The Electron

© 2012   Fernando Caracena

The first elementary particle discovered

Actually, two elementary particles were discovered almost at the same time during the birth of quantum theory: the electron and the photon.

We are bathed in light, and light allows us to see. Toward the late 1800s, light was discovered to be electromagnetic waves, its colors corresponding to different frequencies. Maxwell' s theory of electromagnetism specifies that it takes the acceleration of electric charge to produce electromagnetic waves. The oscillation of charge at a specific frequency then produces a single color of light. Max Planck stumbled across quantum theory by plowing ahead doggedly into impossible until he broke through a mental barrier which was the boundary of classical physics.

In order to analyse electromagnetic radiation from an oven, Planck assumed that radiation was emitted and absorbed in discrete chunks of radiation, which he was going to approach zero at the end of the calculation. But he got the results that agreed perfectly with experiments for finite sized chunks or quanta of energy. So Planck first stumbled upon the photon, but he really did not realize that it was a particle. He though of it more as a mistake in his calculations that he spent much time trying to correct. Enter Albert Einstein who saved the day and put researchers on the right pat to the quantum theory; but even Einstein was dragged "screaming and kicking" into the startling, Dali-esque world painted by quantum theory.

The electron, the first elementary particle discovered as being a fundamental particle of physics, is a major constituent of the atom. It carries the smallest unit of charge that is observable in free particles. In the convention of sign of charge proposed by Benjamin Franklin, the electron carries one negative unit of electric charge (-e). This unit of charge was determined experimentally by the Millikan Oil drop experiment.

Ordinary matter is composed of atoms of various kinds, which may be further combined into larger structures, as either crystals or molecules (theory). The atom itself is an electrically neutral, composite structure made up of electrons orbiting a very small nucleus that contains positive elementary charges in a number exactly equal to that of the orbiting electrons. In its lowest energy state, an atom is electrically neutral. If an extra electron is added to, or taken away from it, the remaining core is left electrically charged (called an ion). Electron "orbits" (see orbitals) are spatial manifestations of quantum states, in which the electron's position is smeared out into a cloud of probability, which cannot be further defined, except by detection--a process that destroys that quantum state. The electron quantum state around the atom acts as a whole unit in the form of a cloud of charge that is spread out in proportion to the quantum probability density. The size of the orbital cloud of electrons defines the size of the atom which, for neutral atoms runs from 0.3 to 3 Angstroms (1 x 10-10 meters).

The atomic nucleus is much smaller than the atomic cloud, ranging in size from from that of hydrogen(1.75 fm) to that of uranium (15 fm). The femtometer is defined as follows:

1 fm=1 ×10−15 m.

Except for hydrogen, the atomic nucleus is composed of two types of particles, called nucleons (neurons and protons), which have about equal mass, about 1836 times more massive than the electrons. The neutron is an electrically neutral particle, and the proton carries one unit of positive charge (+e). The nucleons are held together in the nucleus by the strong nuclear force, which is much stronger than the electrical force, otherwise, all of the charges present in the nucleus being positive or neutral, the nucleus would explode under electrostatic repulsion.

The number of protons in the atomic nucleus, the atomic number (Z), determines its electrical charge (+Z e). Although the neutron is electrically neutral, it does spin, and has a magnetic moment (acts like a small magnet), which suggests that it contains internal components, which are charged and rotate at various radii, but which add up to zero net charge in a way that produces a magnetic field and no electric field. The nucleons are themselves also composites; but the constituent particles of nucleons (quarks) are not seen in a free state outside of a nucleon. In the Standard model, quarks occur in threes to form nucleons, each quark possessing a fractional electric charge units (0, 1/3, 2/3) in positive and negative amounts, which add up to  integral units of charge.  


Light bulbs, Vacuum tubes and electron guns

The idea of the electron as a “particle” that has a mass, charge, position and velocity is a mental construct that unites a lot of experimental knowledge of its large-scale effects. The electron was first imagined as the smallest component of a beam of cathode rays that were observed in semi-evacuated tubes (pictures of Crookes Tubes) invented by the English physicist William Crookes. Cathode rays were produced in Crookes Tubes using an electron gun, which consisted of a hot, negatively charged cathode that emitted the electrons, which were accelerated toward a positively charged anode, beyond which they emerged through a hole into the tube as a faint glowing beam. [If the gas inside the tube was not ordinary air, then the glow could be made in different colors.] The beam could cast shadows away from the cathode along the beam. It could be curved by applying a transverse electric or magnetic field. Further, electron beams could turn paddle wheels placed in their path. This meant that they carried momentum and therefore, could exert a force on impact. Cathode rays also produced luminescence on hitting screens coated with phosphorescent material. Examined microscopically, the luminescence consists of pin-point flashes of light. From this fact and others sited above, physicists concluded that cathode rays were streams of particles, each particle of which had a mass and carried a negative unit of charge. The fact that the beam was bent uniformly by an external magnetic field suggested also that the particles themselves had an identical ratio of mass and charge. Through experiments involving the bending of cathode rays by external magnetic magnetic field, physicists were able to determine the ratio of charge to mass of the electron. One had only to perform another experiment to measure either the charge on the electron or its mass to determine both properties. But first, there is still a bit of history to cover, the thermionic emission of electrons.

In Britain in 1873, Daniel Lordan observed that a red-hot iron sphere having a negative, electric charge would leak its charge into a vacuum; but a positively charged one, retained its charge. Reporting on this effect, he was unable to explain why it happened. This phenomenon is called thermionic emission, which we can now explain. Electrons are normally the only free conductors of electricity. A negatively charged sphere contains an excess of electrons, which are readily boiled off when the sphere is heated up. The electrons are normally the only freely moving charges in a Crookes tube; and therefore, in a near vacuum, there are no positive charges that can boil out of a heated metal sphere to conduct away the positive charge.The positive charges are contained in the nuclei of metal atoms that are held fast in the matrix that make up the metal.

This one-way flow of electrons from a heated negatively charged conductor in a near vacuum was rediscovered on February 13, 1880 by Thomas A. Edison while in the process of developing the electric light bulb, in which he heated a small electrical conductor to incandescence by running an electric current through it. It was necessary to evacuate the air from the bulb air to keep the incandescent filament from burning up.  One type of filament selected for testing was a charred string, which worked well because carbon is a conductor of electricity, but had the major drawback that when it became incandescent, some carbon atoms evaporated from the charred string, and deposited a black coat on the inside of the glass bulb. Edison tried to steer the carbon atoms onto a metal plate by connecting the filament and the plate to opposite terminals of a battery. In the end, he discarded the idea of carbon filaments for light bulbs, but in the process, he rediscovered thermionic emission. When a filament connected to the negative terminal of a battery became hot, an electric current flowed between it and a cool metal plate connected to the positive terminal of a battery, but no current would develop if the polarity of the contacts were reversed (positive hot filament and negative cool plate). Thermionic emission furnished free electrons in an electron gun that became an important feature of Crookes Tubes and Cathode Ray Tubes (CRT)s, which were the main imaging devices of early television.

J. J. Thomson designed an experiment using a cathode ray tube to find the charge to mass ratio of an electron. If either the mass or charge of the electron could be determined then both the mass and charge of the electron would be known. In an offshoot of the atomic theory, physicists experimenting with charge supposed that there was a fundamental unit of charge that was carried by some of the elementary constituents of matter. The charge on the electron was finally measured the American physicist, Robert Millikan. Once these properties were known, the electron was identified as one of the elementary particles of matter.

The Atomicity of Electric Charge

In 1909, the American physicist Robert Millikan designed a clever experiment to measure the amount of charge on an electron using what is now called the Millikan Oil Drop Experiment. It is part of the modern repertoire of experiments that undergraduate physics are expected to complete. The experiment consists of a microscope that points into the interior of a small chamber, containing a fog of oil drops injected from an atomizer. A vertical scale within the field of view measures the vertical position of any drop sighted through the scope. Two flat, horizontal, metal plates, placed one above the other and connected to a battery, produce a vertically aligned electric field in the field of view of the scope. When the electric field is shut off, the small oil droplets float downward at very slow terminal velocities under gravity's pull. Fluid mechanics relates the density and radius of a small sphere to its terminal velocity through air, allowing one to be able to calculate the mass of any oil drop by measuring its terminal velocity. Turning on the electric field and adjusting it, one may also be able to select a charged drop that can be levitated at rest against gravity, or even moved upward. When held in balance at a fixed height, the electric force on the drop is equal and opposite to the force of gravity acting on it. This charge can be inferred from the strength of the applied electric field and the weight of the oil drop. A charged drop may contain several electric charge units, but it acquires and loses charge in discrete amounts that are no smaller than the fundamental unit of charge. The rest of the experiment consists in observing one charged drop at a time, and compiling the results of many observations of its changes in motion as the drop acquires or loses charge. An analysis of the increments in charge charges of the drop reveal various integral multiples of a fundamental unit of charge (e), the charge on the electron being a negative charge unit(-e). Since first performed, the Millikan Oil Drop Experiment has been repeated many times, resulting in a calculated value for the charge on the electron of about -1.6 x 10 -19 Coulombs, and mass of 9.1x10-31 kg. Robert A. Millikan, received the Nobel Prize in 1923 for measuring the charge on the electron, and in 1916, for experimentally confirming Einstein's photoelectric equation.

Conceptualizing an Electron as an Elementary Particle

Taken together, the results of the determination of the ratio of charge to mass and charge on the electron (determined within Crookes Tubes), gave not only the electronic charge (-e), but also its mass (me). It was therefore reasonable to picture a cathode ray as consisting of a stream of small particles (electrons) each having a mass, me, and charge, -e, which travelled outward from the cathode. In this way of thinking, each particle had a trajectory that went along a continuous path from one point to the other. However, quantum experiments have determined that this picture is wrong. This is not how electrons propagate. Their motion involves the idea of waves.

The elementary unit of charge is presently measured as

e = 1.602176565(35)×10−19 coulombs,

where the uncertainty contained in parenthesis (35) extends over the last two digits in this case ±0.00000000035×10−19 C. The rest mass of the electron is 9.109 382 15(45)×10−31 kg .

Later blogs of the physics series will address the quantum mechanical properties of the electron, and some of the atomic theory.


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