The Discovery Of The Electron

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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.
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