# 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

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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.

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

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

## Gonzales, Juan Founding Editor

Gonzales, Juan is from United States and they belong to El Tecolote and they are from  San Francisco, United States got related to this Particular Journal. and Gonzales, Juan deal with the subjects like Hispanic Interest; Local News

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