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Transcript
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) • • • • • 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: – – – – 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