36 Week 1: Discrete Charge and the Electrostatic Field
Particle Symbol Charge Mass-energy (m 0 c^2 )
Quarks
Up quark u +2/3 ∼3 MeV
Up antiquark u ̄ -2/3 ∼3 MeV
Down quark d -1/3 ∼6 MeV
Down antiquark d ̄ +1/3 ∼6 MeV
Leptons
Electron e− -1 511 keV
Positron e+ +1 511 keV
Electron neutrino νe 0 <2 eVTable 1: Charge and Mass of First Generation FermionsHowever, just as a fluid is itself microscopically particulate, composed of quantized elementary
particles, the “elementary” charge (associated with these elementary particles that are the building
blocks of all matter) has experimentally turned out to be discrete and essentially indivisible. Indeed,
we characterize elementary particlesbya unique signature consisting of their (rest) mass, their
charge, and other measurable properties.
There are two kinds of elementary particles observed in nature that form the massive building
blocks of nearly everything we see, usually grouped intofamilies. One family consists of thequarks^27
, which carry a charge that is quantized in units ofe/3, wheree= 1. 6 × 10 −^19 C. The other family
are calledleptons^28 which carry a charge that is quantized in units ofeitself.
Table 1 summarizes the names and charge properties of the first generation of the quarks and
leptons. Note that quarks come in units of 2e/3 and−e/3, but we can never directly observe the
thirds. In ordinary matter, these quarks are found in thebound state(bound together by nuclear
forces we will not discuss here) into thenucleons: theproton(charge +e) andneutron(charge 0). In
fact, a proton is made up of three quarks:uud– where the neutron is also made up of three quarks:
udd. We only see particles with a net charge quantized in units of±eoutside of a nucleon.
Protons are quite massive – they have a rest mass around 938.3 MeV/c^2 (1. 67 × 10 −^27 kg), almost
2000 times larger than that of an electron at 0.511 MeV/c^2 (9. 11 × 10 −^31 kg). Neutrons are just a
hair more massive than a proton (939.6 MeV/c^2 ). Protons and neutrons are bound together by the
strong interaction into an atomic nucleus on the order of 10−^15 meters in diameter. This (positively
charged) nucleus strongly attracts negatively charged electrons via the electrostatic force that is the
first object of our study, which then arrange themselves aroundthe nucleus to create a structured,
electrically neutral object – theatom. Finally, atoms in turn are “glued” together by electrostatic
forces to form molecules, and molecules often stick together to form bulk matter.
As you proceed in your studies in this course, you should keep asimplepicture of an atom in
your mind – a very massive and tiny nucleus surrounded more or less symmetrically surrounded by
a much larger “cloud” of light, relatively mobile electrons to the point of electrical neutrality, with
clusters of atoms bound together into molecules (the object of the study ofchemistry). This picture
will turn out to be enormously useful to us as we seek to understand electronic properties of matter.
Nearly all matter is made up of atoms and hence nothing but protons, neutrons, and electrons.
Nearly all themobilecharge in solid matter is made up ofelectrons, as the nucleus of any given atom
is much more massive and likely to be surrounded by charge or locked insolids into a rigid structure
in such a way that it isn’t terribly mobile, although in fluids ionic charge can move around with
either sign. In semiconductors the mobile charge can also be electron “holes” – de facto positive
charge carriers consisting of regions of electron deficit that moveagainst an otherwise stationary
(^27) Wikipedia: http://www.wikipedia.org/wiki/quark.
(^28) Wikipedia: http://www.wikipedia.org/wiki/lepton. ,