within 2 sphere paradigm) motion. ie for some part of the year, observe in nature planets go “backwards”. • Is the earth the center of the universe or only a Normal Science Response: Ptolemy → Tycho • planet orbiting the sun? • Planets move on circles (epicycles) centered on • Ptolomy vs. Copernicus another circle (deferent = Sun for Tycho) which moves uniformly around the Earth. • Resolved by the simple description of Kepler, the earth and Extraordinary Response: Copernicus → Kepler other planets move in ellipses • • Copernicus: All planets (including the Earth) move • Copernican revolution about the Sun. • Affects our understanding of our place in the • Kepler: abandons paradigm of uniform circular motion: Elliptical orbits (Sun at one focus) with a universe – our world view varying speed (equal areas in equal times) 1

Lecture 26 Review of Course The Classical Revolution The Classical Revolution • Continuing the Copernican revolution • Newton and Maxwell define “Classical Physics” • How do we understand motion on earth? • Newton’s Three Laws • Inertia: • Galileo (mathematician) introduces the experimental method and quantitative mathematical analysis • “Every body continues in its state of rest, or of uniform motion in a right line, unless it is compelled to change that • Describes motion on earth by simple laws state by a force impressed on it.” • Applies also to the heavens – the earth moves! • Force & Acceleration: • Basis for later developments ===> Newton • “The change in motion [rate of change of momentum] is proportional to the motive force impressed; and is made in • Next slide the direction of the right line in which that force is impressed. • Action = Reaction: • “To every action [change of momentum] there is always opposed an equal reaction; or, the mutual actions of two bodies are always equal, and directed to contrary parts.” Forces & Fields Paradigm of Classical Physics: • Forces: Space & Time, Particles & Waves • Free Fall (Galileo -- > Newton) • Experimental method • Galileo: All bodies fall downward with the same acceleration • Newton: F = m a + force of gravity near earth is F g = m g • Galileo, ….. Test theories under controlled conditions • Universal Gravitation ( Kepler-- > Newton) • Space & Time T 2 = k R 3 • Kepler: Described motion of planets • Time is absolute (the same in all reference frames) • Newton: F = m a + Universal law of gravity F m = G m 1 m 2 / R 2 • Particles • Conservation Laws: • Particles have mass which is conserved • Conservation of Momentum: • Particles obey Newton’s equations • Newton’s 3rd & 2nd Laws ⇒ total momentum (p=mv) of interacting objects is conserved (does not change in time). • Examples: Baseballs, electrons • Conservation of Energy: • Waves • Energy changes form but the total is always conserved • Waves are moving patterns • Fields: (Newton == > Faraday == > Maxwell) • Waves show interference effects • Electric Fields created by charges. • Examples: Light ( Maxwell’s equations), Sound • Magnetic Fields created by charges in motion. • Nature is Deterministic • Electromagnetic waves (speed = c) Timeline - Modern Physics Space-Time Revolution Einstein • The Initial Paradigm: Classical Physics Bohr • Light is a wave (thought to be a medium called “ether”) Michelson Rutherford Nuclear Energy Thomson Neutron Stars • The anomaly (crisis) for classical physics: Released De Broglie discovered Planck Schrodinger Michelson-Morley Experiment Heisenberg • Found Speed of light to be the same in all directions even though 2000 the earth is moving around the sun 1900 1950 Expansion Laser • Normal response of Universe Special Invented discovered Relativity • Try to explain the experiment as some anomaly of light Transistor All the Quantum • Extraordinary Response (Revolution) General Invented Quarks Mechanics Relativity • Completely revise our notions of space and time discovered • “Modern Physics” began with a two great revolutions starting around 1900, and ending ???? • See Timeline description of lives of various scientists on WWW pages. 2