The path to the electron (Horst Wahl, QuarkNet lecture, summer 2000) Early histo

The path to the electron (Horst Wahl, QuarkNet lecture, summer 2000) Early histo www.phwiki.com

The path to the electron (Horst Wahl, QuarkNet lecture, summer 2000) Early histo

Gonzales, Juan, Founding Editor has reference to this Academic Journal, PHwiki organized this Journal The path to the electron (Horst Wahl, QuarkNet lecture, summer 2000) Early history of electricity -– beginnings, Franklin, Galvani, Volta Electricity: beginning of quantitative era – Coulomb, Ampère, Faraday Electric field Currents in addition to magnetic field, induction Towards a field theory of electromagnetism Faraday, Maxwell Electromagnetic waves – prediction, properties Electromagnetic waves – observation Discharge tubes, cathode rays Photoelectric effect (Hertz, Hallwachs) Studies of nature of cathode rays Measurements of e/m of cathode rays Lorentz, Wiechert, Kaufmann, Thomson Further studies of photoelectric effect (Thomson, Lenard) Explanation of photoelectric effect, measurement of h (Einstein, Millikan) Electricity – history Early history Greeks discovered about 600BC that amber, when rubbed with wool, attracts other objects “Electric phenomena” named after “electron”, Greek word as long as amber; studied by many through ages; real progress in underst in addition to ing only gained in 18th century; Charles Dufay (1745): there are two types of electricity Benjamin Franklin (1706-1790) (US politician, diplomat, scientist, writer,printer) lightning as electrical phenomenon lightning rod coined name ”positive” in addition to “negative” as long as the two kinds of electric charge Luigi Galvani (1737-1798) (Prof. of Anatomy at U. of Bologna) “De viribus electricitatis in motu musculari commentarius” (1791) electric phenomena in muscular motion (experiments with froglegs) Aless in addition to ro Volta (1745-1827) electrophorus (1775) straw electroscope (1781) condensator (1782) relation between chemical reactions in addition to electricity (1796) “Voltaic cell” (battery) (1800) History of electricity—beginning of the quantitative era Charles Augustin de Coulomb (1736-1806) like charges repel, unlike charges attract each other; discovered “Coulomb’s Law”, using torsion balance invented by him. André Marie Ampère (1775-1836) (Prof. Physics at École Polytechnique, Paris) La théorie des phénomènes électrodynamiques” (1826) attraction in addition to repulsion of electric currents, direction of magnetic field of a current, explanation of magnetism as due to “molecular currents”. Michael Faraday (1791-1867) (bookbinder’s apprentice, self-taught chemist in addition to physicist, prof. of physics in addition to chemistry) “Experimental researches in electricity” (1844-1845) “Experimental researches in chemistry in addition to physics” (1859) concept of “electric field”, field lines (lines of as long as ce) induction (1831) basic laws of electrochemistry (1833-1834) investigations of dielectrics studies of gas discharges diamagnetism magnetic rotation of plane of polarization of light (1845)

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ELECTRIC FIELD “field of as long as ce”: exists in a region of space when an appropriate object (called the “test object” or “probe”) placed at any point in the region experiences a as long as ce. as long as ce depends on a property of the test object (e.g. charge, ), the “test charge”; “field strength” = ( as long as ce experienced by test object) divided by (test charge), = “ as long as ce per unit test charge”; as long as electrostatic as long as ce, this field strength is called “electrostatic field” or “electric field”; field can be visualized by “lines of as long as ce” or “field lines”, which give the direction of the field at every point, i.e. the as long as ce experienced by a test-charge at any point in space is in the direction tangent to the line of as long as ce at that point; the density (concentration) of field lines corresponds to the magnitude of thefield strength: the denser the concentration of lines, the stronger the field; the farther apart the lines, the weaker the field; electrostatic field lines begin on positive in addition to end on negative charges; field lines do not cross; originally, field lines were invented (by Faraday) as means of visualization, but eventually were regarded as st in addition to ing as long as an invisible physical reality – the electric field; In modern view, all as long as ces (“interactions”) are due to fields, described by “gauge field theories”. Currents in addition to magnetic fields electric current = ordered flow of electric charge; unit of current = 1 Ampère = 1A = 1 Coulomb/second; all charges generate electric fields – moving charges also generate magnetic fields a straight current carrying wire generates a cylindrical magnetic field in the space surrounding it (magnetic field lines are circles around the wire) a current carrying wire loop generates a magnetic field similar to that of a bar magnet (magnetic dipole field) magnetic as long as ce on moving charge – “Lorentz as long as ce”: F = q v B (B is the magnetic field strength, v the velocity of the charge q) as long as ce is perpendicular to both magnetic field in addition to velocity no as long as ce when motion parallel to magnetic field electric fields act on all charges – magnetic fields act only on moving charges unit of magnetic field = 1 Tesla = 1 T 1 Tesla = 1 Newton / (Ampère meter) Electromagnetic induction flux of the field: flux of the field through a surface = the total net number of field lines penetrating the surface. as long as a uni as long as m field B, the flux is just the product of the field strength in addition to the “effective” area of the surface; the effective area is the area “offered” to or “penetrated” by the field lines (i.e. the equivalent area perpendicular to the field). all other things equal, the flux is maximal if the surface is perpendicular to the field direction; it is = zero if the surface is parallel to the field direction. Faraday’s law of induction When the magnetic flux through the surface enclosed by a wire loop changes, an “electromotoric as long as ce” (voltage) is “induced” in the wire loop (electric field) the induced voltage is equal to the rate of change of the flux: V = – /t Lenz’ rule: the direction of the induced electric field is such as to counteract the effect that produced it (energy conservation!!) ways to change the flux: vary the field strength move the wire loop in in addition to out of the field area (or move the wire loop in a non-uni as long as m field) change the area enclosed by the wire loop (e.g. by de as long as ming it) change the angle between the wire loop in addition to the field direction (e.g. by rotating the wire loop) induction is the basis of the “generators of electricity” that run in electric power plants.

Towards a field theory of electromagnetism 1831: Michael Faraday (1791 – 1867): electromagnetic induction “Lines of as long as ce” concept of electric in addition to magnetic “fields” 1856: James Clerk Maxwell (1831-1879): paper “On Faraday’s lines of as long as ce” Express Faraday’s ideas in mathematical as long as m Show that field concept gives valid alternative to Ampère’s treatment based on central as long as ces 1856-1857: Wilhelm Weber (1804-1891) in addition to Rudolph Kohlrausch (1809-1858): Measurements of electric charges using electrostatic in addition to magnetic as long as ces Comparison indicates that electric currents travel with speed of light 1861-1862: Maxwell’s papers “On physical lines of as long as ce”: provide mathematical as long as mulation of Faraday’s as long as ce lines, study properties of ether; Conclude that electromagnetic fields advance with speed v = (0 0)-½ Measurements of 0 in addition to 0 v c, the speed of light Conclusion: “light consists in the transverse undulations of the same medium which is the cause of electric in addition to magnetic oscillations” 1864: Maxwell’s paper: “A dynamical theory of the electromagnetic field” Ignores the model previously proposed as long as the ether, but keeps the mathematical treatment; Asserts that equations valid without any assumptions about nature of medium equations “Maxwell’s equations” describe interplay between electric in addition to magnetic fields in addition to their relation to charges in addition to currents M.e. lead to “wave equation” as long as “electromagnetic waves” propagating with speed c = (0 0)-½ Biographical Note: James Clerk Maxwell (1831-1879), (Prof.Physics in Aberdeen, London, Cambridge) theory of heat, kinetic gas theory (Maxwell-Boltzmann velocity distribution), theory of electricity in addition to magnetism Heinrich Hertz (1857-1894) (Prof. Physics Karlsruhe, Bonn) wrote influential book on Maxwell’s theory experimental observation of electromagnetic radiation (1887) (radio waves) influence of UV light on electric discharges Electromagnetic waves – prediction MAXWELL’S EQUATIONS: are four differential equations summarizing nature of electricity in addition to magnetism: ( as long as mulated by James Clerk Maxwell around 1860): (1) Electric charges generate electric fields. (2) Magnetic field lines are closed loops; there are no magnetic monopoles. (3) Currents in addition to changing electric fields produce magnetic fields. (4) Changing magnetic fields produce electric fields. Together with the equation as long as the Lorentz as long as ce, these equations describe all electromagnetic phenomena (i.e. all electromagnetic phenomena can be derived from them.) from Maxwell’s equations one can derive another equation which has the as long as m of a “wave equation”. This differential equation was known from mechanics to have solutions which describe wave phenomena in mechanics.

Electromagnetic wave equation From the analogy between wave equation as long as mechanical waves in addition to the wave equation in terms of electric in addition to magnetic fields, Maxwell concluded that there should be also solutions to the wave equation derived from his equations – “electromagnetic waves”, corresponding to the propagation of oscillations of the electric in addition to magnetic fields. speed of electromagnetic waves is also derived from this wave equation, expressed in terms of constants which appear in the relation between charge in addition to electric field (k = 1/(4) in Coulomb’s law) in addition to between current in addition to magnetic field ( in Ampère’s law). This speed turns out to be = the speed of light! Conclusion in addition to prediction: light is just a as long as m of electromagnetic radiation there should be other as long as ms of electromagnetic radiation (different frequencies) which can be produced by making charges “wiggle”; This was experimentally verified by Heinrich Hertz: (built devices to generate in addition to to receive e.m. waves – first human-made radio waves) Electromagnetic waves: electromagnetic radiation = coupled, oscillating electric in addition to magnetic fields moving through space at the speed of light; magnetic in addition to electric fields “feed on each other”, obeying Maxwell’s 3rd in addition to 4th laws e.m. waves do not need material carrier – move through vacuum (- no “ether”); e.m. waves are transverse waves – electric field perpendicular to magnetic field, both perpendicular to direction of propagation; speed of light 300 000 km/sec = 186 000 miles/second (this is the speed of light in vacuum) (speed of light in air is very similar) electromagnetic waves generated by accelerating charges Electromagnetic spectrum: Discharge tubes 1855- 1857: Heinrich Geissler (1815-1879) (Bonn) Mercury pump (can reach 10-3 torr) Build discharge tube (glass tube with two electrodes, filled with gas at very low pressure) at lower pressure than ever be as long as e (“Geissler tube”) (big improvement over tubes built previously by Humphrey Davy) 1858: Geissler in addition to Julius Plücker (1801-1868): Detailed study of discharges, pressure dependence See influence of magnet on discharges 1869: Johann Hittorf (1824-1914) (Münster) determined that discharge in a vacuum tube was accomplished by the emission of rays ( named “glow rays” by him, later termed “cathode rays”) capable of casting a shadow of an opaque body on the wall of the tube. rays seemed to travel in straight lines in addition to produce a fluorescent glow where they passed through the glass. Rays deflected by magnetic field 1870’s: William Crookes (1832-1919) (London): detailed investigation of discharges; Confirms Hittorf’s findings about deflection in magnetic field Concludes that rays consist of particles carrying negative charge

Electromagnetic waves- Observation 1886 – 1887: Heinrich Hertz (1857-1894) (Karlsruhe) Built apparatus to generate in addition to detect electromagnetic waves predicted by Maxwell’s theory High voltage induction coil to cause spark discharge between two pieces of brass; once spark as long as ms conducting path between two brass conductors charge oscillated back in addition to as long as th, emitting e.m. radiation Circular copper wire with spark gap used as receiver; presence of oscillating charge in receiver signaled by spark across the spark gap Experiment successful – detected radiation up to 50 ft away Established that radiation had properties reminiscent of light: was reflected in addition to refracted as expected, could be polarized, speed = speed of light Photoelectric effect 1887: Heinrich Hertz: In experiments on e.m. waves, unexpected new observation: when receiver spark gap is shielded from light of transmitter spark, the maximum spark-length became smaller Further investigation showed: Glass effectively shielded the spark Quartz did not Use of quartz prism to break up light into wavelength components find that wavelenght which makes little spark more powerful was in the UV Hertz’ conclusion: “I confine myself at present to communicating the results obtained, without attempting any theory respecting the manner in which the observed phenomena are brought about” Photoelectric effect– further studies 1888: Wilhelm Hallwachs (1859-1922) (Dresden) Per as long as ms experiment to elucidate effect observed by Hertz: Clean circular plate of Zn mounted on insulating st in addition to ; plate connected by wire to gold leaf electroscope Electroscope charged with negative charge – stays charged as long as a while; but if Zn plate illuminated with UV light, electroscope loses charge quickly Electroscope charged with positive charge: UV light has no influence on speed of charge leakage. But still no explanation Calls effect “lichtelektrische Entladung” (light-electric discharge)

Cathode rays 1894: Hertz in addition to Philipp Lenard (1862-1947): Further investigations of cathode rays using discharge tubes: Cathode rays penetrate through thin Al window ate end of tube, Cause fluorescence over distance of few centimeters in air Deflected by magnetic field No deflection by electric fields (later explained due to insufficiently good vacuum) 1895: Wilhelm Röntgen (1845-1923) (Würzburg) Uses discharge tubes designed by Hittorf in addition to Lenard (but improved pump) to verify Hertz’ in addition to Lenard’s experiments Discovers X-rays – as long as get about cathode rays! Röntgen in addition to X-rays: From Life magazine,6 April 1896 H in addition to of Anna Röntgen Studies of the nature of cathode rays 1895: Jean Perrin (1870-1942) (Paris): Modifies cathode ray tube – adds “Faraday cup” which is connected to electrometer Shows that cathode rays carry negative charge 1896: Hendrik A Lorentz (1853-1928) (Leiden) Formulates atomistic interpretation of Maxwell’s equations in terms of electrically charged particles (called “ions” by him) “Lorentz as long as ce” = as long as ce exerted by magnetic field on moving charged particles 1896: Pieter A. Zeeman (1865-1943) (Amsterdam) Observes broadening of Na D line in magnetic field measures broadening vs field strength 1896: Explanation of this effect by Lorentz: based on light emitted by “ions” orbiting within Na atom Calculates expected broadening f (e/m)B By comparing with measured line broadening, obtains estimate of e/m of “ions” in Na atom: e/m 107 emu/g 1011 C/kg (cf modern value of 1.76×10 C11/kg) 1897: three experiments measuring e/m, all with improved vacuum: Emil Wiechert (1861-1928) (Königsberg) Measures e/m – value similar to that obtained by Lorentz Assuming value as long as charge = that of H ion, concludes that “charge carrying entity is about 2000 times smaller than H atom” Cathode rays part of atom Study was his PhD thesis, published in obscure journal – largely ignored Walther Kaufmann (1871-1947) (Berlin) Obtains similar value as long as e/m, points out discrepancy, but no explanation J. J. Thomson

1897: Joseph John Thomson (1856-1940) (Cambridge) Improves on tube built by Perrin with Faraday cup to verify Perrin’s result of negative charge Conclude that cathode rays are negatively charged “corpuscles” Then designs other tube with electric deflection plates inside tube, as long as e/m measurement Result as long as e/m in agreement with that obtained by Lorentz, Wiechert, Kaufmann, Bold conclusion: “we have in the cathode rays matter in a new state, a state in which the subdivision of matter is carried very much further than in the ordinary gaseous state: a state in which all matter is of one in addition to the same kind; this matter being the substance from which all the chemical elements are built up.“ Thomson’s paper on cathode rays James Joseph Thomson (1856- 1940): 3rd Cavendish professor at Cambridge (after Maxwell in addition to Rayleigh) (1884- 1919) Master of Trinity College (1918-1940)

Further studies of photoelectric effect 1899: J.J. Thomson: studies of photoelectric effect: Modifies cathode ray tube: make metal surface to be exposed to light the cathode in a cathode ray tube Finds that particles emitted due to light are the same as cathode rays (same e/m) 1902: Philipp Lenard Studies of photoelectric effect Measured variation of energy of emitted photoelectrons with light intensity Use retarding potential to measure energy of ejected electrons: photo-current stops when retarding potential reaches Vstop Surprises: Vstop does not depend on light intensity energy of electrons does depend on color (frequency) of light

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1905: Albert Einstein (1879-1955) (Bern) Gives explanation of observation relating to photoelectric effect: Assume that incoming radiation consists of “light quanta” of energy hf (h = Planck’s constant, f=frequency) electrons will leave surface of metal with energy E = hf – W W = “work function” = energy necessary to get electron out of the metal When cranking up retarding voltage until current stops, the highest energy electrons must have had energy eVstop on leaving the cathode There as long as e eVstop = hf – W Minimum light frequency as long as a given metal, that as long as which quantum of energy is equal to work function 1906 – 1916 Robert Millikan (1868-1963) (Chicago) Did not accept Einstein’s explanation Tried to disprove it by precise measurements Result: confirmation of Einstein’s theory, measurement of h with 0.5% precision 1923: Arthur Compton (1892-1962)(St.Louis): Observes scattering of X-rays on electrons

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