A charged particle is a particle which carries electric charge. Such particles can be affected by electromagnetic forces and as such are very important building blocks of the universe as we know it. The charge of charged particles is measured in terms of the elementary charge, e, which is 1.602*10-19C. This is the smallest amount of charge that has been observed in a self-contained system, and all other stable objects have an integer multiple of this charge.

#### The Nature of Charge

In the last paragraph, I threw around the term 'electric charge', or simply 'charge', without explaining what it is. Charge is the degree to which a particle interacts with the electromagnetic force. It is an intrinsic property of a particle, like mass, and it has two primary effects. The first is to be affected by electromagnetic fields, experiencing a force in accordance to the Lorentz force law. The second effect is to generate an electromagnetic field in accordance with Maxwell's Equations. This is a purely classical formulation of electromagnetism and is sufficient for the purposes of this writeup. A fully quantum theory of electromagnetism is quantum electrodynamics or QED, which is beyond the scope of this writeup.

#### Charged Elementary Particles

Of the twelve elementary fermions, nine are charged: the electron, the muon, the tau lepton, and all six quarks. The up-type quarks (up, strange, and top) have a charge of +2/3 e, the down-type quarks (down, charm, and bottom) have a charge of -1/3 e, and the three leptons (electron, muon, and tau) have a charge of -e. Each of these particles has an antiparticle with equal and opposite charge. The quarks get away with having fractional charge by never being seen outside combinations that have integer charge.

Although there are eighteen different charged elementary particles in total, only three are commonplace. These are the electron, the up quark, and the down quark. The other six particles, and all nine antiparticles, are rare and generally have short lifetimes, either by decay or mutual annihilation.

Individual, free quarks are never seen. Instead, the quarks bundle up with gluons into clusters called hadrons. There are two types of hadron, baryons and mesons. A baryon is a collection of three quarks, and a meson is a combination of a quark and an antiquark (not necessarily the antiquark of the first quark). In either case, the combination cannot have a fractional charge. Mesons will always have +e, -e, or 0 charge, while baryons can also have +2e or -2e.

The only truly stable charged hadron is the proton, consisting of two up quarks and a down quark, having a charge of +e. The antiproton, with a charge of -e, does not spontaneously decay, but due to the prevalence of protons in the known universe has a short lifetime from mutual annihilation.

#### The Role of Charged Particles

Charged particles, specifically the proton and electron, are very important in building the observed universe. Between nuclear scale (10-15m) and ordinary microscopic scale (10-6m), the electromagnetic force is the most important force. The electromagnetic attraction between protons and electrons is the force that binds together atoms.

Charged particles are also responsible for chemical bonding. In a covalent bond, electrons that are electromagnetically bound to one nucleus also become electromagnetically bound to another nucleus. This has the indirect effect of binding the two nuclei together. Another case is ionic bonding, where an electron or electrons is transferred from one atom to another, leaving ions with net, opposite electric charges. These two ions are then electrostatically attracted to each other. Other forms of chemical bonds are more complicated but are essentially electromagnetic in nature.

Charged particles are important in particle physics because most methods of acceleration and detection of subatomic particles rely on the charge of the particle. Only charged particles can be accelerated by a particle accelerator; uncharged particles are not affected by the accelerator's electric and magnetic fields. Energetic charges are also a form of ionising radiation, so they generate detectable trails of ions when passing through matter. Such detectors can be reasonably small and have a high resolution, unlike detectors for uncharged particles, which need large amounts of material for proper detection, and are correspondingly low-resolution.

On a more macroscopic scale, charges and electromagnetic forces are responsible for the 'solidity' of objects. The electrons on the surface of one object repel the electrons at the surface of the other object to produce a force that holds the two bodies separate. These forces are generally referred to as 'contact forces'. As tension is derived from the strength of chemical bonds, this means that most of the forces encountered in basic mechanics are actually electromagnetic effects. Thus, charged particles also have an important role in almost all macroscopic physics, as well as in microscopic physics.

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This writeup is copyright 2003-2004,2008 D.G. Roberge and is released under the Creative Commons Attribution-NoDerivs-NonCommercial licence. Details can be found at http://creativecommons.org/licenses/by-nd-nc/2.5/ .

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