Download Electricity

Static electricity and magnetism
Harry Varvoglis
University of Tübingen &
University of Thessaloniki
Initial evolution of the field
• The evolution of concepts in Electricity and Magnetism
was slower than in the chapters of Mechanics or
Optics.
• The reason was that the corresponding phenomena do
not appear in everyday life.
• Therefore there was a lack of experimental results,
inasmuch experiments with static electricity are
affected by atmospheric humidity and the electric
current was not known before the 19th century.
• That is why the first concepts were related to
magnetism, which was studied through the use of
natural magnets.
Thales from Miletus (630-543 BC)
•Observation of
“magnet” rocks, i.e.
rocks from the area of
Magnesia, in Asia
Minor, which were
“natural magnets”.
•Electrification by
friction.
Pierre Peregrinus de Maricourt
(floruit 1269)
•The first to understand that
there exist two different “kinds”
of “magnetic charges”.
•He is responsible for the
“historical mistake” to name
north magnetic pole the pole of
a magnetic needle facing the
north geographic pole.
•But in this way, by definition,
Earth’s north magnetic pole lies
in the south geographical pole!
William Gilbert – contemporary of Galileo
Doctor of Elizabeth I
Construction and use of terrellae
Magnetic poles appear in pairs
Differences between electricity and magnetism
Modern electromagnetic terrella
Museo Galileo, Florence
Otto von Guericke (1602-1686)
First electrostatic generator
Von Guericke’s experiment
to demonstrate the
magnitude of atmospheric
pressure.
(from Deutsches Museum)
Electrostatic generator
~1850, Deutsches Museum
19th century (~1860)
electrostatic generator
Museo Galileo, Florence
Stephen Grey (1666-1736)
electrification by conduction
He understood the differences
between conductors and insulators.
Charles du Fay (1698-1739)
•Discovered the difference
between positive and
negative electric charges.
•He named them vitreous
(positive, when glass is
rubbed with silk) and
resinous (negative, when
amber is rubbed with fur).
Du Fay’s discoveries
From his paper: “A Discourse
concerning Electricity”,
Phil. Trans., 1734
•All bodies can be electrically charged by heating and rubbing, except
metals and soft/liquid bodies.
•All bodies, including metal and liquid, can be charged by influence
(induction).
•Glass is as satisfactory as silk as an insulator.
•Thread conducts better wet than dry.
•There are two states of electrification, Vitreous and Resinous.
•Bodies electrified (charged) with vitreous electricity attract bodies
electrified with resinous electricity and repel other bodies electrified
with vitreous electricity.
John Canton (1718-1772)
• He discovered that it is
possible to produce
both “vitreous” and
“resinous” electricity
from the same glass
rode, provided one is
rubbing it with a
different material each
time.
Ewald Georg von Kleist (1700-1748)
•He thought that
Electricity is kind
of a fluid, that
can be “poured”
into a jar!
•He stopped
experiments
after receiving
strong shocks.
Pieter van Musschenbroek (1692-1761)
Leyden Jars
One of the first Leyden Jars
~1775
Museo Galileo
Florence
Benjamin Franklin (1706-1790), his kite
and the lightning rod
The two kinds of electricity (vitreous and resinous) are simply opposite forms of
the same “thing”.
First reference of a law F ~ 1/r2
A common misconception
• From a letter of Franklin to a friend in England:
• When rain has wet the kite twine so that it can conduct the
electric fire freely, you will find it streams out plentifully
from the key at the approach of your knuckle, and with this
key a phial, or Leiden jar, maybe charged: and from electric
fire thus obtained spirits may be kindled, and all other
electric experiments [may be] performed which are usually
done by the help of a rubber glass globe or tube; and
therefore the sameness of the electrical matter with that of
lightening completely demonstrated.
• He did not wait, until struck by a lightning! Professor Georg
Wilhelm Richmann of St. Petersburg, who did the
experiment in this way a few months later, was killed!
Joseph Priestley (1733-1804)
•Chemist, discovered oxygen
•He repeated Franklin’s experiment,
in which we se that there is no net
electric force in the interior
of a metal box.
•In analogy to earlier results for
the gravitational filed, he
deduced that the electrical force
should scale like 1/r2.
Henry Cavendish (1731-1810)
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An eccentric English aristocrat.
He was said to be silent and solitary.
He used to wear always a greyish green
coat (today named Cavendish green).
He communicated with his female
servants only by notes, because he was
especially shy of women.
After his death, Maxwell found in his
manuscripts that:
– He had defined the first unit of
capacitance, that of a sphere with a radius
of 1 in .
– He had calculated the capacitance of a
plate capacitor.
•
He also discovered hydrogen and had
understood:
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–
–
–
the concept of electric potential
the form of Coulomb’s law,
Ohm’s law
the “mechanical theory of heat”
Charles Augustin de Coulomb
•Invented the torsion balance, F = k Δθ .
•One measures the constant k from the free oscillations of the balance.
•Using the torsion balance, Coulomb measured the force between two charges,
and found experimentally the law F = k q1.q2/r2
Did Cavendish measure G?
• Cavendish used Coulomb’s
torsion balance, in order to
measure the force of
gravitational attraction between
two spheres.
• He could have reported the
measurement of G.
•But, due to the system of units he used, he
reported only the measurement of the Earth’s
average density.
•Using the SI system, his value of G differs by
only 1% from the presently accepted value.
A unified mathematical formulation of
electricity, magnetism and gravitation
• Philosophical point of view: establishment of
action at a distance.
• Practical point of view: establishment of the
concept of potential (Lagrange).
• Calculation of the potential (Laplace equation)
• Solution of Laplace’s equation (Poisson –
Green)
• Alternative formulation of Gravitation by
Gauss and Green