Electricity and Magnetism
From Parlor
Games to
Maxwell’s
Equations
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 finalized
as quickly as E & M
– Heat, light, energy all slow to be “finalized”
– Laws of motion, astronomy (celestial mechanics)
“finalized” more quickly
Task of lecture

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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
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Technology without any theoretical explanation
Gilbert’s De magnete, 1600
– Experiments showed E and M unrelated
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Amber draws many bodies, magnet only iron
Amber draws only light bodies, magnet heavy bodies
– Offered two separate theories (Aristotelian language)
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“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 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
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

1 fluid
Like charges repel
 Electric fluid repels
itself
 Opposites attract
 Ordinary matter has
 Ordinary matter has
some electric fluid
+ and - charge
+ +
- - - Glass rod
+
_ +
+
+
+
+
Both theories
+
explain the
+
+
+
phenomenon!
+

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 (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)
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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
N
S
S
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
Bar magnets
N
S
Electromagnet
with
changing
polarity
-
N
S
+
Battery
Principle of the galvanometer, a
new electromagnetic instrument!
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
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 +

Fe
q
Electric field, E = Fe/q
E has direction!
+
+
-
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!
Maxwell’s unification of E&M
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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
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–
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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
Electric current
flows from A to B;
rotating cells represent magnetic field;
rotating spheres flow
of electricity
Heuristic model,
not Cartesian
ontology