Angstrom 10‘10 meter

Angstrom units Meters 1 X 10- Calories, gram Liter-atmospheres 4.1.30 X 10"-... 

The numerical values of most physical quantities are expressed in terms of units. The distance between two points, for example, can be specified by the number of meters (or feet, Angstroms, etc.). Similarly, time cap be expressed in seconds, days or, say, years. However, the number of days per year varies from one year to another. The quantities, distance (length) and time, as well as mass, are usually chosen to be primary quantities. In terms of them Newton s second law for the force on an object, can be written as force = mass, distance/(time)2. The definition of the primary quantities allows dimensional expressions to be written, such as [force] — MLT-2 in the present example. Note, however, that in everyday life one speaks of the weight of an object (or a person). Of course the weight is not the mass, but rather the force acting on the object by the acceleration due to gravity [acceleration] = LT 2. 

FIGURE 13.5 Surface area of solid (A) and porous particle-based (B) packings. Surface area expressed as square meters per gram, particle size as micrometers, and pore size as angstrom units. 

The four hydrogen atoms occupy equivalent positions with respect to the central carbon atom in methane. The C-H bond length in methane is 1.107 A (1 A [angstrom] = 1 x 10 ° meters), a very short distance indeed. The four atoms of hydrogen are arranged about the central carbon atom at the comers of a regular tetrahedron. 

A angstrom (metric unit)—Ten billion angstroms equal 1 meter... 

If a fiber is less than a few micrometers thick, mineralogists may emphasize its fineness with the term fine-fibrous. Particles with diameters of a few mils (1 mil = 25 micrometers see Table 1.1) must be examined under a microscope. Electron microscopy is employed for the finest materials, and high-resolution electron microscopy is required to image fibrils with diameters of a few angstroms (1 angstrom = 10 ° meter or 0.1 of a nanometer). At any resolution, the aspect ratio can be estimated or measured as part of the identification of the fiber. 

An atomic unit of length used in quantum mechanical calculations of electronic wavefunctions. It is symbolized by o and is equivalent to the Bohr radius, the radius of the smallest orbit of the least energetic electron in a Bohr hydrogen atom. The bohr is equal to where a is the fine-structure constant, n is the ratio of the circumference of a circle to its diameter, and is the Rydberg constant. The parameter a includes h, as well as the electron s rest mass and elementary charge, and the permittivity of a vacuum. One bohr equals 5.29177249 x 10 meter (or, about 0.529 angstroms). 

Figure 7-6 Different size scales in a packed bed catalytic reactor. We must consider the position z in the bed, the flow around catalyst pellets, diffusion within pores of pellets, and adsorption and reaction on reaction sites. These span distance scales from meters to Angstroms.

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