Section 2 Physical principles

The ratio of elastically to inelastically scattered electrons and, thus, their importance for imaging or analytical work, can be calculated from basic physical principles consider the differential elastic scattering cross section... 

This section briefly reviews prediction of the native structure of a protein from its sequence of amino acid residues alone. These methods can be contrasted to the threading methods for fold assignment [Section II.A] [39-47,147], which detect remote relationships between sequences and folds of known structure, and to comparative modeling methods discussed in this review, which build a complete all-atom 3D model based on a related known structure. The methods for ab initio prediction include those that focus on the broad physical principles of the folding process [148-152] and the methods that focus on predicting the actual native structures of specific proteins [44,153,154,240]. The former frequently rely on extremely simplified generic models of proteins, generally do not aim to predict native structures of specific proteins, and are not reviewed here. 

In this section we will describe the physical principles of measurements for gravimeters of the first group. It is our pleasure to note that Prof. W. Torge gave very extensive and detailed description of both types of gravimeters that greatly helped us to prepare this and the next sections. 

In the following chapter this brief outline of representation theory will be applied to several problems in physical chemistry. It is first necessary, however, to show how functions can be adapted to conform to the natural symmetry of a given problem. It will be demonstrated that this concept isof particular importance in the analysis of molecular vibrations and in the th iy of molecular orbitals, among others. The reader is warned, however, that a serious development of this subject is above the level of this book. Hence, in the following section certain principles will be presented without proof. 

There are probably more than five million species on the Earth many of which are very similar. Moreover, as many as now exist have probably been lost. In our opinion it is not to be expected that an explanation can be given for particular species although they are connected by morphology or DNA/RNA or protein sequences. We shall turn to the problem as to why there have been and are so many species in Chapter 11. We wish to look at evolution from the point of view of very general chemical and physical principles which is the same thermodynamic approach that we used in the analysis of formation of clouds or the ozone layer in Section 3.4. We observe immediately that the major groups in Table 4.2 have all advanced and have not displaced one another. 

Basic principles of MRS. The overall physical principles and characteristics of magnetic resonance spectroscopy (MRS) are identical to those described previously in the MRI section. In fact, magnetic resonance spectroscopy can simply be thought of as just another way of expressing the NMR signals that are recorded during an NMR experiment. Whether it is an MRI or and MRS experiment, virtually all of the same equipment is used and all of the basic NMR principles still apply. The prime difference that separates basic MRS from modern-day MRI is that in MRS, the... 

The arguments treated in the two preceding sections were developed in terms of simple equilibrium thermodynamics. The weathering of rocks at the earth s surface by the chemical action of aqueous solutions, and the complex water-rock interaction phenomena taking place in the upper crust, are irreversible processes that must be investigated from a kinetic viewpoint. As already outlined in section 2.12, the kinetic and equilibrium approaches are mutually compatible, both being based on firm chemical-physical principles, and have a common boundary represented by the steady state condition (cf eq. 2.111). 

Progress in the Raman spectroscopic study of carbohydrates became possible during the past few years owing to the introduction of laser sources. Before discussing the results of laser-Raman spectroscopy applied to carbohydrates, we shall give a brief recapitulation of the physical principles of the Raman effect. Experimental techniques of infrared spectroscopy have been described in previous reviews,116,17 but no such description has been given for the Raman method. That is why the Description Section, which follows, will include the physical fundamentals of the method, as well as the sampling techniques. 

The components of a thermoelectric cooler are indicated by the cross section of a typical unit shown in Fig. 1. Therm oelectiic coolers such as this are actually small heat pumps that operate on the physical principles well established over a century ago. Semiconductor materials with dissimilar characteristics are connected electrically in senes and thermally in parallel, so that two junctions are created. The semiconductor materials are n- and /i-type and are so named because either they have more electrons than necessary to complete a perfect molecular lattice structure (ri-type), or not enough electrons to complete a lattice structure (/7-type). The extra electrons in the -type material and the holes left in the /7-type material are called carriers and they are the agents that move the heat energy from the cold to the hot junction. 

In the first section will be presented XAS from the physical principles to data analysis and measurements. Then section 2 will be devoted to a discussion of a few examples to illustrate the power and limitations of XAS for gaining structural information. Examples are focused on EXAFS studies on nanocrystalline materials. Detailed reviews for applications on other fields of materials science or for presenting the complementary information available by the study of the X-ray Absorption Near Edge Structure (XANES) part of the X-ray absorption spectrum can be found in a number of books [3-5], A brief overview of the recent development of the technique regarding the use of X-ray microbeams available on the third generation light sources will be finally presented in the last section. 

This chapter is organized along the themes of physical principles, technological realization and security applications of XDI, whose historical development is traced in the remainder of this section. Section 2 reviews the principles underlying the two fields of physics that are involved in XDI XRD on the one hand and X-ray tomography on the other. 

The physical principles of XRD have to be complemented with those underlying radiological imaging in order to complete the description of XDI for extended objects. Attenuation effects are much more significant as a source of signal degradation in XDI than in XRD, which only deals with small samples, and multiple scatter effects have to be explicitly accounted for as described in this section. 

This chapter has summarized developments in XDI along the axes of physical principles, technological realization and security applications. This threefold structure will be adopted for discussing the future outlook of XDI in this section. 

A. Pross, Theoretical and physical principles of organic reactivity, Wiley, New York, 1995, ISBN 0471555991, Section 10.4.1. 

We now consider application of percolation theory to describing mercury intrusion into porous solids. First we briefly recall the main physical principles of mercury porosimetry (in particular, the Washburn equation). These principles are treated in detail in many textbooks [e.g., Lowell and Shields 49)]. The following discussions (Sections IV,B and IV,C) introduce general equations describing mercury penetration and demonstrate the effect of various factors characterizing the pore structure on this process. Mercury extrusion from porous solids is briefly discussed in Section IV,D. 

---