Physics 29: 1-11 Molecules and Solids
Text Reading: Giancoli, Physics - Principles with Applications, Chapter 29: Sections 1 to 11
Text Reading: Giancoli, Physics - Principles with Applications, Chapter 29: All.
For those of you who have already had chemistry, this chapter is mostly review.
- 29.1 Electrons in the outer shells of atoms are attracted to any nearby positive charge, so they exert attractive force not only on their own atom's nucleus, but on nearby atomic nuclei as well. Covalent bonds form between atoms whose nuclei attract electrons in each other more or less equally. Ionic bonds form when the attraction of one atom for electrons overwhelms the attraction of another atom for its electrons. Rather than two distinct types of bonds, ionic and covalent bonds represent the extremes of a range of possible electrical force combinations that result in atoms sticking together in bonds.
- 29.2 The energy required to pull two charged atoms (ions) apart, where separation approaches infinity, is represented by the work required to overcome the Coulomb force FE = kQq/r2. We can also look at this work as energy stored in the bond when it formed: the binding energy of the bond. In most situations, chemical reactions require energy or work to break existing bonds (activation energy) before the individual atoms can reform and store energy (binding energy) as new molecule combinations. In biological systems, energy can be stored in the chemical bonds between adenine diphosphate (ADP) and a phosphate ion (P) to for adenine triphosphate (ATP). A cell can break this bond to recover and use the binding energy for other reactions.
- 29.3 Wan der Waals forces occur when charge is unevenly distributed across a molecule, forming a dipole where positive and negative charge areas lie near one another. In this situation, the positive side of one molecule will be attracted to negative locations on other molecules, or, as in the case of large proteins, to negative locations on the same molecule. In bonds between hydrogen and strongly electronegative elements like nitrogen, oxygen, and fluorine, hydrogen's electron remains between the hydrogen and N, O, or Fl atom, leaving the hydrogen's positive nuclei unshielded. Such hydrogens form stronger-than-average van der Walls bonds, called hydrogen bonds. Complementary DNA strands held together by hydrogen bonds can be easily manipulated for replication or "reading" required for protein synthesis.
- 29.4 As with individual atoms, electrons are still confined to discrete quantum states, so molecular spectra form unique signatures that can be used to determine molecular structure.
- 29.5 In ionic solids and metals, atoms do not form discrete molecules, but extended symmetrical relationships, where each atom may be bound to some extent to several other atoms, forming lattice structures. In metals, electrons in lower orbits may actually overlap locations of the binding electrons, and transfer easily between atoms, creating current that carries energy and charge.
- 29.6 In some solids, the possible energy levels are so close together that electrons can slip easily from one level to another. When the highest energy level is only partially occupied, electrons can slip in and out, traveling easily. Materials with such bands are good conductors. If the highest energy level is occupied, electrons have difficulty crossing a forbidden zone gap from lower bands into the high-level conducting band so these materials are insulators. In semiconductors, this gap is small enough to allow some electrons to make the jump.
- 29.7 Semiconductor properties can be fine tuned by adding small impurities, such as arsenic, which supply extra electrons to the primary semiconductor metals, usually germanium and silicon. The bands carrying the extra electrons may lie near the conduction band (n-type) or near the valence band (p-type).
- 29.8 When an inactive n-type semiconductor is joined to an inactive p-type seminconductor to form a diode, electrons will move from the n-type to the p-type metal until an equilibrium is reached. Current applied from a source such as a battery flows readily through the diode, and diode properties can be selected to limit the amount of current and force it to flow in one direction only. Electrons flowing across the band gap emit photons (light-emitting diodes or LEDs).
- 29.9 Under the right circumstances (a diode connected in reverse bias), the electron field causes atoms to ionize, increasing the amount of current. Diodes appropriately aligned and doped allow us to create photovoltaic cells, which convert light energy to electric current, and LEDs, which use current to create light with high efficiency. Organic LEDs use organic compounds with semiconductor properties.
- 29.10 Transistors are three-part systems, with either a p-type semicondictor sandwiched between two n-types (npn) or an n-type conductor sandwiched between two p-types. One component is the collector, one the base, and one the emitter. Transistors can act as amplifiers or as gates, and can be miniturized to create complex circuits in very small volumes.
- 29.11 Integrated circuits use impurities embedded in silicon crystals to create circuits with millions to billions of elements packed within a few square millimeters.
||E: rotational energy (Quantized!)
I: Inertial moment
ω: angular velocity
l: rotational angualr momentum quantum number
||E: vibration energy|
v: vibrational quantium number
Read the following weblecture before chat: Molecules and Solids
Unfortunately, the best simulations for solid state physics concepts are Java applets which no longer run on most browsers. Watch this space. In the meantime, check the weblecture for video and demonstration sites.
Chat Preparation Activities
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- Mastery Exercise: The Moodle Mastery exercise for the chapter will contain sections related to our chat topic. Try to complete these before the chat starts, so that you can ask questions.
- Required: Complete the Mastery exercise with a passing score of 85% or better.
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If you want lab credit for this course, you must complete at least 12 labs (honors course) or 18 labs (AP students). One or more lab exercises are posted for each chapter as part of the homework assignment. We will be reviewing lab work at regular intervals, so do not get behind!
- Lab Instructions: There is no lab for this unit.
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