Electricity and MagnetismFrom Parlor

Games toMaxwell’sEquations

Electrical children, 1748

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 finalizedas quickly as E & M

– Heat, light, energy all slow to be “finalized”– Laws of motion, astronomy (celestial mechanics)

“finalized” more quickly

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

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)

18th-century electrostatics The apparatus

Glass and amberrods

Electric machines Leyden jars--a

“terrible instrument”discovered byaccident in 1746

The phenomena Attraction and

repulsion mademore visible

Conductors andinsulators

Sparks and shocks Lightning as

electricity– First known research

death in physics, 1753

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

Electroscope action as example 2 fluids

Like charges repel Opposites attract Ordinary matter has

+ and - charge

+-

+

+

_ +-

-

+ +- -

+++

- Glass rod

++ +

+

1 fluid Electric fluid repels

itself Ordinary matter has

some electric fluid

Both theoriesexplain thephenomenon!

Quantification of electrostatics Quantity of charge (Q)

– Quantified with electroscope, first electrical phenomenon tobe so

Intensity of charge (T=tension)– Length of sparks, length of wire melted (Leyden jar)

Force (Fe) measured by Coulomb in 1785– Coulomb’s Law: Fe = kc Q1Q2/R2

Form analogous to Newton’s law of gravity!– Coulomb = measure of charge (a big unit)– Electrical forces in atoms much stronger than gravitational

forces Fe is 1041 > Fg, between proton and electron in hydrogen atom

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

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. 1800

– From parlor to factory– From qualitative to quantitative laws– Linked to heat, light, muscle action– Boundaries still fluid (animal magnetism)

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 SNS

Faraday’s research, 1820-50 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!)

Faraday’s motors

N

S

S

N

Battery

+-

Bar magnets

Electro-magnetwithchangingpolarity

Principle of the galvanometer, anew electromagnetic instrument!

Electromagnetic induction Magnetism makes electricity, 1831

Theory-driven: magnet causes “electrotonicstrain” 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

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

Electric field, E = Fe/q

+q

FeA

E has direction!+

+ -

Faraday’s field theory, cont. Magnetic fields, named B

Exist wherever force of magnetic origin isexerted on a test magnet (have direction)

N S

Thus, a field defines properties of space!

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)

Maxwell’s aether

Heuristic model,not Cartesianontology

Electric currentflows from A to B;rotating cells repre-sent magnetic field;rotating spheres flowof electricity