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What We’ve Learned So Far

What We’ve Learned So Far. A Review of Topics on Test 2. What Have We Learned About Optical and Electric Storage?. Laser light is focused through a (circular) lens onto the surface of a CD The central max of the diffraction pattern must be no larger than one bit if data is to be resolved.

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What We’ve Learned So Far

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  1. What We’ve Learned So Far A Review of Topics on Test 2

  2. What Have We Learned About Optical and Electric Storage? • Laser light is focused through a (circular) lens onto the surface of a CD • The central max of the diffraction pattern must be no larger than one bit if data is to be resolved d sinq = 1.22 l tan q = y/D • Capacitors store charge Q in proportion to the voltage V between the plates: C = Q/V = e0A/d • Capacitors are used in RAM

  3. What Have We Learned About Magnetic Storage? • Two domains magnetized in same direction is a 0 • Two domains magnetized in opposite directions is a 1 • Direction of magnetization changes at start of new bit. • Magnetic data is written by running a current through a loop of wire near the disk • As magnetic data passes by coil of wire, changing field induces currents according to Faraday’s Law:

  4. What Have We Learned About Magnetoresistance? • Charges traveling through magnetic field experience magnetic force (provided velocity and field are not aligned): FB = qv x B • In a current-carrying wire, this force results in more frequent collisions and thus an increased resistance: Magnetoresistance • Electrons traveling through magnetized material undergo spin-dependent scattering • When magnetic field is present in magnetic superlattice, scattering of electrons is cut dramatically, greatly decreasing resistance: Giant magnetoresistanced

  5. What Have We Learned About Atoms? • ENERGY IS QUANTIZED • Electrons can absorb energy and move to a higher level; they can emit light and move to a lower level • In hydrogen the emitted light will have energy E = (13.6 ev)(1/nf2 – 1/ ni2) • The wavelength is given by l = hc/E = 1240(nm eV)/E • Energy levels of nearby atoms are slightly shifted from each other, producing bands of allowed energies • Electrons move from the locality of one atom to the next only if an energy state is available within the same band

  6. What have we learned about Resistance? • In many, ohmic, materials, current is proportional to voltage: V = iR • Resistance is proportional to the length of an object and inversely proportional to cross-sectional area: R = rL/A • The constant of proportionality here is called the resistivity. It is a function of material and temperature.

  7. What Have We Learned About Solids? • In conductors, the valence band is only partially-full, so electrons can easily move • In semiconductors and insulators, the valence band is completely full, so electrons must gain extra energy to move • semiconductors have smaller band gap, insulators have larger band gap • Conductors have a partially-filled valence band • The primary effect of higher temperature on resistance is to increase R due to more collisions at higher temperatures • Semiconductors have a completely-filled valence band • The primary effect of temperature on resistance is due to this requirement: the higher the temperature, the more conduction electrons

  8. What Have We Learned About Semiconductors? • Can dope semiconductors to increase conductivity: p-type uses atoms with 3 valence electrons, so empty “hole”, n-type uses atoms with 5 valence electrons, so extra conduction electron • In p-n junction, conduction electrons travel from n side (and holes travel from p side) to p (n) side to combine with holes (electrons) • p-side becomes negatively charged; n-side becomes positively charged: potential difference • If put negative terminal on p-side, increases potential difference and get no current (reverse bias) • If put positive terminal on p-side, lower potential difference and get current

  9. What Have We Learned About Semiconductor Devices? • When electrons and holes combine at p-n junction, excess energy emitted as light (LED) • energy of light depends on bandgap • fairly monochromatic wavelength emitted in all directions • Can pump enough electrons to conduction band to achieve population inversion, and could mirror surfaces to encourage stimulated emission: this produces semiconductor laser see applet for more info about laser operation: http://www.phys.ksu.edu/perg/vqm/laserweb/Ch-3/F3s5p1.htm

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