The Discovery Of The Electron

The Discovery Of The Electron
The electron was discovered in 1895 by J.J. Thomson in the form of
cathode rays, and was the first elementary particle to be identified. The
electron is the lightest known particle which possesses an electric charge. Its
rest mass is Me <approximately equal> 9.1 x 10 -28 g, about 1/1836 of the mass
of the proton or neutron.


The charge of the electron is -e = -4.8 x 10^-10 esu <elec trostatic
unit). The sign of the electron’s charge is negative by convention, and that of
the equally charged proton is positive. This is somewhat a unfortunate
convention, because the flow of electrons in a conductor is opposite to the
conventional direc tion of the current.


The most accurate direct measurement of e is the oil drop experiment
conducted by R.A. Milikan in 1909. In this experiment, the charges of droplets
of oil in air are measured by finding the electric field which balances each
drop against its weight. The weight of each drop is determined by observing its
rate of free fall through the air, and using Stokes’ formula for the viscous
drag on a slowly moving sphere. The charges thus measured are integral multiples
of e.


Electrons are emitted in radioactivity <as beta rays> and in many other
decay processes. The electron itself is completely stable. Electrons contribute
the bulk to ordinary matter; the volume of an atom is nearly all occupied by the
cloud of elec trons surrounding the nucleus, which occupies only about 10^-13 of
the atom’s volume. The chemical properties of ordinary matter are determined by
the electron cloud.


The electron obeys the Fermi-Dirac statistics, and for this reason is
often called a fermion. One of the primary attributes of matter, impenetrability,
results from the fact that the elec tron, being a fermion, obeys the Pauli
exclusion principle.


The electron is the lightest of a family of elementary particles, the
leptons. The other known charged leptons are the muon and the tau. These three
particles differ only in mass; they have the same spin, charge, strong
interactions, and weak interactions. In a weak interaction a charged lepton is
either unchanged or changed into and uncharged lepton, that is a neutri no. In
the latter case, each charged lepton is seen to change only into the
corresponding neutrino.


The electron has magnetic properties by virtue of (1) its orbital motion
about the nucleus of its parent atom and (2) its rotation about its own axis.

The magnetic properties are best described through the magnetic dipole movement
associated with 1 and 2. The classical analog of the orbital magnetic dipole
moment of a small current-carrying circuit. The electron spin magnetic dipole
moment may be thought of as arising from the circulation of charge, that is, a
current, about the electron axis; but a classical analog to this moment has much
less meaning than that to the orbital magnetic dipole moment. The magnetic
moments of the electrons in the atoms that make up a solid give rise to the bulk
magnetism of the solid.
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