Transcript Notes24

Conceptual Physics
Study Notes & Questions: Conductors, Etc. (Chap. 24)
1)
In conductors, the material’s valence electrons are free to move
between neighboring atoms/molecules—electrons can flow freely. The
electrons repel one another so current flows on the outer surface of
the material—not inside. As they move they jostle atoms, they meet
some “resistance,” dissipating KE and raising the material’s
temperature. The better the conductor, the less resistance. (p515)
2)
In insulators, the atoms/molecules hold their valence electrons tightly,
preventing electrons from moving through the material. The
resistance of insulators is very high. (p517)
3)
In semiconductors, the material is essentially insulating, but thermal
activity (molecular vibrations) boosts a few electrons’ orbitals high
enough to mix with neighboring valence shells. Pure semiconductor
allow only a tiny current to flow, which increases with temperature.
(p517)
4)
Superconductors only work at low temperatures where low thermal
activity permit highly ordered crystalline structures. Under certain
quantum mechanical conditions, electron wave functions can pass
through material with no electrical resistance. (p519) Currents
generated in a superconductor will persist indefinitely. Magnetic fields
created by these currents will also persist indefinitely.
5)
Electrons orbiting atomic nuclei generate small magnetic fields—
these are intrinsic to all atoms. Furthermore, electrons and protons
spin, creating their own intrinsic magnetic fields. These tiny atomic
magnets tend to be grouped together in magnetic domains about
1000 atoms in diameter. The domains tend to be randomly oriented,
canceling one another out, so that no large scale magnetism is
observed.
6)
In certain ferromagnetic metals (iron, nickel, cobalt), domains can be
aligned, creating large magnetic fields. (p525). Paramagnetism is
induced magnetism when some materials are placed in an external
magnetic field (p526). In diamagnetism, atoms acts to counteract an
external magnetic field (p527).
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Conceptual Physics
Study Notes & Questions: Week 15—Semiconductors (Chap. 25)
1)
Pure semiconductors are good insulators, although some thermally
induced conductivity occurs. Their valance electrons’ orbitals almost,
but not quite, mix with neighboring atoms (which would allow currents
to flow.) However if tiny amounts of alternative atoms are introduced
into the semiconductor crystal, their valence orbitals do mix enough to
allow current to flow. This is called doping (p532).
2)
In n-type semiconductors, electrons carry current. In p-type
semiconductors, holes—that is valence orbital vacancies—can
perform the same function as carrying current. (p532).
3)
Diodes are formed when n-type and p-type semiconductors are
placed adjacent to one another. At the contact surface, electrons and
holes, forming a depletion zone that has a small electric field across
it, allow electrons to cross from n-to-p but not p-to-n. (NOTE: the
current flow direction is actually in the opposite direction of electron
flow.)
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4)
A diode rectifies AC current, only allowing current to flow in one
direction (p534).
5)
A transistor is a sandwich of p, n & p (or n, p & n) semiconductors.
Normally, this construction is insulating—a voltage placed across the
two end plates will not flow. A small metal electrode placed above the
center material injects extra electrons in the center semiconductor
which dramatically changes its conduction properties. Current now
flows freely between the two end plates. By adjusting the center
electrode voltage up and down, the large current between the two end
plates can be adjust up and down. (p535)
6)
In computer language, a bit is a “quantum of information.” In a
physical system, like an integrated circuit (p538), a bit is a high or low
voltage at a particular electrode position, at a particular moment in
time. (p540). In a magnetic data storage medium, a bit of data is
stored as a particular orientation of a magnetic field in a small patch
of surface material. (p544)
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