Electricity and Magnetism

Transcription

1 Electricity and Magnetism From Parlor Games to Maxwell s Equations Electrical children, 1748

2 E & M as finalized physics Four moments in any topic of physics Identify the relevant phenomena Quantity relevant aspects of phenomena Create explanatory theories Apply the theories in technology Most areas of classical physics not finalized as quickly as E & M Heat, light, energy all slow to be finalized Laws of motion, astronomy (celestial mechanics) finalized more quickly

3 Task of lecture Pre-sciences of electricity and magnetism-- separate phenomena 18th-century phenomena and theories Marriage of E & M in 19th century Field theories Mathematical descriptions An aether returns! A European network of researchers involved

4 E & M--unrelated phenomena Electrum = amber in Greek Attractive quality, ignored by Aristotle Named electric by Gilbert in 1600 Magnesia = Turkish area rich in iron Compass invented in 4th-century China Technology without any theoretical explanation Gilbert s De magnete, 1600 Experiments showed E and M unrelated Amber draws many bodies, magnet only iron Amber draws only light bodies, magnet heavy bodies Offered two separate theories (Aristotelian language) Innate sympathy for magnets (special powers) Effluvia for electricity (moist particles released by rubbing attract dry particles in bodies)

5 18th-century electrostatics The apparatus Glass and amber rods Electric machines Leyden jars--a terrible instrument discovered by accident in 1746 The phenomena Attraction and repulsion made more visible Conductors and insulators Sparks and shocks Lightning as electricity First known research death in physics, 1753

6 18th-century theories 2-fluid theory (Dufay, Nollet), 1740s Fluids flow in both directions, toward (paper bits) and away (sparks) from electrics Opposites attract; likes repel 1-fluid theory (Franklin), 1746 Electric fluid repels itself, attracts ordinary matter, is CONSERVED All ordinary matter contains some electric fluid Surplus or lack of electric fluid, created by rubbing, produces phenomena No closure on this debate in 18c Phenomena generally not quantified Some phenomena ignored, others stressed On-going disagreement over shape of lightning rods

7 Electroscope action as example 2 fluids Like charges repel Opposites attract Ordinary matter has + and - charge _ - Glass rod fluid Electric fluid repels itself Ordinary matter has some electric fluid Both theories explain the phenomenon!

8 Quantification of electrostatics Quantity of charge (Q) Quantified with electroscope, first electrical phenomenon to be so Intensity of charge (T=tension) Length of sparks, length of wire melted (Leyden jar) Force (F e ) measured by Coulomb in 1785 Coulomb s Law: F e = k c Q 1 Q 2 /R 2 Form analogous to Newton s law of gravity! Coulomb = measure of charge (a big unit) Electrical forces in atoms much stronger than gravitational forces F e is > F g, between proton and electron in hydrogen atom

9 Current electricity discovered Galvani s accidental discovery of animal electricity, 1791 in Bologna Muscle twitches when: Scalpel on nerve and electric machine sparks Muscle hung on iron fence with brass hook Muscle placed in iron-zinc arc Concluded that animals generate a special electricity, with muscles acting like Leyden jars Volta s battery, 1800 in Pavia Rejects animal electricity ; claims that muscle is a detector, not a generator, of electricity Electricity produced by 2 dissimilar metals in contact Zinc-silver-cardboard piles worked best Electric potential pushes charge in circuit

10 Current electricity exploited Technological applications by 1805 Carbon arcs in circuit produce light Decomposition of chemical compounds (hydrogen from water, chlorine gas from saltwater) Competing theories of current in pile Shifts in electrical studies ca From parlor to factory From qualitative to quantitative laws Linked to heat, light, muscle action Boundaries still fluid (animal magnetism)

11 Marriage of E & M in 1820s Oersted s non-accidental discovery of electromagnetism, 1820 in Copenhagen Moving current in wire makes magnetism Hypothesized circular force in space (not central force like gravity!) Ampere s electromagnet, 1820 in Paris Theorizes magnet as current loops in atoms, reducing magnetism to electricity N N S S

12 Faraday s research, First professional physicist we have discussed in Physics 1 Self-taught at evening science lectures Full-time employee of Royal Institution from 1813 Visual thinker, not a mathematician Electromagnetism makes motion, 1821 Mercury-magnetic motor Electric motor with electromagnet as armature (also a galvanometer!)

13 Faraday s motors Bar magnets N S Electromagnet with changing polarity N S - Battery + Principle of the galvanometer, a new electromagnetic instrument!

14 Electromagnetic induction Magnetism makes electricity, 1831 Theory-driven: magnet causes electrotonic strain in conductor or medium Moving bar magnet makes electricity Electromagnet turned on/off makes electricity Invents dynamo, 1830s Mechanical work rotates electromagnet which generates electric current

15 Faraday s field theory, 1838 New conceptual tools: strained space Object (A) generates field throughout space Field of (A) exerts force on charged test element (q) placed in field A + q F e Electric field, E = F e /q E has direction! + + -

16 Faraday s field theory, cont. Magnetic fields, named B Exist wherever force of magnetic origin is exerted on a test magnet (have direction) N S Thus, a field defines properties of space!

17 Maxwell s unification of E&M 1st prof exp. physics, Cambridge 1871 Unified math description of E, B fields produced by given currents and charges, with 4 equations Summarized many experimental results Coulomb s law and idea of E fields Changing E field produces B fields No free magnetic charges (no free poles) Electric charge is conserved Changing B field produce E fields Forces between current-carrying wires Light moves at speed of c (300,000,000 m/sec)

18 Maxwell s aether Electric current flows from A to B; rotating cells represent magnetic field; rotating spheres flow of electricity Heuristic model, not Cartesian ontology