DQW

Albert, you are asking a lot of good, yet very specific, questions in several dispersed areas of general/physical chemistry. While a forum like this can answer these questions individually, it can not help you get a ground-up understanding of the basic concepts that will clarify several of these doubts with a single fell swoop. The underlying principles are what you want to learn, and the best way to do that (outside of classes) is from a good book. I recommend Physical Chemistry, by Atkins. That is one book you will not regret buying - but at least check it out at your library.

I share Martin's excitement upon looking into t' Hooft's "theorist" page. Thanks, Martin - that page is already looking like a treasure (and it's currently under construction) !

Xenu is priceless ! I personally find the whole "scientific conspiracy" thing (that blike talked about) quite tiresome (and not just in the case of ID or creationism, but in general, for every new crackpot idea out there).

If what you are interested in the the weak field in the region indicated, then yes, mumetal will do just that. But a side effect, is that this will also considerably weaken the field on the other side, though not as much as on the side that the mumetal covers.

Quick counterexample : [imath]NaOH + HCl \longrightarrow NaCl + H_2O [/imath]

When handling indium, you need only be careful if dealing with powder or dust. I've made a few indium o-ring seals in the course of my work, and for safety and to prevent contamination, I wear gloves, but that's as far as I go in terms of precaution. I would - if at all - liken it to bismuth. Even though bismuth is toxic by itself, you need to be most careful when handling powders or dust (as the room temperature vapor pressure of both metals is way below concern levels). As far as toxicity goes, indium is still something of a question mark; so it's rarely a bad move to be more careful than prescribed.

Lowering the activation energy does not affect the reaction enthalpy.

An excellent analogy. Thanks for the effort. I hope the OP gains an intuitive feel for reaction energetics from it. That's exactly what I was talking about. The probability goes like [imath]e^{-E_a/RT} [/imath], but even a very large activation only creates a very small probability. So, when asking whether a reaction will happen, it is important to understand what one means by 'happen'. Typically, though, this is taken to mean an equilibrium constant that is noticably larger than 1 or a free energy change that is significantly negative (of course, one follows from the other).

Yes, this is true of both indium and mercury - both of them are hardly toxic in the pure elemental form, but both form very toxic oxides.

This is the traditional method of shielding. However, it does not do the kind of "shielding" the OP is talking about. Mumetal shields regions outside it by restricting a large fraction of the field to itself. It is an excellent soft ferromagnet with a susceptibility of around 105 (large, positive susceptibility). What the OP wants, is a material that will "completely reverse the magnetic field" - a perfect diamagnet (large, negative susceptibility). The geometry described in the picture, however, can be achieved (roughly) either way. With a SC, the field lines pass through air (more likely liquid nitrogen or its cold vapor); with mumetal, the field lines pass through the metal. The first is harder to implement (no room temperature solution), but the second restricts the field lines to the inside of a metal.

Research on thermoelectrics is still quite active. To the OP : You want to look up thermoelectricity or thermopower. Here are a few links to get you started : http://www.nanothermel.org/public_main.htm http://www.rhul.ac.uk/Physics/Nanophysics/thermopower.htm

The picture you've drawn can not be achieved as proposed...and even something close to what you've drawn can be made only with a superconductor (SC). SCs are your perfectly diamagnetic materials. They have a susceptibility [imath] \chi _m = -1[/imath] making [imath] B_{in} = H + M = H + \chi _m H = H ( 1+ \chi _m) = 0 [/imath] Bismuth, while being a relatively strong diamagnet (compared to other normal diamegnets) has a susceptibility of only about [imath] \chi _m = -10^{-4} [/imath], so it will allow 99.99% of the field right through it.

There are SAT test preparation books that have vocabulary lists that will help you practice for the SAT-I verbal section. Here's a look at the Barron's book - look at pgs 139 - 244 More links : http://www.freevocabulary.com/ http://www.english-test.net/sat/vocabulary/meanings/488/sat-test.php http://rnewton.home.infionline.net/sat/sateng.html

GaN has a blue photoemission peak, so yes, it is used to make blue LEDs. GaAs is used in high frequency devices like CD/DVD players (due to much higher mobilities than Si) and in heterostructures, for all kinds of fundamental and applied research.

In general, all reactions will occur above 0K. Some reactions tend to occur more completely than others, and some, more rapidly than others. The completion of a reaction is linked to the thermodynamics (specifically, the Gibbs' Free Energy) and the rapidity is related to its kinetics (specifically the rate constant, which is a function of reaction conditions - temperature, pressure, catalysis, etc. - as well as molecular geometry and the molecularity of the reaction).

While Zukav's book makes for interesting reading, it can hardly teach you any QM. Do not try to learn physics from a book written by someone who is not a physicist and has had no formal training in physics (Zukav studied International Relations at Harvard) - that will most likely lead to more misconceptions than revelations.

I second Tom on Resnick and Eisberg - that's a very good beginning QM book. Another good book at about the same level is Gasiorowicz.

No, it is not valid. You are assuming that the force on the upper magnet is independent of its position above the (fixed) SC ? What is the basis for such an assumption ?

You will not find any gallium in an LED - only gallium nitride, which is a hard ceramic that melts at 2500C or about 4500F. For gallium, see this thread (particularly posts #26, #34 and definitely, #63) : http://www.scienceforums.net/forums/showthread.php?t=9680

I'll go through something of an introduction before I attempt to answer the question itself. If two substances melt at temperatures [imath]T_A[/imath] and [imath]T_B[/imath] one can only expect their mixture to melt at the exact same above temperatures (notice that I'm saying that a thing melts at two temperatures) if the the two components do not interact with each other in any way. For instance' date=' consider a misture of snow and gallium pellets. If you slowly heat this mixture up, the snow (ice) will melt at about 0C, and from then on, up to about 30C you will have a mixture of solid gallium in water, at which point the gallium will melt, giving rise to a liquid-liquid mixture. No matter whether this mixture is 90% snow or 90% gallium, you will see essentially this same behavior. This mixture clearly has two melting points - one where the first component melts (often called a solidus temperature or curve) and another where the second does (called the liquidus temperature or curve). Between these temperatures, there's a solid-liquid mixture. An alloy is nothing but an intimate mixture between metals (as primary components). In this context, there are two (pertinent) different kinds of alloys : (i) isomorphous alloys or solutions, and (ii) eutectic alloys. An isomorphous solution is a substitutional alloy formed between metals with similar crystal structures(lattice parameter and symmetry) (eg : Cu-Ni or Ag-Au alloys). In an alloy of say, 70%Cu-30%Ni, the Ni atoms occupy exactly the lattice positions of the Cu atoms - hence the term 'substitutional'. In such alloys the liquidus temperature and solidus temperatures often lie between the two melting points and are a function of the composition because the components are intimately interacting with each other. An isomorphous alloy is what one may refer to as a "homogeneous mixture" or a solid "solution". [img']http://www.ce.berkeley.edu/~paulmont/CE60/alloys/img003.gif[/img] Solder is a eutectic alloy, not an isomorphous solution. A eutectic is formed between metals of very different latice parameters and crystal structures (eg : Sn-Pb or Ag-Cu). A eutectic alloy is called a two-phase alloy because if you look at it under a microscope, you can distinguish two different types of regions each with its own composition and crystal structure. Each of these phases is in fact, just an isomorphous alloy. So here's what happens when I try to alloy Pb with Sn : I start with pure Sn and slowly add Pb to it. At very low concentrations of Pb, the Pb atoms actually occupy the positions of the Sn atoms in the Sn lattice. But Pb atoms being larger than Sn atoms, this will strain the lattice locally. Beyond a certain concentration, it becomes infeasible to substitute the bulky Pb atoms in the spaces meant for Sn. Instead, it becomes easier to make a new phase which is predominantly Pb, with a small portion of the Sn atoms substituting for the Pb. So, a eutectic alloy consists of two distincet phases - each phase rich in one of the components. But each of these phases is now strained because of the distortion resulting from different radii. This strain causes a lowering of the melting temperatures. In addition to the strain, there is also a lowering that results from the poor bonding between the two phases. If the alloy is predominantly one of the two phases (say 85%A-15%B or 15%a-85%B) there will not be much lowering due to poor interphase bonding because there is so little of one phase (ie: the alooy is mostly single-phase). While the first factor plays a dominant role near the extreme compositions, the second factor is what is important at intermediate compositions. Among the various compositions that can be made between Sn and Pb, the one with the lowest melting point is called the eutectic composition (62%Sn-38%Pb).

Could you please elaborate ?

This is really a question of nomenclature. For instance, a crytalline solid is not considered a molecule.