1 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).

2 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.)

3 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)