Tuesday, August 14, 2007

Capacitors and Capacitance.

Capacitors

The capacitor's function is to store electricity, or electrical energy.
The capacitor also functions as a filter, passing alternating current (AC), and blocking direct current (DC).

Capacitance

This is a measure of a capacitor's ability to store charge. A large capacitance means that more charge can be stored. Capacitance is measured in farads, symbol F. However 1F is very large, so prefixes are used to show the smaller values.

Three prefixes (multipliers) are used, µ (micro), n (nano) and p (pico):

  • µ means 10-6 (millionth), so 1000000µF = 1F
  • n means 10-9 (thousand-millionth), so 1000nF = 1µF
  • p means 10-12 (million-millionth), so 1000pF = 1nF

Tuesday, July 31, 2007

Botany

branch of biology that deals with the study of plants, including their structure, properties, and biochemical processes. Also included are plant classification and the study of plant diseases and of interactions with the environment. The principles and findings of

botany have provided the base for such applied sciences as agriculture, horticulture, and forestry.

Plants were of paramount importance to early man; he depended upon them as sources of food, shelter, clothing, medicine, ornament, tools, and magic. Today it is known that, in addition to their practical and economic values, green plants are indispensable to all life on Earth: through the process of photosynthesis, plants transform energy from the sun into the chemical energy of food, which makes all life possible. A second unique and important capacity of green plants is the formation and release of oxygen as a by-product of photosynthesis. The oxygen of the atmosphere, so absolutely essential to many forms of life, represents the accumulation of over 3,500,000,000 years of photosynthesis by green plants.

Although the many steps in the process of photosynthesis have become fully understood only in recent years, even in prehistoric times man somehow recognized intuitively that some important relation existed between the sun and plants. Such recognition is suggested by the fact that, in primitive tribes and early civilizations, worship of the sun was often combined with the worship of plants.

Earliest man, like the other anthropoid mammals (e.g., apes, monkeys), depended totally upon the natural resources of his environment, which, until he developed methods for hunting, consisted almost completely of plants. The behaviour of pre-Stone Age man can be inferred by studying the botany of aboriginal peoples in various parts of the world. Isolated tribal groups in South America, Africa, and New Guinea, for example, have extensive knowledge about plants and distinguish hundreds of kinds according to their utility, as edible, poisonous, or otherwise important in their culture. They have developed surprisingly sophisticated systems of nomenclature and

classification, which approximate the binomial system (i.e., generic and specific names) found in modern biology. The urge to recognize different kinds of plants and to give them names thus seems to be as old as the human race.

In time plants were not only collected by primitive man but also grown by him. This domestication resulted not only in the development of agriculture but also in a greater stability of human populations that had previously been nomadic. From the settling down of agricultural peoples in places where they could depend upon adequate food supplies came the first villages and the earliest civilizations.

Because of the long preoccupation of man with plants, a large body of folklore, general information, and actual scientific data has accumulated, which has become the basis for the science of botany.

Theophrastus, a Greek philosopher who studied first with Plato and then became a disciple of Aristotle, is credited with founding botany. Only two of an estimated 200 botanical treatises written by him are known to science: originally written in Greek about 300 BC, they have survived in the form of Latin manuscripts, De causis plantarum and De historia plantarum. His basic concepts of morphology, classification, and the natural history of plants, accepted without question for many centuries, are now of interest primarily because of Theophrastus' independent and philosophical viewpoint.

Pedanius Dioscorides, a Greek botanist of the 1st century AD, was the most important botanical writer after Theophrastus. In his major work, an herbal in Greek, he described some 600 kinds of plants, with comments on their habit of growth and form as well as on their medicinal properties. Unlike Theophrastus, who classified plants as trees, shrubs, and herbs, Dioscorides grouped his plants under three headings: as aromatic, culinary, and medicinal. His herbal, unique in that it was the first treatment of medicinal plants to be illustrated, remained for about 15 centuries the last word on medical botany in Europe.

From the 2nd century BC to the 1st century AD, a succession of Roman writers—Cato, Varro, Virgil, and Columella—prepared Latin manuscripts on farming, gardening, and fruit growing but showed little evidence of the spirit of scientific inquiry for its own sake that was so characteristic of Theophrastus. In the 1st century AD,

Pliny the Elder, though no more original than his Roman predecessors, seemed more industrious as a compiler. His Historia naturalis—an encyclopaedia of 37 volumes, compiled from some 2,000 works representing 146 Roman and 327 Greek authors—has 16 volumes devoted to plants. Although uncritical and containing much misinformation, this work contains much information otherwise unavailable, since most of the volumes to which he referred have been destroyed.

The printing press revolutionized the availability of all types of literature, including that of plants. In the 15th and 16th centuries, many

herbals were published with the purpose of describing plants useful in medicine. Written by physicians and medically oriented botanists, the earliest herbals were based largely on the work of Dioscorides and to a lesser extent on Theophrastus, but gradually they became the product of original observation. The increasing objectivity and originality of herbals through the decades is clearly reflected in the improved quality of the woodcuts prepared to illustrate these books.

In 1552 an illustrated manuscript on Mexican plants, written in Aztec, was translated into Latin by Badianus; other similar manuscripts known to have existed seem to have disappeared. Whereas herbals in China date back much further than those in Europe, they have become known only recently and so have contributed little to the progress of Western botany.

The invention of the optical lens during the 16th century and the development of the compound microscope about 1590 opened an era of rich discovery about plants; prior to that time, all observations by necessity had been made with the unaided eye. The botanists of the 17th century turned away from the earlier emphasis on medical botany and began to describe all plants, including the many new ones that were being introduced in large numbers from Asia, Africa, and America. Among the most prominent botanists of this era was

Gaspard Bauhin, who for the first time developed, in a tentative way, many botanical concepts still held as valid. In 1665

Robert Hooke published, under the title Micrographia, the results of his microscopic observations on several plant tissues. He is remembered as the coiner of the word cell, referring to the cavities he observed in thin slices of cork; his observation that living cells contain sap and other materials too often has been forgotten. In the following decade,

Nehemiah Grew and

Marcello Malpighi founded plant anatomy; in 1671 they communicated the results of microscopic studies simultaneously to the Royal Society of London, and both later published major treatises.

Experimental plant physiology began with the brilliant work of

Stephen Hales, who published his observations on the movements of water in plants under the title Vegetable Staticks (1727). His conclusions on the mechanics of water transpiration in plants are still valid, as is his discovery—at the time a startling one—that air contributes something to the materials produced by plants. In 1774,

Joseph Priestley showed that plants exposed to sunlight give off oxygen, and

Jan Ingenhousz demonstrated, in 1779, that plants in the dark give off carbon dioxide. In 1804 Nicolas de

Saussure demonstrated convincingly that plants in sunlight absorb water and carbon dioxide and increase in weight, as had been reported by Hales nearly a century earlier.

The widespread use of the

microscope by plant morphologists provided a turning point in the 18th century—botany became largely a laboratory science. Until the invention of simple lenses and the compound microscope, the recognition and classification of plants were, for the most part, based on such large morphological aspects of the plant as size, shape, and external structure of leaves, roots, and stems. Such information was also supplemented by observations on more subjective qualities of plants, such as edibility and medicinal uses.

In 1753

Linnaeus published his master work, Species Plantarum, which contains careful descriptions of 6,000 species of plants from all of the parts of the world known at the time. In this work, which is still the basic reference work for modern plant taxonomy, Linnaeus established the practice of binomial nomenclature—that is, the denomination of each kind of plant by two words, the genus name and the specific name, as Rosa canina, the dog rose. Binomial nomenclature had been introduced much earlier by some of the herbalists, but it was not generally accepted; most botanists continued to use cumbersome formal descriptions, consisting of many words, to name a plant. Linnaeus for the first time put the contemporary knowledge of plants into an orderly

system, with full acknowledgment to past authors, and produced a nomenclatural methodology so useful that it has not been greatly improved upon. Linnaeus also introduced a "sexual system" of plants, by which the numbers of flower parts—especially stamens, which produce male sex cells, and styles, which are prolongations of plant ovaries that receive pollen grains—became useful tools for easy identification of plants. This simple system, though effective, had many imperfections. Other classification systems, in which as many characters as possible were considered in order to determine the degree of relationship, were developed by other botanists; indeed, some appeared before the time of Linnaeus. The application of the concepts of Charles Darwin (on evolution) and Gregor Mendel (on genetics) to plant taxonomy has provided insights into the process of evolution and the production of new species.

Systematic botany now uses information and techniques from all the subdisciplines of botany, incorporating them into one body of knowledge. Phytogeography (the biogeography of plants), plant ecology, population genetics, and various techniques applicable to cells—cytotaxonomy and cytogenetics—have contributed greatly to the current status of systematic botany and have to some degree become part of it. More recently, phytochemistry, computerized statistics, and fine-structure morphology have been added to the activities of systematic botany.

The 20th century has seen an enormous increase in the rate of growth of research in botany and the results derived therefrom. The combination of more botanists, better facilities, and new technologies, all with the benefit of experience from the past, has resulted in a series of new discoveries, new concepts, and new fields of botanical endeavour. Some important examples are mentioned below.

New and more precise information is being accumulated concerning the process of photosynthesis, especially with reference to energy-transfer mechanisms.

The discovery of the pigment phytochrome, which constitutes a previously unknown light-detecting system in plants, has greatly increased knowledge of the influence of both internal and external environment on the germination of seeds and the time of flowering.

Several types of plant hormones (internal regulatory substances) have been discovered—among them auxin, gibberellin, and kinetin—whose interactions provide a new concept of the way in which the plant functions as a unit.

The discovery that plants need certain trace elements usually found in the soil has made it possible to cultivate areas lacking some essential element by adding it to the deficient soil.

The development of genetical methods for the control of plant heredity has made possible the generation of improved and enormously productive crop plants.

The development of radioactive-carbon dating of plant materials as old as 50,000 years is useful to the paleobotanist, the ecologist, the archaeologist, and especially to the climatologist, who now has a better basis on which to predict climates of future centuries.

The discovery of alga-like and bacteria-like fossils in Precambrian rocks has pushed the estimated origin of plants on Earth to 3,500,000,000 years ago.

The isolation of antibiotic substances from fungi and bacteria-like organisms has provided control over many bacterial diseases and has contributed biochemical information of basic scientific importance as well.

 

the branch of biology that deals with plants. It involves the study of the structure, properties, and biochemical processes of all forms of plant life, including trees. Also included within its scope are plant classification and the study of plant diseases and of the interactions of plants with their physical environment. Over the years various specialized branches of botany have developed, and the principles and findings of botany, moreover, have provided the base on which depend such applied plant sciences as agriculture, horticulture, and forestry.

The science of botany traces back to the ancient Greco-Roman world but received its modern impetus in Europe in the 16th century, mainly through the work of various physicians and herbalists. These professionals, in seeking plants useful in medicine, began seriously to observe plants themselves, as reflected in the woodcuts with which their herbal books were illustrated.

In the 17th century, as a result of the earlier revival of learning and of increased facilities for travel and study in Europe and Asia, many more plants became known, and some botanists turned from medical botany to attempts to name and catalog all known kinds of plants. The most celebrated early work of this kind was Pinax theatri botanici (1623; "Illustrated Exposition of Plants") by the Swiss scientist Gaspard Bauhin, who listed and described about 6,000 species.

In the 18th century the greatest figure in botany was the Swedish scientist

Carolus Linnaeus. His most valuable and lasting contributions were his careful descriptions of approximately 6,000 species arranged in genera (the same arrangement used today), his collation of the species that he knew with the names and descriptions of previous botanists, and his rules of nomenclature. He established binomial nomenclaturei.e., the naming of each species by two words, of which the first is the name of the genus to which it belongs and the second is a qualifying word, usually an adjective ( e.g., the dog rose is Rosa canina).

Even in this early period, botany was becoming specialized. While many botanists were occupied only with the classes and names of plants, the foundations of anatomy, morphology, and physiology were being laid. The important field of genetics was initiated in the 19th century, principally through the work of the Austrian botanist Gregor Mendel.

Today the principal branches of botanical study are morphology, physiology, ecology, and systematics (the identification and ranking of all plants). Various subdisciplines include bryology (the study of mosses and liverworts), pteridology (the study of ferns and their relatives), paleo botany (the study of fossil plants), and palynology (the study of modern and fossil pollen and spores).

Area of Study

For convenience, but not on any mutually exclusive basis, several major areas or approaches are recognized commonly as disciplines of botany; these are morphology, physiology, ecology, and systematics.

Morphology

Morphology deals with the structure and form of plants and includes such subdivisions as: cytology, the study of the cell; histology, the study of tissues; anatomy, the study of the organization of tissues into the organs of the plant; reproductive morphology, the study of life cycles; and experimental morphology, or morphogenesis, the study of development.

Physiology

Physiology deals with the functions of plants. Its development as a subdiscipline has been closely interwoven with the development of other aspects of botany, especially morphology. In fact, structure and function are sometimes so closely related that it is impossible to consider one independently of the other. The study of function is indispensable for the interpretation of the incredibly diverse nature of plant structures. In other words, around the functions of the plant, structure and form have evolved. Physiology also blends imperceptibly into the fields of biochemistry and biophysics, as the research methods of these fields are used to solve problems in plant physiology.
Ecology

Ecology deals with the mutual relationships and interactions between organisms and their physical environment. The physical factors of the atmosphere, the climate, and the soil affect the physiological functions of the plant in all its manifestations, so that, to a large degree, plant ecology is a phase of plant physiology under natural and uncontrolled conditions; in fact, it has been called "outdoor physiology." Plants are intensely sensitive to the forces of the environment, and both their association into communities and their geographical distribution are determined largely by the character of climate and soil. Moreover, the pressures of the environment and of organisms upon each other are potent forces, which lead to new species and the continuing evolution of larger

Systematics

Systematics deals with the identification and ranking of all plants; it includes classification and nomenclature (naming) and enables the botanist to comprehend the broad range of plant diversity and evolution.

Other Disciplines

In addition to the major subdisciplines, several specialized branches of botany have developed as a matter of custom or convenience. Among them are bacteriology, the study of bacteria; mycology, the study of fungi;

algology or phycology, the study of algae; bryology, the study of mosses and liverworts; pteridology, the study of ferns and their relatives; and paleobotany, the study of fossil plants. Palynology is the study of modern and fossil pollen and spores, with particular reference to their identification; plant pathology deals with the diseases of plants; economic botany deals with plants of practical use to man; and ethnobotany covers the use of plants by aboriginal peoples, now and in the distant past.

Botany also relates to other scientific disciplines in many ways, especially to zoology, medicine, microbiology, agriculture, chemistry, forestry, and horticulture, and specialized areas of botanical information may relate closely to such humanistic fields as art, literature, history, religion, archaeology, sociology, and psychology.

Fundamentally, botany remains a pure science, including any research into the life of plants and limited only by man's technical means of satisfying his curiosity. It has often been considered an important part of a liberal education, not only because it is necessary for an understanding of agriculture, horticulture, forestry, pharmacology, and other applied arts and sciences, but also because an understanding of plant life is related to life in general.

Other Subdiciplines

In addition to the major subdisciplines, several specialized branches of botany have developed as a matter of custom or convenience. Among them are bacteriology, the study of bacteria; mycology, the study of fungi;

algology or phycology, the study of algae; bryology, the study of mosses and liverworts; pteridology, the study of ferns and their relatives; and paleobotany, the study of fossil plants. Palynology is the study of modern and fossil pollen and spores, with particular reference to their identification; plant pathology deals with the diseases of plants; economic botany deals with plants of practical use to man; and ethnobotany covers the use of plants by aboriginal peoples, now and in the distant past.

Botany also relates to other scientific disciplines in many ways, especially to zoology, medicine, microbiology, agriculture, chemistry, forestry, and horticulture, and specialized areas of botanical information may relate closely to such humanistic fields as art, literature, history, religion, archaeology, sociology, and psychology.

Fundamentally, botany remains a pure science, including any research into the life of plants and limited only by man's technical means of satisfying his curiosity. It has often been considered an important part of a liberal education, not only because it is necessary for an understanding of agriculture, horticulture, forestry, pharmacology, and other applied arts and sciences, but also because an understanding of plant life is related to life in general.

Because man has always been dependent upon plants and surrounded by them, he has woven them into his designs, into the ornamentation of his life, even into his religious symbolism. A Persian carpet and a bedspread from a New England loom both employ conventional designs derived from the forms of flowers. Medieval painters and great masters of the Renaissance represented various revered figures surrounded by roses, lilies, violets, and other flowers, which symbolized chastity, martyrdom, humility, and other Christian attributes.

Morphological Aspects

The invention of the compound microscope provided a valuable and durable instrument for the investigation of the inner structure of plants. Early plant morphologists, especially those studying cell structure, were handicapped as much by the lack of adequate knowledge of how to prepare specimens as they were by the imperfect microscopes of the time. A revolution in the effectiveness of microscopy occurred in the second half of the 19th century with the introduction of techniques for fixing cells and for staining their component parts. Before the development of these techniques, the cell, viewed with the microscope, appeared as a minute container with a dense portion called the nucleus. The discovery that parts of the cell respond to certain stains made observation easier. The development of techniques for preparing tissues of plants for microscopic examination was continued in the 1870s and 1880s and resulted in the gradual refinement of the field of nuclear cytology, or karyology. Chromosomes were recognized as constant structures in the life cycle of cells, and the nature and meaning of meiosis, a type of cell division in which the daughter cells have half the number of chromosomes of the parent, was discovered; without this discovery, the significance of Mendel's laws of heredity might have gone unrecognized. Vital stains, dyes that can be used on living material, were first used in 1886 and have been greatly refined since then.

Improvement of the methodology of morphology has not been particularly rapid, even though satisfactory techniques for histology, anatomy, and cytology have been developed. The embedding of material in paraffin wax, the development of the rotary microtome for slicing very thin sections of tissue for microscope viewing, and the development of stain techniques are refinements of previously known methods. The invention of the phase microscope made possible the study of unfixed and unstained living material—hopefully nearer its natural state. The development of the electron microscope, however, has provided the plant morphologist with a new dimension of magnification of the structure of plant cells and tissues. The fine structure of the cell and of its components, such as mitochondria and the Golgi apparatus, have come under intensive study. Knowledge of the fine structure of plant cells has enabled investigators to determine the sites of important biochemical activities, especially those involved in the transfer of energy during photosynthesis and respiration. The scanning electron microscope, a relatively recent development, provides a three-dimensional image of surface structures at very great magnifications.

For experimental research on the morphogenesis of plants, isolated organs in their embryonic stage, clumps of cells, or even individual cells are grown. One of the most interesting techniques developed thus far permits the growing of plant tissue of higher plants as single cells; aeration and continuous agitation keep the cells suspended in the liquid culture medium.

Physiological Aspects

Plant physiology and plant biochemistry are the most technical areas of botany; most major advances in physiology also reflect the development of either a new technique or the dramatic refinement of an earlier one to give a new degree of precision. Fortunately, the methodology of measurement has been vastly improved in recent decades, largely through the development of various electronic devices. The phytotron at the California Institute of Technology represents the first serious attempt to control the environment of living plants on a relatively large scale; much important information has been gained concerning the effects on plants of day length and night length and the effects on growth, flowering, and fruiting of varying night temperatures. Critical measurements of other plant functions have also been obtained.

Certain complex biochemical processes, such as photosynthesis and respiration, have been studied stepwise by immobilizing the process through the use of extreme cold or biochemical inhibitors and by analyzing the enzymatic activity of specific cell contents after spinning cells at very high speeds in a centrifuge. The pathways of energy transfer from molecule to molecule during photosynthesis and respiration have been determined by biophysical methods, especially those utilizing radioactive isotopes.

An investigation of the natural metabolic products of plants requires, in general, certain standard biochemical techniques—e.g., gas and paper chromatography, electrophoresis, and various kinds of spectroscopy, including infrared, ultraviolet, and nuclear magnetic resonance. Useful information on the structure of the extremely large cellulose molecule has been provided by X-ray crystallography.

Ecological Aspects

When plant ecology first emerged as a subscience of botany, it was largely descriptive; today, however, it has become a common meeting ground for all the plant sciences, as well as for other sciences. In addition, it has become much more quantitative. As a result, the tools and methods of plant ecologists are those available for measuring the intensity of the environmental factors that impinge on the plant and the reaction of the plant to these factors. The extent of the variability of many physical factors must be measured. The integration and reporting of such measurements, which cannot be regarded as constant, may therefore conceal some of the most dynamic and significant aspects of the environment and the responses of the plant to them. Because the physical environment is a complex of biological and physical components, it is measured by biophysical tools. The development of electronic measuring and recording devices has been crucial for a better understanding of the dynamics of the environment. Such devices, however, produce so much information that computer techniques must be used to reduce the data to meaningful results.

The ecologist, concerned primarily with measuring the effect of the external environment on a plant, adapts the methodology of the plant physiologist to field conditions.

The plant sociologist, on the other hand, is concerned with both the relation of different kinds of plants to each other and the nature and constitution of their association into natural communities. One widely used technique in this respect is to count the various kinds of plants within a standard area in order to determine such factors as the percentage of ground cover, dominance of species, aggressiveness, and other characteristics of the community. In general, the plant sociologist has relatively few quantitative factors to measure and must therefore take a subjective and intuitive approach, which, nevertheless, gives extremely useful results and some degree of predictability.

Some ecologists are most concerned with the inner environment of the plant and the way in which it reacts to the external environment. This approach, which is essentially physiological and biochemical, is useful for determining energy flow in ecosystems. The physiological ecologist is also concerned with evaluating the adaptations that certain plants have made toward survival in a hostile environment.

In summary, the techniques and methodology of plant ecology are as diverse and as varied as the large number of sciences that are drawn upon by ecologists. Completely new techniques, although few, are important; among them are techniques for measuring the amount of radioactive carbon-14 in plant deposits up to 50,000 years old. The most important new method in plant ecology is the rapidly growing use of computer techniques for handling vast amounts of data. Furthermore, modern digital computers can be used to simulate simple ecosystems and to analyze real ones.

Taxonomical Aspects

Experimental research under controlled conditions, made possible by

botanical gardens and their ranges of greenhouses and controlled environmental chambers, has become an integral part of the methodology of modern plant taxonomy.

A second major tool of the taxonomist is the herbarium, a reference collection consisting of carefully selected and dried plants attached to paper sheets of a standard size and filed in a systematic way so that they may be easily retrieved for examination. Each specimen is a reference point representing the features of one plant of a certain species; it lasts indefinitely if properly cared for, and, if the species becomes extinct in nature—as hundreds have—it remains the only record of the plant's former existence. The library is also an essential reference resource for descriptions and illustrations of plants that may not be represented in a particular herbarium.

One of the earliest methods of the taxonomist, the study of living plants in the field, has benefitted greatly by fast and easy methods of transportation; botanists may carry on fieldwork in any part of the world and make detailed studies of the exact environmental conditions under which each species grows.

During the present century, many new approaches have been applied to the elucidation of problems in systematic botany. The transmission electron microscope and the scanning electron microscope have added to the knowledge of plant morphology, upon which classical taxonomy so much depends.

Refined methods for cytological and genetical studies of plants have given the taxonomist new insights into the origin of the great diversity among plants, especially the mechanisms by which new species arise and by which they then maintain their individuality in nature. From such studies have arisen further methods and also the subdisciplines of cytotaxonomy, cytogenetics, and population genetics.

Phytochemistry, or the chemistry of plants, one of the early subdivisions of organic chemistry, has been of great importance in the identification of plant substances of medicinal importance. With the development of new phytochemical methods, new information has become available for use in conjunction with plant taxonomy; thus has arisen the modern field of chemotaxonomy, or biochemical systematics. Each species tends to differ to some degree from every other species, even in the same genus, in the biochemistry of its natural metabolic products. Sometimes the difference is subtle and difficult to determine; sometimes it is obvious and easily perceptible. With new analytical techniques, a large number of individual compounds from one plant can be identified quickly and with certainty. Such information is extremely useful in adding confirmatory or supplemental evidence of an objective and quantitative nature. An interesting by-product of chemical plant taxonomy has resulted in understanding better the restriction of certain insects to specific plants.

Computer techniques have recently been applied to plant taxonomy to develop a new field, numerical taxonomy, or

taximetrics, by which relationships between plant species or those within groups of species are determined quantitatively and depicted graphically. Another method measures the degree of molecular similarity of deoxyribonucleic acid (DNA) molecules in different plants. By this procedure it should be possible to determine the natural taxonomic relationships among different plants and plant groups by determining the extent of the relationship of their DNA's: closely related plants will have more similarities in their DNA's than will unrelated ones.
Additional Reading

Specific references on botany include Merritt Lyndon Fernald, Gray's Manual of Botany, 8th ed. (1950, reprinted 1988), taxonomic references for more than 8,000 species of plants found in North America; Arthur W. Galston, Peter J. Davies, and Ruth L. Satter, The Life of the Green Plant, 3rd ed. (1980), a well-organized and interesting presentation of functional botany; Michael Crawley (ed.), Plant Ecology (1986), a discussion of factors that influence the distribution and abundance of plants; David K. Northington and J.R. Goodin , The Botanical World (1984), an overview of plant morphology, physiology, and role within an ecosystem; Donna N. Schumann, Living with Plants: A Gardener's Guide to Practical Botany , 2nd ed. (1992); Kingsley R. Stern, Introductory Plant Biology, 6th ed. (1994), basic information on plant structure, function, reproduction, and evolution; James D. Mauseth , Botany: An Introduction to Plant Biology (1991), and Plant Anatomy (1988), an integration of plant structure and function; Lincoln Taiz and Eduardo Zeiger (eds.), Plant Physiology (1991), essays summarizing important principles; R.P.F. Gregory, Biochemistry of Photosynthesis, 3rd ed. (1989), covering the details of photosynthesis; G. Robin South and Alan Whittick, Introduction to Phycology (1987), a systems approach to the study of algae, profusely illustrated; Susan Isaac, Fungal-Plant Interactions (1992), a synthesis of fungal physiology, plant pathology, and biology; M.M. Yeoman (ed.), Plant Cell Culture Technology (1986), a discussion of the application of plant cell and tissue cultures in industry; and S.H. Mantell, J.A. Matthews, and R.A. McKee, Principles of Plant Biotechnology (1985), relating the principles to crop improvement and to the use of natural plant processes in industry.


Monday, July 30, 2007

Gravitation

Gravitation is a natural phenomenon by which all objects attract each other. In everyday life, gravitation is most familiar as the agency that endows objects with weight. Gravitation is responsible for keeping the Earth and the other planets in their orbits around the Sun; for keeping the Moon in its orbit around the Earth; for the formation of tides; for convection (by which hot fluids rise); for heating the interiors of forming stars and planets to very high temperatures; and for various other phenomena that we observe. Gravitation is also the reason for the very existence of the Earth, the Sun, and most macroscopic objects in the universe; without it, matter would not have coalesced into these large masses, and life, as we know it, would not exist.
Modern physics describes gravitation using the general theory of relativity, but the much simpler Newton's law of universal gravitation provides an excellent approximation in most cases.
In scientific terminology gravitation and gravity are distinct. "Gravitation" is the attractive influence that all objects exert on each other, while "gravity" specifically refers to a force which all massive objects (objects with mass) are theorized to exert on each other to cause gravitation. Although these terms are interchangeable in everyday use, in theories other than Newton's, gravitation is caused by factors other than gravity. For example in general relativity, gravitation is due to spacetime curvatures which causes inertially moving objects to tend to accelerate towards each other. Another (discredited) example is Le Sage's theory of gravitation, in which massive objects are effectively pushed towards each other.

Saturday, July 7, 2007

Units and dimensions

Units and dimensions
Introduction
Base units and derived units
The SI system of units.

Introduction
In science, a type of question often asked is how much? how big? In order to answer such questions it is important to have systems of measurement which are consistent and understood by all.
A dimension is a property that can be measured such as distance, time, temperature, speed.
A unit is a basic division of a measured quantity and it enables to say how much of the quantity we have - 10 miles, 2 hours etc.

Base units and derived units
Basic units are units that are defined by reference to some external standard. This external standard is arbitrary but is a matter of common agreement.
Derived units are units that are defined by reference to combinations of the base units.

The SI system of units.
The SI system is an internationally agreed system of units based on seven base units. These are listed in table 1 below. Some of the more important derived units are listed in table 2.
Table 1
Base units of the SI system of unitsQuantity Unit Symbol Mass kilogramme kg Length metre m Time second s Mole mole mol Temperature kelvin K Electric current ampere A Light intensity candela cd

Table 2
Some derived units in the SI systemQuantity Unit Symbol Volume cubic metre m3 Force Newton = kg m s-2 N Pressure Pascal = N m-2 Pa Work, Energy Joule = N m J Power Watt = J s-1 W Molar concentration Molar = mol dm-3 or mol L-1 M

Multiples of the basic units are used to avoid having to write very large or very small numbers. These are listed in table 3.
Table 3
Multipliers for SI unitLarge quantities Small Quantities Prefix Symbol multiplier Prefix Symbol multiplier deca D 10 deci d 10-1 hecta h 100 centi c 10-2 kilo k 103 milli m 10-3 Mega M 106 micro m 10-6 Giga G 109 nano n 10-9 Tera T 1012 pico p 10-12 Exa E 1015 femto f 10-15

Dimensional consistency
All equations relating physical quantities should be dimensionally consistent. That is when the units on both sides of an equation are worked out they should be identical.
Example 1consider the ideal gas equation, Pv = nRT
Writing the units for each term in the equation
LHS: RHS:
i.e. LHS units = RHS units
If an equation contains additive terms, then each term in the equation must be dimensionally consistent.
Example 2The equation for the distance travelled by a body moving with uniform acceleration is;
s = u.t +1/2 a.t2
Writing the units for each term in the equation
i.e. Units of all three terms in the equation are the same.
source: wikipedia

Wednesday, July 4, 2007

Vectors And Scalars

To better understand the science of propulsion it is necessary to use some mathematical ideas from vector analysis. Most people are introduced to vectors in high school or college, but for the elementary and middle school students, or the mathematically-challenged:

DON'T PANIC!.

There are many complex parts to vector analysis and we aren't going there. We are going to limit ourselves to the very basics. Vectors allow us to look at complex, multi-dimensional problems as a simpler group of one-dimensional problems. We will be concerned mostly with definitions The words are a bit strange, but the ideas are very powerful as you will see.
Math and science were invented by humans to describe and understand the world around us. We live in a (at least) four-dimensional world governed by the passing of time and three space dimensions; up and down, left and right, and back and forth. We observe that there are some quantities and processes in our world that depend on the direction in which they occur, and there are some quantities that do not depend on direction. For example, the volume of an object, the three-dimensional space that an object occupies, does not depend on direction. If we have a 5 cubic foot block of iron and we move it up and down and then left and right, we still have a 5 cubic foot block of iron. On the other hand, the location, of an object does depend on direction. If we move the 5 cubic foot block 5 miles to the north, the resulting location is very different than if we moved it 5 miles to the east. Mathematicians and scientists call a quantity which depends on direction a vector quantity. A quantity which does not depend on direction is called a scalar quantity.

Vector quantities have two characteristics, a magnitude and a direction. Scalar quantities have only a magnitude. When comparing two vector quantities of the same type, you have to compare both the magnitude and the direction. For scalars, you only have to compare the magnitude. When doing any mathematical operation on a vector quantity (like adding, subtracting, multiplying ..) you have to consider both the magnitude and the direction. This makes dealing with vector quantities a little more complicated than scalars.
On the slide we list some of the physical quantities discussed in the Beginner's Guide to Propulsion and group them into either vector or scalar quantities. Of particular interest, the forces which operate on a flying aircraft, the weight, thrust, and aerodynmaic forces, are all vector quantities. The resulting motion of the aircraft in terms of displacement, velocity, and acceleration are also vector quantities. These quantities can be determined by application of Newton's laws for vectors. The scalar quantities include most of the thermodynamic state variables involved with the propulsion system, such as the density, pressure, and temperature of the propellants. The energy, work, and entropy associated with the engines are also scalar quantities. There are some quantities, like speed, which have very special definitions for scientists. By definition, speed is the scalar magnitude of a velocity vector. A car going down the road has a speed of 50 mph. Its velocity is 50 mph in the northeast direction. It can get very confusing when the terms are used interchangeably!

While Newton's laws describe the resulting motion of a solid, there are special equations which describe the motion of fluids, gases and liquids, through the propulsion system. For any physical system, the mass, momentum, and energy of the system must be conserved. Mass and energy are scalar quantities, while momentum is a vector quantity. This results in a coupled set of equations, called the Navier-Stokes equations, which describe how fluids behave when subjected to external forces. These equations are the fluid equivalent of Newton's laws of motion and are very difficult to solve and understand. A simplified version of the equations called the Euler equations can be solved for some fluids problems.
source: wikipedia
  

Friday, June 29, 2007

statics

Statics


Mechanics
The study of forces acting on bodies.
3 Branches of Mechanics:

1.Statics

2.Dynamics

3.Strength of Materials

Statics
The study of rigid bodies that are in equilibrium.

Force
A "push" or "pull" exerted by one body on another, such as:
A person pushing on a wall
Gravity pulling on a person

Scalar
A quantity possessing only a magnitude such as mass, length, or time.

Vector
A quantity that has both a magnitude and direction such as velocity or force.

Force
Force is a vector quantity, therefore a force is completely described by:
a.Magnitude
b.Direction

Point of Application
Types of vectors used in statics:

Vector Addition - the parallellogram law.

Resolution of forces into components.
The net effect of a number of forces on one point can be the same as the effect of one force.

Free Body Diagram
A free body diagram is a sketch of the body and all the forces acting on it.
3 steps in drawing a free body diagram:

1.Isolate the body, remove all supports and connectors.

2.Identify all EXTERNAL forces acting on the body.

3.Make a sketch of the body, showing all forces acting on it.
       
Equilibrium
A body is in equilibrium if the sum of all the external forces and moments acting on the body is zero.

Steps in solving a statics problem.
1.Draw a free body diagram.
2.Choose a reference frame. Orient the X & Y axes. (Most often X is chosen in the horizontal direction and Y is chosen in the vertical direction.)
3.Choose a convenient point to calculate moments around.
4.Apply the 3 equilibrium equations and solve for the unknowns.

Problem
Two children balance a see-saw in horizontal equilibrium. One weighs 80 pounds, and the other weighs 60 pounds and is sitting 4 ft. from the fulcrum. Find the force the fulcrum applies to the beam and the distance to the fulcrum to the 80 lb. child. (Neglect the mass of the see-saw.)Work
work refers to an activity involving a force and movement in the directon of the force. A force of 20 newtons pushing an object 5 meters in the direction of the force does 100 joules of work.
 Energy
energy is the capacity for doing work. You must have energy to accomplish work - it is like the "currency" for performing work. To do 100 joules of work, you must expend 100 joules of energy.
 Power
power is the rate of doing work or the rate of using energy, which are numerically the same. If you do 100 joules of work in one second (using 100 joules of energy), the power is 100 watts.
 
  Work Energy Principle
The change in the kinetic energy of an object is equal to the net work done on the object.
This fact is referred to as the Work-Energy Principle and is often a very useful tool in mechanics problem solving. It is derivable from conservation of energy and the application of the relationships for work and energy, so it is not independent of the conservation laws. It is in fact a specific application of conservation of energy. However, there are so many mechanical problems which are solved efficiently by applying this principle that it merits separate attention as a working principle.

For a straight-line collision, the net work done is equal to the average force of impact times the distance traveled during the impact.

Average impact force x distance traveled = change in kinetic energy
If a moving object is stopped by a collision, extending the stopping distance will reduce the average impact force. Car crash example Seatbelt use Auto stopping distance
Large truck-small truck collision Two trucks, equal momentum Impact force of falling object

Work-energy principle for angular quantities
The rate of doing work is equal to the rate of using energy since the a force transfers one unit of energy when it does one unit of work. A horsepower is equal to 550 ft lb/s, and a kilowatt is 1000 watts. 
source:wikipedia

Circular motion

Circular Motion
When what goes around comes around...
Imagine that a particle is subject to a force of constant magnitude but whose direction may change. The particle's acceleration at any instant would be in the direction of the force at that instant. The change in the particle's velocity over a very short time would be a vector in the direction of the average acceleration. The new velocity at the end of this tiny time interval would be the vector sum of the original velocity and the change in velocity. The displacement of the particle during the little time slice would be given by the average velocity times the Dt. Now suppose that the changing direction of the force was such that the force was always perpendicular to the velocity. The Central Force display illustrates this situation.
Notice that in this example that the force bends the path of the particle into a circle and that the force vector and therefore the acceleration always points toward the center of that circular path. The magnitude of the velocity along the path remains constant. Under these conditions the particle is said to be undergoing uniform circular motion where "uniform" means the speed of the particle is constant. We have evidently caught this system in a delicate balance where in each Dt the force deflects the particle just enough from the trajectory it would have followed, a straight line in the direction of the velocity, that it ends up on a circular path. The question now is what must be the relationship among the acceleration, velocity and radius of the circle for us to get this nice result. Here we are going to work some tricks that you might leave you thinking, "There is no way I would have thought of this on my own!". What we are going to do is a typical physicist's ploy of looking around for any relationship among the variables in which we are interested. Then seeing if there is any logic that leads to the relationship we want. One of the things that make this seem like magic is that we do not show you all the false leads and dead ends that were tried before this line of reasoning presented itself. The other thing is, this business gets easier with experience. Having worked out a few of these connections helps in working out new ones.
This sort of thing, by the way, drives us mathematicians crazy. We like things to follow absolutely one step after the other so that we are driven inevitably to the correct solution. This business of jumping in sort of in the middle of a problem with some idea what the answer is going to be and using a mixture of physics, logic, geometry and intuition to get a result that then may be tested by experiment is really a physicist thing.
Using the image at the left, taken from the Central Force display, Take a good look at the little red/cyan/blue triangle made up of original velocity, change in velocity and new velocity vectors. A blue copy of the new velocity vector was placed at the base of the original velocity vector. Now compare that to the figure made up of the two gray radius lines and the arc included between them. Except for the fact that the second figure has a curved line for one side the two are similar triangles, meaning that the angles in the two triangles are the same. By taking Dt sufficiently small, the effect of the curvature may be made negligible. Now the thing about similar triangles is that the ratios of corresponding sides are equal. So the ratio of the cyan side over the red side in the small triangle is equal to the ratio of the arc length over the radius in the larger figure. The length of the cyan side is the magnitude of the change in velocity, Dv. The length of the red side is the magnitude of the velocity, v. The length of the arc is the magnitude of the velocity times the time increment v*Dt. And the length of the gray line is just the radius of the circle, r. So we get the following relationship. Dv / v = v * Dt / r. We were interested in the relationship among acceleration, velocity and radius that gave us this nice circular motion. The magnitude of the acceleration, a, is Dv / Dt so let's divide both sides of the preceding equation by Dt. Then to get acceleration by itself on one side of the equation, multiply both sides by v. These maneuvers get us this relationship, a = v2 / r, which ties together the acceleration, velocity and radius as we set out to do. Any time we find a particle in uniform circular motion it has an acceleration of magnitude v2 / r with a direction always perpendicular to the velocity. Of course being perpendicular to the velocity which is tangent to the circle, the acceleration vector points toward the circle's center. An acceleration of the sort we have been talking about, one that points toward the center of the circular motion of a particle, is called centripetal (center seeking) acceleration. Any particle whose direction is changing is undergoing a centripetal acceleration of magnitude v2 / r where r is the radius of curvature of the particle's path. The direction of the centripetal acceleration is along the radius of curvature.
Now let's imagine a particle whose path is curved but not circular. We know that one component of the acceleration must be v2 / r in the direction of r, where r is the radius of curvature of the path. This component of the acceleration contributes only to the change in direction of the particle since it is perpendicular to the path and therefore can not affect the speed of the particle along the path. If the total acceleration includes a component tangent to the path then the speed of the particle is affected. The Curved Path display illustrates these acceleration components. In the Curved Path display you will see the path of a particle which is moving along the x axis at constant speed and subject to a force toward the x-axis proportional to the y displacement. Now let's go back and look at circular motion, but not uniform circular motion. Consider a weight attached to a rod of negligible mass which is suspended from a pivot so the rod and weight could swing freely in the vertical plane. The Pendulum Accelerations display shows you such an arrangement.
There is a lot to learn from this little pendulum display. Perhaps the first lesson is that the physics of everyday objects like a pendulum can get pretty messy, and we haven't even got to the friction part of the story yet. The second lesson is that we must pay careful attention to what we are really seeing. Because of the path and speed of the pendulum weight, we know that the radial and tangential components of its acceleration are as displayed. If they were any different the weight would have some other motion. What this display does not show you is the actual forces which result in these net accelerations. The only forces acting on the pendulum weight are gravity and the force applied by the rod. Somehow these must always add up to the total acceleration times the mass of the pendulum weight, in accordance with Newton's second law.
Near the bottom of the swing, the tension in the rod must be sufficient to both support the weight and curve its path into a circle when it is moving its fastest. Near the 180 degree position, the rod will actually go into compression, supporting the weight when its speed is near zero. You will encounter many interesting problems based on a pendulum like this. For example, what would have to be the speed of the pendulum at the top of its loop for the force exerted by the rod to be zero? Try to work this one out based on what you now understand.
In the next lesson in this course we will introduce the ideas of work and energy.
source: wikipedia

Wednesday, June 27, 2007

Topics studied under Physics in Nepal for grade XI

Unit one: Mechanics1. Measurements: - Measurements of physical quantity system of units: SI unit, Dimensions: Main uses of dimensional equations.2. Scalars & Vectors: - Graphical re-presentation of, addition, subtraction, & resolution of vectors: scalars and vector products.3. Kinematics: - Displacement, velocity, speed, acceleration, equations of motions, motion under gravity, projectile motion.4. Laws of motions: - Newton’s laws of motion: inertia, linear momentum: force, impulse, conservation of linear momentum: momentum and explosive force parallelogram of forces: triangle laws of forces: Lami theorem; coplanar forces; Moment of forces: parallel forces: Torque due to a couple: Center of gravity, center of mass: Solid friction: Laws of solid friction and Verification.5. Work, Energy and Power: - Work: Power, energy: Kinetic energy, Potential energy. Conservative and non-conservative forces: elastic and inelastic collision, conservation of energy.6. Circular Motion: - angular velocity: acceleration in circle: centripetal force Motion of car and cyclist round a banked track.7. Gravitation: - Newton’s law of gravitation: variation in value of ‘g’ due to altitude, depth and rotation of the earth, gravitational potential: escape velocity: parking orbits: Weightlessness: P.E. and K.E. of satellite.8. Simple Harmonic Motion:- Equation of S.H.M: Period, Simple pendulum: spring and mass : phase9. Introduction to Rotational Dynamics: - Moment of inertia: Torque on the rotating body: work done by a couple: Angular momentum and its conservation. Kinetic energy of rolling object10. Hydrostatics: - Fluid pressure, Pascal’s law of transmission of fluid pressure: Archimedes’ principle: density and specific gravity and their determination: principle of flotation: condition of equilibrium of floating bodies.