Landscape design concepts (part-2)

Environmental impact assessment

Most site-planning considerations extend well beyond the property lines and often beyond the horizon. They deal with such regional influences as topography, land use and traffic way patterns activity centers and destinations; these set the broad framework within which each project must find its fit. Each specific site fee has its physical characteristics and givens to which all lines and forms must relate and upon which they will exert a negative or a positive impact. Such off-site and on-site background factors are investigated to the extent deemed necessary and recorded on a set of reference maps for use throughout the planning process.

Once the topographic survey has been obtained, it is often useful to prepare a series of keyed overlays at the same scale. Each overlay may display information on the particular aspect of the site. The title of such overlay sheets might be, for example, “Soils”, “Slopes” (by range of gradients), “Vegetation”, “traffic”, “Hydrology”, “Utilities”, etc. The sheets might simply indicate in a sharper focus the information recorded on the engineering survey. Or each might include, in addition to topographic data, pertinent notations from many other sources.

Eco-environs plan set (background planning – design data)

On projects of larger scope or greater complexity or on those requiring a lengthy planning process, the list of such reference and overlay sheets may be long. The sheets are often bound as a permanent set, constantly updated and backed by files or shelves of supporting correspondence and reports. Such clearly displayed and well-organized background material is useful not only in conceptualizing and checking the studies as they progress but also in giving depth and strength to the plan presentations.

Further, since most extensive development proposals and all federally aided construction projects require the submission of an environmental impact assessment in some form, these sets may be considered a graphic checklist of environmental concerns.

When such environmental considerations are defined and explored early, they become not only a useful test but also a sound basis for the evolving studies and resulting plan solution. The negative impacts of the project can thus be reduced and the attributes significantly increased during the planning process. The many benefits of such a systematic approach cannot be overemphasized.

The feel of the land

Graphic survey information and supporting reference data are essential, but they must be supplemented by at least one and preferably repeated visits to the site. Only by actual site observation can we get the “feel” of the property, sense its relationship to the surrounding areas and become fully aware of the lay of the land. Only in the field can we sense the dynamic lines that are the site’s bounding roads, the insistent line of pedestrian approach, the arc of the sun, the prevailing breeze, the good views, the ugly vies, the sculptural landforms, the springs, the trees, the rock outcrops, the usable areas, those features to be preserved if possible and those to be eliminated – in short, the character of the site. We must climb from hollow to hill, kick at the sod, and dig into the soil. We must look and listen and fully sense those qualities that are peculiar to this specific landscape area.

Whatever we can see along the lines of approach is an extensional aspect of the site. Whatever we can see from the site (or will see in the probable future) is part of the site. Anything that can be heard smelled, or felt from the property is property is part of the property. Any topographical feature, natural or built, that has any effect on the property or its use must be considered as a planning factor.

In our present power-happy and schedule-conscious era, this vitally important aspect of developing a simpatico feeling for the land and of learning to know and understand the land – learning to analyze the total project site – is too often overlooked. And too often our completed work gives tragic evidence of our haste and neglect.

In Japan, historically, this keen awareness of the site has been of great significance in landscape planning. Each structure has seemed a natural outgrowth of its site, preserving, accentuating and extending its best features. Studying in Japan, the author was struck by this consistent quality and once asked an architect how he achieved it in his work.

Identify all proposed uses or actions that would have a significant impact upon the environment.

In the appropriate frame of the matrix place a square for a negative impact and a circle for one that is seen to be beneficial.

Within each square or circle place a number, from 1 to 10, to represent the magnitude and importance (local to regional) of each impact: 10 represents the greatest effect, 1 the least. While the arithmetic sum is not to be considered an absolute identification of the project’s worth, it is a telling decision factor.

In text to accompany the completed chart discuss any unusual, potent, hazardous or lasting impacts inherent in the project.

In separate sections describe those means by which in the project planning and design the negative consequences have been mitigated and the benefits increased.     

Landscape design concepts (part-1)

Specification for topographic survey Property

Lawson farm and portion of Beeler Mill property.

General

Surveyor shall do all work necessary to determine accurately the physical conditions existing on the site. Surveyor shall prepare a map of the given a given area in ink on plastic drafting film at scale of 1 inch to feet. Four black-line prints of the survey map shall be furnished.

Datum

Elevations shall be referenced to any convenient and permanent bench mark with an assumed elevation of 100.0 feet.

Information required

1. Title of survey, property location, scale, north point, certification and date.

2. Tract boundary lines, courses and distances. Error of traverse closure shall not exceed 1:10,000. Calculate and show acreage.

3. Building setback lines, easements and right-of-way.

4. Names of on-site and abutting parse owners.

5. Names and locations of existing streets on or abutting the tract. Show right-of-way, type and width of surfacing and center-line of gutters.

6. Position of buildings and other structures, including foundations, piers, bridges, culverts, wells and cisterns.

7. Location of all site construction, including wells, fences, roads, drives, curbs, gutter, steps, walks, trails, paved areas, etc. indicating types of materials or surfacing.

8. Locations, types, sizes and direction of flow of existing storm and sanitary sewers on or contiguous to the tract, giving top and invert elevations of manholes and inlet and invert elevations of other drainage structures; location, ownership, type and size of water and gas mains, manholes, valve boxes, meter boxes, hydrants and other appurtenances, locations of utility poles and telephone lines and fire-alarm boxes. For utilities not traversing the site indicate by key plan if necessary, the nearest off-site leads, giving all pertinent information on types, sizes, inverts and ownership.

9. Location of water bodies, streams, springs, swamps or boggy areas and drainage ditches or s wales.

   10. Outline of wooded areas. Within areas so noted, show all trees that 

         have a trunk diameter of 4 inches or greater at waist height, giving

         approximate trunk diameters and common names of the trees.

 

   11. Road elevations. Elevations shall be taken at 50 foot intervals along

         center-lines of roads, flow line of gutter on property side and tops and

         bottoms of curbs. The pertinent grades of abutting street and road

         intersections shall also be shown.

  

   12. Ground surface elevations shall be taken and shown on a 50 foot 

         grid system as well as at the top and bottom of all considerable

         breaks in grade, whether vertical as in walls or sloping as in banks.

         Show all floor elevations for buildings. Spot elevations shall also be

         indicated at the finished grade of building corners, building entrance

         platforms and all walk intersections. In additions to the elevations

         required, the map shall show contours at 2-foot vertical intervals.

         All elevations shall be to the nearest tenth of a foot. Permissible

         tolerance shall be 0.1 foot for spot elevations and one-half of the

         contour interval for contours.

The conceptual plan

A seed of use - a cell of function - wisely applied to a respective site will be allowed to develop organically, in harmonious adaptation to the natural and the planned environment.

We have by now developed a comprehensive program defining the proposed nature of our project. We have become fully aware of all features of the total environs. Up to this point, the planning effort has been one of research and analysis. It has been painstaking and perhaps tedious, but this phase is of vital importance because it is the only means by which we can achieve full command of the data on which our design will be based. From this point on, the planning process becomes one of integration of proposed uses, structures and site.

Plan concepts

If structure and landscape development are contemplated, it is impossible to conceive one without the other for it is the relationship of structure to site and site to structure that gives meaning to each and to both.

This point perhaps raises the question of who on the planning team – architect, landscape architect, engineer or other – is to do the “conceiving”. Strangely, this problem, which might seemingly lead to warm debate, seldom arises for an effective collaboration brings together experts in various fields of knowledge who, in a free interchange of ideas, develop a climate of perspective awareness and know-how. In such a climate, plan concepts usually evolve more or less spontaneously. Since the collaboration is arranged and administered by one of the principals (who presumably holds the commission), it is usually this team leader who coordinates the planning in all its aspects and gives it expressive unity. It is the work of the collaborators to advance their assigned planning tasks and to aid in the articulation of the main design idea in all ways possible. 

Radiation (part-2)

Black body and black body radiation

By Prevost’s theory of exchanges, bodies in temperature equilibrium with their surroundings emit radiation of the same character and intensity as that which they absorb. Lampblack, for examples, possesses the power of absorbing nearly all the radiation which is incident upon it and retains this power as its temperature is raised. Such a body having the power to absorb radiation of all wave-lengths completely is termed a black body. No material surface absorbs all the radiant energy incident on it. Lampblack reflects about 2% and so it approximates very closely to a black body. A black body is realized experimentally by having a uniform temperature enclosure with an aperture, small in comparison with the internal dimensions of the enclosure, through which the radiation may emerge. The emission of thermal radiation from and the absorption of radiation from and the absorption of radiation of all wave-lengths passing into, such an enclosure is complete. This radiation from such an enclosure is known as black body radiation or more appropriately full radiation. The smaller the aperture, the more completely is the radiation black. So a correction is to be applied for the lack of blackness due to the finite size of the aperture. This is due to the fact that some of the radiation coming from the wall is able to escape out and the state of thermodynamic equilibrium does not hold. This is avoided by Fery in his particular type of black body.

Application of Kirchhoff’s law to astrophysics

Kirchhoff’s law was responsible for the birth of two entirely new branches of science, astrophysics (physics of the suns and stars) and spectroscopy.

In 1680 Newton had shown that the sunlight can be decomposed by means of a prism into seven colors of the rainbow, but Fraunhofer, who repeated the experiment in 1801 with better instruments, found to hiss surprise that the spectrum was not continuous but crossed by dark lines. He could not account for the origin of these dark lines, neither did any of his contemporaries. But he realized their great importance, measured them and catalogued their wave-lengths. He also investigated the light from stars and found similar dark lines in their spectra. Other physicists, notably, Fizeau observed that if the solar spectrum is examined side by side with the spectrum from a sodium flame, the two yellow lines of the sodium spectrum occupy the same place as the Fraunhofer D-lines in the solar spectrum. Similar is the case with the hydrogen spectrum. Although these facts paved the way for a possible solution, the dark lines of Fraunhofer remain unexplained, until Kirchhoff completely solved the problem on the basis of his law. 

 

Kirchhoff arrived at the same law from thermodynamic reasoning and apply it to explain the dark lines as follows:

Sodium can emit the D-lines when it is excited; hence it can also absorb the same light when white light falls upon it and allows other light to pass through it unaffected. The gases in the outer cooler mantle round the sun, therefore, deprive the continuous spectrum from the central mass of the lines they themselves can emit and give rise to the dark lines. The D-lines (absorption spectrum of sodium) in the Fraunhofer spectrum prove that there is sodium in the sun’s atmosphere. Similarly, the other dark lines testify to the presence of their respective elements in the sun’s atmosphere. The presence of dark lines in the spectra of stars is accounted for in similar manner. It is by the analysis of such dark lines, one is able to tell of what elements the analysis of such dark lines, one is able to tell of what elements the cooler atmosphere surrounding the sun and stars are composed.

Kirchhoff’s discovery is of much greater importance than the mere success in explaining the fraunhofer bands. It clearly asserts that every different type of atom, when it is properly excited emits light of definite wavelength which is characteristic of the atom. 

Radiation pyrometers

Pyrometers are instruments for measuring high temperatures by means of radiation. In most thermometers the elements must be brought into contact with the body and if the temperature is too high the element is likely to melt. The radiation method does not depend upon contact with the body and so has not this limitation, but does assume that the body is a perfect black body. Frequently this will not be so and as a result the measured temperature will be lower than the actual.           

Radiation

Introduction

When we hold one end of a metal rod in our hand insert the other end in a gas flame, heat reaches our hand conduction along the rod. If our hand is held above the flame heat reaches it by way of the upward-moving convection currents of hot gas. When we hold our hand at one side of hot object our hand still becomes warm, even though conduction through the air is negligible and the hand is not in the path of convection currents. Energy now reaches the hand by radiation.

The term radiation refers to the continual emission of energy from the surface of all bodies. The energy is called radiant energy and is carried by electromagnetic waves. Here also – just as in the case of light waves – we assume that there exists an all pervading hypothetical medium – either – through which the wave is propagated. Being electromagnetic waves, thermal radiation travels with the velocity of light, viz, 3 x 10¹º cm/sec. in free space. When they fall on a body that is not transparent to them they are in part reflected and in part absorbed. A material medium absorbing thermal radiation gets heated.

It has been found that the wave-length of thermal radiations lie on the larger side of the wave-length of red light. They, therefore, fall under the class of infra-red radiation. In view of their large wave-lengths the Angstrom, the familiar unit of measurement of optical wave, is not a convenient unit in this region.

Properties of radiation

The fact that at an eclipse the heat and the light of the sun are cut off simultaneously indicates that radiant heat and light travel through space with the same velocity. This suggests that radiant heat and light are essentially the same in nature, except that they differ in their wave-lengths. Radiant heat is invisible light. The following is the summary of the properties of thermal radiation in a homogeneous medium.

(1)    They travel in straight lines.

(2)    They obey the laws of reflection and refraction of light.

(3)    They exhibit the phenomena of interference, diffraction ad polarization.

(4)    They travel through empty space with the velocity of light in all directions.

(5)    The amount of heat received per second per unit area of a given surface, i.e. the intensity of radiation, by absorption of thermal radiation emitted by a source of heat at a constant temperature is inversely proportional to the square of the distance between the source and the absorbing surface. This is known as inverse square law.

(6)    They produce heat in a body by which they are absorbed.

(7)    They do not heat the medium through which they pass.

(8)    They produce photo-electric current in cesium photo-electric cell.

Diathermanous and athermanous substances

Those substances which allow heat radiations to pass through them are called diathermanous whereas those which absorb heat radiations are called athermanous or a diathermanous. These terms are analogous to the terms transparent and opaque in light. Among solids, quartz, rock salt and sylvine are diathermanous. In general, those bodies which are transparent to light are diathermanous. However, there are some exceptions. Dry air is transparent to heat whereas moist air is opaque to it. A solution of iodine in carbon disulphide is transparent to heat but is opaque to visible light. Further, it was shown by Tyndall that glasses like hydrogen, oxygen, nitrogen are diathermanous whereas ammonia, absolute alcohol, carbon dioxide and water vapor are athermanous.

Temperature radiation

If a body is maintained at a constant temperature and continuously emits radiation without undergoing any change in its structure, the radiation is called pure temperature radiation. At low temperature a body emits radiations primarily of longer wave-lengths, but with the increase of temperature the proportion of shorter wave-length radiations increases. This radiation becomes apparent as visible light when the body is red hot. Further increase in the temperature increases the proportion of short wave-length radiation, the body ultimately becoming white hot and emitting radiations all visible wave-lengths. The distribution of energy among the various wave-lengths depends upon the nature of the surface as well as the temperature of the body.

Prevost’s theory of exchanges

Before 1792 scientists were in belief that hot bodies emit hot radiations and cold bodies unit emit cold radiations. This means that if we stand near a fire we get hot radiations from the fire and feel warmth. It is observed that if a body at a high temperature is placed inside a constant temperature enclosure, it radiates heat and the temperature of the body falls till it is the same as the enclosure. This implies that the body stops radiating heat when it is at the same temperature of the surroundings. Again on lowering the temperature of the enclosure, it will be observed that once again the body starts radiating heat.

Pierre Prevost of Geneva in 1792 discarded the old assumptions and put forward his theory of exchange which states that all bodies radiate heat continuously at all the temperatures above absolute zero respective of their surroundings. This means that a process of exchange of heat is in action between the body and its surroundings, in which each one radiates heat and at the same time absorbs heat.

In the case of a hot body, placed in an enclosure at a lower temperature, the heat radiated by the body per second is greater than the amount of heat received by the body per second from the surroundings, as a result the temperature of the body falls till the body attains the temperature of the enclosure. In the case of a cold body placed in an enclosure at a higher temperature, the amount of heat received by the body per second is greater than the amount of heat radiated by the same body per second; as a result the temperature of the body rises till it is the same as that of the enclosure. In the case of a body at the temperature of the enclosure, the amount of heat radiated per second by the body is equal to the amount of heat received by the body per second from the surroundings, as such there is neither gain nor loss of heat and the body remains at the same temperature of the enclosure. Thus a state of dynamic equilibrium is said to have reached.  

Heat Engines

Practical forms of heat engines

Mechanical work can be transformed to heat in various ways. Heat can similarly be converted to mechanical work by machines known as heat engines. Heat engine is a mechanical device by which heat can be converted to mechanical work. There are two chief classes of heat engines in practical uses:

(1)    External combustion heat engine: In this type of engines the combustion of the fuel takes place outside the engine proper. Steam-engine falls under this category.

(2)    Internal combustion heat engine: In this type of engines the combustion of the fuel takes place inside the engine proper. Petrol engine, Diesel oil engine, etc. fall under this category.

Condensing and non-condensing engines

Condensing engines

non-condensing engines

In steam engine described above, the spent steam is led to the atmosphere through the exhaust in puffs. This type is known as non-condensing engine. Locomotive engines are of this type. In some forms of engines, the exhaust steam is condensed in a vacuum chamber, called condenser, where low temperature is maintained by a jet of cold water. Marine engines are of this condensing type. These engines are somewhat more efficient than the other type of engines. The condensed steam is put back into the boiler.

Steam turbine

In steam engine the to-an-fro motion of the piston is converted to rotatory motion by means of a crank and rod. A piston comes to rest at the end of each stroke. This causes much shaking and disagreeable vibrations.

Steam turbine is fundamentally different from all types of steam engines in that the heat energy present in the steam first used to set the steam itself in motion. The high pressure steam comes out of boiler through a number of nozzles and strikes the curved blades on the rim of wheel. The kinetic energy of the steam pushes the blades forward. The speed of the wheel is enormous; sometimes the speed reaches 70,000 revolutions per minute if the pressure of the steam from the boiler is reduced to atmospheric pressure in one stage. So with the help of large reduction gears, the speed is lowered to the desired value.

Advantage

In a turbine there is no reciprocating motion and so it works smoothly without jerking. Moreover, it occupies much less space. The efficiency of a turbine is much greater than that of a steam engine. It is much less complicated than steam engine. Hence turbines are used in steam ships, electric generating stations, etc.       

Diesel engine

Diesel engines are now-a-days considered as the most popular type of internal combustion engines. Modern Diesel engines are widely used foe marine population, for heavy lorries and buses, for generating electricity, driving factory machinery, pumping and many other similar purposes. It is also in use in railway locomotives. In fact, the steam engine is gradually becoming obsolete with the advent of high-power, high-efficiency Diesel engine.

Operation

The whole operation of a diesel engine may be divided into four strokes.

First stroke (suction-stroke)

During this stroke, the exhaust and the fuel valves are closed, the air-inlet valve is opened and the piston moves outwards. A charge of air is sucked in at nearly atmospheric pressure.

Second stroke (compression-stroke)

During this stroke, all the valves are closed and the piston is moving inwards. As a result, the air previously sucked in is compressed to a high pressure of about 500 lbs. per square inch.

Third stroke (working or power-stroke)

The air and exhaust-valves are closed and the fuel valve is opened just before the beginning of this stroke and remains open during a small part of the stroke. The high compression during the second stroke raises the temperature of the air to about 1000°C which is above the ignition point of the fuel (i.e. the temperature at which the fuel takes fire). At this moment a small charge of fuel is forced into the combustion chamber. Coming in contact with the heated air the fuel burns with great rapidity and the subsequent expansion of the gaseous product imparts the necessary power-stroke and the piston moves outwards. Towards the end of the power-stroke the exhaust – valve opens and the spent gas in the cylinder begins to go out.

Forth stroke (exhaust-stroke)

The air and the fuel valves are closed, the exhaust-valve is opened and due to inward motion of the piston, spent gases escape through the exhaust-pipe. The initial state is thus reached and the operation is repeated.

Thermodynamics (part-3)

Statement of the second law of thermodynamics

The first law of thermodynamics is a statement of the principle of conservation of heat energy. The second law deals with the condition and possibility of those energy transformations. The second of thermodynamics has been stated by different scientists in different ways. Of all the statements, the one due to Clausius is the most convenient for general use.

Rudolf J. E. Clausius

(1)    Negative stand point (Clausius): “It is impossible for a self-acting machine, unaided by any external agency, to convey heat from one body to another at a higher temperature.”

That is to say, heat will not pass spontaneously from a cold body to a warmer one. In order to make it do so it is necessary to do external work of some sort. In the case of a refrigerator (ammonia ice plant), heat is absorbed from the brine solution at a lower temperature and rejected into water at a higher temperature. This does not happen of its own accord. An external agency (the pump) has to do work in order to achieve this.

(2)    Positive stand point (Edser): “Heat flows of itself from higher to lower temperature.”

Lord Kelvin (1824-1907)

(3)    Lord Kelvin stated the law in the form “It is impossible, by means of inanimate material agency to drive mechanical effect from any portion of matter by cooling it below the temperature of the coolest body of its surroundings.”

This can be readily understood from the fact that a heat engine cannot work already when the temperature of the source and sink are equalized and much more so when the source cools down to a temperature lower than that of the coldest body in the surroundings.

Max Planck (1858-1947) German scientist

     

(4)    Planck stated the law in the form “It is impossible to construct an engine which will work in a complete cycle and produce no effect expects the raising of a weight and the cooling of a heat reservoir.”

(5)    Kelvin and Plank stated the law as follows: “It is impossible to construct an engine which operating in a cycle will produce no effect other than extraction of heat from a reservoir and performance of an equivalent amount of work.”

James Clerk Maxwell (1831–1879)

(6)    Maxwell stated the law as follows: “It is impossible to produce any difference in temperature and pressure in any isolated mass originally at uniform temperature and pressure and pressure without some expenditure of energy.”

Arguments can be advanced to show that these various statements of the second law are essentially equivalent to one another.

It is not possible to prove the law by the experimental verification of a great number of predictions based upon it.

The law explains our failure to utilize the immense quantity of heat energy in our surroundings. For example, we cannot run an engine on the heat content of the oceans because we have no large sink at a lower temperature into which the engine could discharge heat.

It is possible to deduce from the second law of thermodynamics that the reversible engine is the most efficient and that its efficiency depends on the operating temperatures and not on the substance used.

To be more fruitful in application the second law may be stated in precise mathematical way. This requires the introduction of a law new physical quantity called entropy.

Impossibility of perpetual motion machine of the second kind

A cyclic device which would continuously abstract heat from a single reservoir and convert the heat completely to mechanical work is called a perpetual motion machine of the second kind. Such a machine would not violate the first law (the principle of conservation of energy) since it would not create energy but economically it would be just as valuable as if it did so, because of the existence of heat reservoirs such as the oceans or the earth’s atmosphere from which heat could be abstracted continuously at no cost. Hence the second law is sometimes stated, “A perpetual motion machine of the second kind is impossible.”

Carnot’s theorem

Assuming the truth of the second law of thermodynamics, we may deduce two important results which are usually taken together to constitute Carnot’s theorem.

(a)    Working between the same initial and final temperature, no engine can be more efficient than a reversible engine.

(b)    The efficiency of all reversible engines working between the same limits of temperature is the same. 

Entropy

One of the objects of the second law of thermodynamics is to predict the direction in which a thermal process will take place. This is best done by introducing some physical quantity which would be a function of thermodynamic coordinates and which would serve the purpose for determination of the direction of occurrence of a thermal process. In mechanics and in electricity we define the quantities potential and potential energy, for determining the direction of occurrence of an event. In these cases a mass or a charge – as the case may be – moves in such a direction that its potential energy approaches a minimum. In thermodynamical processes we must search for a quantity which tells us about the direction of flow of heat and which could efficiently define the thermodynamical state of any working substance. The required quantity was supplied by Clausius who called it entropy which we denote by the symbol S.

Statement of the second law of thermodynamics in terms of entropy

The statement of the second law of thermodynamics in terms of entropy is in a way, a restatement of the principle of increase of entropy. The law of Clausius may be stated as follows:

“Every physical or chemical process in nature takes place in such a way that the sum of the entropies of all bodies taking part in the process increases. In the limiting case of a reversible process the sum of the entropies remains constant.”

Thermodynamics (part-2)

Heat depends on the path

We have seen that, in general, the work done on or by a system is not a function of the coordinates of the system but depends on the path by which the system is brought from the initial to the final state. Exactly the same is true of the heat transferred to or from a system. The heat Q is not a function of thermodynamic coordinates but depends on the path. An infinitesimal amount of heat, i.e. dQ is not an exact differential.  

Impossibility of perpetual motion of the first kind

perpetual motion

When a system is carried through a cyclic process change of internal energy, i.e. du is equal to zero and so dQ=dW. That is, the net heat flowing into the system is equal to the net work done by the system. This means that it is impossible to construct a cycles, will put out more energy in the form of work than is absorbed in the form of heat. A machine which would create energy out of nothing is called a perpetual motion machine of the first kind. The first law is sometimes stated, “A perpetual motion machine of the first kind is impossible”. 

Reversible and irreversible processes

reversible process

(1)    A reversible process is one which can be retraced in the opposite direction so that the working substance passes through exactly the same states in all respects as in the direct process. Moreover, the thermal and mechanical effects at each stage should be exactly reversed. That is, if heat is absorbed by a substance in the direct process to produce external work, the substance will give out an equal quantity of heat in the reverse process when the same amount of external work is done on it. In practice, no change is completely reversible, but changes which occur slowly are normally almost so. The following examples will clarify the process.

(a)    When heat is added to a given mass of a gas at constant pressure, it expands and performs some external work. If the same amount of work be done on the gas, it will give out the same quantity of heat. It is assumed that were is no friction to be overcome during the process as work done in overcoming friction is wasted. The process must be slow otherwise oscillations and eddy currents will be setup and energy wasted in producing them is not recoverable. It is also to be noted that no heat must be lost by conduction, convection and radiation during the operation; as such losses cannot be reversed.

(b)    In a reversible isothermal operation, heat is absorbed by the gas as it expands and does external work and is given out when it is compressed by the same amount and work is done on the gas in the reverse process.

(c)    A given mass of ice changes to water when a certain amount of heat is absorbed by it and the same mass of water changes to ice when the same quantity of heat is removed from it.

(d)    Evaporation is reversible, as a water changes to steam on absorbing heat and steam changes to water on losing heat.

(e)    All isothermal and adiabatic changes are reversible when performed slowly. When a gas is compressed isothermally its volume decreases and on releasing the pressure the gas regains is original volume if there is no friction.

irreversible process

(2)    An irreversible process is such that it cannot be retraced in the opposite direction by reversing the controlling factors. All changes which occur suddenly like the explosion etc. may be considered as irreversible. Some examples of irreversible process are:

(a)    sudden unbalanced expansion of a gas, either isothermal or adiabatic,

(b)    Joule-Thomson expansion,

(c)    Heat produced by friction,

(d)    Heat generated when a current flows through an electrical resistance,

(e)    Exchange of heat between bodies at different temperatures by conduction or radiation.

Thermodynamics

Introduction

Thermodynamics, in a literal sense, is a branch of physics which deals with the relation between heat and mechanical energy. Its foundations were, in fact, laid by experiments in which mechanical energy was converted into heat, as in the researches. Rumford and Joule and conversely heat converted into work as in the steam engines introduced between the times of Rumford and Joule. Broadly speaking, thermodynamics deals with the transformation of heat energy to other forms of energy, such as electrical, magnetic and radiant etc.

Scope of thermodynamics

The object of the science of thermodynamics is the study of all naturally falls under two headings, the study of energy relations and the study of the manner in which energy changes take place. The guiding principles for the two cases are the first and second law of thermodynamics.

Carnot

William Thomson

The first law of thermodynamics (Joule, 1843) states the connection between heat and mechanical work and leads up to the principle of conservation of energy. The second law of thermodynamics (Carnot, 1824, Clausius, 1850 and W.Thomson, 1851) is concerned with the convertibility of energy and gives us a method for determining how the amount of work obtainable from a certain quantity of heat depends on the temperatures between which the operation takes place. It provides a mode of defining the absolute or thermodynamic scale of temperature. These two laws from the basis of the classical thermodynamics and from these two laws a large number of problems on heat may be solved. In 1906, Nernst published a new theory which states that the absolute zero of temperature is unattainable. This is now known as the third law of thermodynamics.

Thermodynamics equilibrium

A body or system is said to be in thermodynamic equilibrium when it is in equilibrium from the mechanical, thermal and chemical points of view. Mechanical equilibrium means that there is no unbalanced force either in the interior of the system or between the system and its surroundings. Thermal equilibrium means that all the parts of the body or system are at the same temperature as that of the surroundings. When a body or system does not undergo any changes or chemical composition it is said to be in chemical equilibrium.

Indicator diagram

Indicator diagram

James Watt

The method of representing the work done during a cycle is due to James Watt, who invented an instrument called the indicator, by means of which an engine can automatically draw a diagram representing pressure changes in relation to volume changes. Such a diagram, for an actual machine is accordingly called an indicator diagram, but should be distinguished from the pressure-volume diagram which unlike an indicator diagram, relates to a constant mass of a substance.

Internal energy or intrinsic energy

If a substance absorbs a certain quantity of heat without any performance of external work, all the heat goes to increase the store of energy within the substance. This store of energy in the internal energy of the substance. In practice, it is not possible to measure the internal energy but changes in internal energy can be readily measured. An increase in the internal energy can be brought about by the communication of energy to the body either as heat or by the performance of work on the body. We use the symbol u to denote internal energy.

The internal energy of a body in a given state depends upon the configuration and motion of the molecules. Thus internal energy may be regarded partly as potential energy and partly as kinetic energy arising out of the fact that the ultimate particles of the body are in a state of invisible chaotic motion. As no mutual force of attraction or repulsion is exerted by the ultimate particles of a perfect gas, all the energy is kinetic. Experimentally, Joule also proved that for a perfect gas the internal energy is solely kinetic and is a function of temperature only (Mayer’s hypothesis).

The change in internal energy of a body between two different states depends only upon the initial and final states and not upon the process by which the change is effected.

Energy equivalent of the first law of thermodynamics

The heat supplied to a system will be spent in two ways:

(1)    in doing external work and

(2)    in changing the internal energy of the system. That is,

dQ=dW+du, expressing all the quantities in work’s unit, where dQ is the quantity of heat given to a system, du is the change in the internal energy and dW is the external work done.

It may be pointed out that the expression of dQ in work’s unit can be found by multiplying the heat supplied in calories by J, the mechanical equivalent of heat. Accordingly, the first law of thermodynamics may be stated as follows: “The change in the internal energy of any system during a physical process is equal to the algebraic sum of the heat communicated to the system and the work done by external agencies on the system, both being expressed in the same unit” (Clausius).

The quantity du is defined with the help of principle of conservation of energy by considering a thermally insulated system for which du=-dW, since dQ=0. Changes in the internal energy can thus be measured in terms of the external work done on the system. In the case of a simple gas expanding against a uniform external pressure P, dW=PdV, where dV is the change in volume.