the discovery of the electron

Upload: aaron-walker

Post on 14-Apr-2018

215 views

Category:

Documents


0 download

TRANSCRIPT

  • 7/30/2019 The Discovery of the Electron

    1/4

    The Discovery of the Electron

    At the end of the 19th century, there was no generally accepted model of the atom. Most physicists

    believed that the atom was indivisible, although the discovery of radioactivity cast doubt on that in the

    minds of some physicists.

    At the same time it was generally believed that electric charge, like mass, was infinitely divisible.

    To explain the connection between electricity and matter, some scientists in the late 19th century

    argued that there had to be a fundamental unit of electricity. In 1891 the Irish physicist, George Stoney,

    introduced the term electron to describe this smallest unit of negative charge.

    In 1897 J. J. Thomson, an English physicist, conducted a series of experiments on cathode rays and after

    observing that the beam of light in the cathode ray tube is attracted to a positive charge and repelled by

    a negative charge he concluded that the rays consist of a stream of small, electrically negatively chargedparticles which have a mass over a thousand times less than that of a hydrogen atom. Thomson has

    discovered the electron. From this point onward, it becomes increasingly clear that atoms are not

    fundamental particles, but in fact are made up of smaller particles.

    As a result of his experiments, Thomson was able to measure the charge to mass ratio of the electron;

    he could not however, measure accurately the charge or mass independently.

    The measurement of the electron's charge independently was achieved by Robert Andrews Millikan by

    his famous experiment from 1909 and with Thomson's results also a value for the electron mass was

    obtained.

    This experiment is called the "oil-drop experiment" and it was the first successful scientific attempt to

    detect and measure the effect of an individual subatomic particle.

    For this and his work on the photoelectric effect Robert Millikan won the 1923 Nobel Prize in physics.

    http://192.129.24.144/licensed_materials/00770/papers/7028005/70280159.pdfhttp://www.aip.org/history/electron/jjrays.htmhttp://galileo.phys.virginia.edu/classes/252/photoelectric_effect.htmlhttp://galileo.phys.virginia.edu/classes/252/photoelectric_effect.htmlhttp://www.aip.org/history/electron/jjrays.htmhttp://192.129.24.144/licensed_materials/00770/papers/7028005/70280159.pdf
  • 7/30/2019 The Discovery of the Electron

    2/4

    The scheme of the experiment is as follows: An atomizer sprayed a fine mist of oil droplets into the

    upper chamber. Some of these tiny droplets fell through a hole in the upper floor into the lower

    chamber of the apparatus. Millikan first let them fall until they reached terminal velocity due to the air

    resistance within the container. Using the microscope, he measured their terminal velocity, and by use

    of a formula, calculated the mass of each oil drop.

    When the electric field is off at terminal velocity force is equal to zero

    Where Fv is the upward force andFg is the downward force

    As the viscous force is not known we use stokes law

    Where rd is the radius of the droplet is the viscosity of air and is the velocity of the droplet withthe electric field off we do not know the volume or mass of the droplet

    As m=v.

    Radius of oil droplet is

    http://kp.phy.tu-dresden.de/boston/downloads/millikan3.pdfhttp://kp.phy.tu-dresden.de/boston/downloads/millikan3.pdf
  • 7/30/2019 The Discovery of the Electron

    3/4

    Next, Millikan applied a charge to the falling drops by irradiating the bottom chamber with x-rays. This

    caused the air to become ionized, which basically means that the air particles lost electrons. A part of

    the oil droplets captured one or more of those extra electrons and became negatively charged. By

    attaching a battery to the plates of the lower chamber he created an electric field between the plates

    that would act on the charged oil drops; he adjusted the voltage till the electric field force would just

    balance the force of gravity on a drop, and the drop would hang suspended in mid-air. Some drops have

    captured more electrons than others, so they will require a higher electrical field to stop. Now the

    electric force is acting upwards and the viscous and gravitational forces are acting downwards. He

    measured the terminal velocity of the droplet.

    articlesthat did not capture any of the extra electrons were not affected by the electrical field and fellto the bottom plate due to gravity.

    When a drop is suspended, its weight m g is exactly equal to the electric force applied, the product of

    the electric field and the charge q E.

    The values ofE (the applied electric field), m (the mass of a drop which was already calculated by

    Millikan), and g (the acceleration due to gravity), are all known values. So it is very easy to obtain the

    value ofq, the charge on the drop, by using the simple formula:

    m g = q E

    Millikan repeated the experiment numerous times, each time varying the strength of the x-rays ionizing

    the air, so that differing numbers of electrons would jump onto the oil molecules each time. He

    obtained various values for q.

    The charge q on a drop was always a multiple of 1.59 x 10-19

    Coulombs. This is less than 1% lower than

    the value accepted today: 1.602 x 10-19

    C.

  • 7/30/2019 The Discovery of the Electron

    4/4

    Source:http://www.juliantrubin.com/bigten/millikanoildrop.html

    http://www.juliantrubin.com/bigten/millikanoildrop.htmlhttp://www.juliantrubin.com/bigten/millikanoildrop.htmlhttp://www.juliantrubin.com/bigten/millikanoildrop.htmlhttp://www.juliantrubin.com/bigten/millikanoildrop.html