The first law of thermodynamics, an expression of the principle of conservation of energy, states that energy can be transformed (changed from one form to another), but cannot be created or destroyed. 
No. Energy cannot be created or destroyed, it can only change form. Energy may be transformed into heat energy. The laws of thermodynamics also state that two bodies who remain in contact transfer heat energy until they reach an equilibrium. Due to this heat energy may be absorbed within air particles. Good luck attempting to get this energy back once permeated through the air :D 

An example is how chemical energy in petrol is converted into kinetic (movement) energy to drive a car, and how slowing down the car (by braking, crashing or just letting friction slow it down) converts the kinetic energy into heat. 

Although energy is never destroyed, it tends towards 'lower quality' and less useful energy every step on the way. As an example, there is no easy way to collect the low grade heat energy from slowing down a car to put it back on the tank. The energy is still there, but in tiny pieces spread all over the place in a form we cannot use. 

This is the second law of thermodynamics, saying that in any closed system (where the system does not get new energy from the outside) the entropy tends to increase. (Entropy is a mathematical definition of disorder - higher entropy means many small pieces instead of a few large ones) 
One scenario for how the Universe will eventually end is that the stars burn out and leave just a cold and dark empty space. In this scenario all the energy from the Big Bang, which today forms our Sun, the planets and the stars, will still be there. It will just be spread very, very thinly across the whole of the Universe instead of being concentrated in relatively few big lumps (the Sun and the stars) the way it is today. :)

Read more: http://wiki.answers.com/Q/Energy_is_never_created_or_destroyed#ixzz22eo4IvvT

law of Conservation of Energy

Conservation of energy

The law of conservation of energy, first formulated in the nineteenth century, is a law of physics. It states that the total amount of energy in an isolated system remains constant over time. The total energy is said to be conserved over time. For an isolated system, this law means that energy can change its location within the system, and that it can change form within the system, for instance chemical energy can become kinetic energy, but that energy can be neither created nor destroyed.

In the twentieth century, the definition of energy was broadened. It was found that particles that have rest mass, and those that do not, are subject to interconversions. There can occurcreation and annihilation of (ponderable) matter particles, and imponderable non-matter particles. Matter is then not conserved. Matter particles (such as electrons) can be converted to non-matter (such as photons), or even into potential or kinetic energy. In such a transformation process of an isolated system that is alternatively described by these apparently distinct quantities, neither the mass nor the energy changes over time. Conservation of total energy, and conservation of total mass, each still holds as a law in its own right. When stated alternatively, in terms of mass and of energy, they appear as the apparently distinct laws of the nineteenth century.

A consequence of the law of conservation of energy is that no intended "perpetual motion machine" can perpetually deliver energy to its surroundings.^[2] Any delivery of energy by such a device would result in delivery of mass also, and the machine would lose mass continually until it eventually disappeared.

The law of conservation of energy

The law of conservation of energy: The total amount of energy in the universe is constant, although energy can be transformed from one form to another. The basic unit of energy is the joule. A watt is one joule per second. Thus a 100 watt electric bulb uses 100 joules per second of electric energy. That energy must come from somewhere and must be paid for. One pays for electric energy in kilowatt-hours which is 1,000 watts being delivered per hour. Thus a kilowatt-hour is 3,600,000 joules, since "kilo" means a thousand, and there are 3,600 seconds in an hour. At 10 cents per kilowatt hour, a joule is really cheap. 10 joules would lift a one kilogram weight one meter.

Schemes for powering cars or houses that require violating the law of conservation of energy can't work. Here's an example.

Several times I have received in email the following idea for powering cars with hydrogen from water. Run the car on hydrogen obtained by splitting water by electrolysis using the car's generator to get the electricity.

Here's why it won't work. The amount of energy you get from burning hydrogen and running a generator produces at most the amount of energy required to replace the energy used to split the water. There would be none left over to power the car. Actually, you would get considerably less energy than is needed to get more hydrogen to replace that burned. Most likely it would be about 20 percent.

Splitting water to get hydrogen is not an original source of energy. Rather it is a way of tranforming energy into a form more usable for a certain purpose. For example, nuclear energy can be used to split water (H2O) into hydrogen and oxygen. The oxygen is released into the air, and the hydrogen is liquefied and used to run cars in the same way gasoline is used to run cars. [The process is quite inefficient compared to running the car directly with a small nuclear reactor. Unfortunately, a nuclear reactor in a car would kill the occupants. Ships can use nuclear reactors, because they can afford the tons of shielding required to keep the neutrons away from the crew.] Solar electricity can also be used to split water, but solar energy is expensive.

What are the sources of energy that have been used?

Human muscle

A hard working human can put out about 100 watts, enough for a light bulb. To light an average home at night would require quite a few slaves turning generators. The energy for human muscle power is chemical energy from food. This energy comes from the sun and is absorbed by the crops. If you pay 5 cents per kilowatt-hour (kwh), you are getting for your 5 cents what could be generated by a man working hard for 10 hours.

Horse power

Horses are a better source of energy than humans. A horse can put out about 8 times as much energy as a human and is easier to keep working.

Coal, oil and natural gas

Chemically coal is carbon (C) with impurities. Natural gas is CH4, i.e. carbon and hydrogen, oil and the gasoline made from oil are hydrocarbons, compound of hydrogen and carbon of various compositions. Octane is C8H18 for example. When burned, oxygen from the air turns the carbon into CO2, carbon dioxide, and the hydrogen into H2O, water, usually in the form of steam. Energy is released because the fuel + oxygen from the air has more energy than the products of burning the fuel. You can get the fuel back from the combustion products by reversing the chemical reaction of burning, but then you must supply the energy. All these fuels are the remains of plants that got energy from sunlight when they grew millions of years ago. The supplies of these fuels won't last more than a few hundred years, maybe quite a bit less. At present, burning these fuels is the largest source of energy for our civilization.

Water power

The sun evaporates water from the ocean, and some of it falls as rain or snow at high altitudes. Water at high altitude has potential energy compared to water at low altitude, and this energy can be obtained by damming the water and running it through electric generators. In the developed countries, most of the good sites for dams, lots of water at a high altitude, are already in use. A large expansion of water power isn't possible.

Direct solar power and wind

These work and have been politically favored in the advanced countries since the 1970s. However, they are still too expensive to compete with only moderate subsidies. The enthusiasts remain optimistic after all these years. My own opinion is that they will remain so expensive that if our civilization had to rely on them the standard of living would go down a lot. Some enthusiasts think they wouldn't mind that.

Nuclear energy

This generates about 20 percent of the world's electricity. Its expansion is temporarily held up by the influence of environmentalist ideology. See the Nuclear FAQ for details. Nuclear energy is humanity's best hope for the long term.

Energy and power

These terms are often confused, because in ordinary language they are sometimes synonyms. However, the distinction is important for science and engineering and is simple to state. Power is the rate at which energy is generated or used. The basic unit of energy is the joule, and the basic unit of power is the watt. A watt is one joule per second. Thus a hundred watt light bulb is using 100 joules of energy per second, turning the electric energy into light and heat. A 1,000 megawatt power plant is generating a billion joules per second. It gets that energy from burning fuel or from splitting atoms of uranium and plutonium. To get 1,000 megawatts it actually uses 3,000 megawatts from the fuel. The leftover 2,000 megawatts is inevitably produced as leftover heat as shown by the second law of thermodynamics.

A variety of units are used for power and energy. Kilowatts and megawatts are a thousand watts and a million watts respectively. A horsepower is 746 watts by James Watts's optimistic definition, but few horses can put out that power for long. Energy is measured in kilowatt-hours. A kilowatt-hour is therefore 3,600,000 joules. Another unit of energy is the British thermal unit or BTU, sometimes used in engineering. It is the amount of energy needed to raise one pound of water one degree Farenheit.

This is a very sketchy exposition. An elementary physics book will do much better.

law of conservation of energy

Conservation of energy

From Wikipedia, the free encyclopedia

This article is about the law of conservation of energy in physics. For sustainable energy resources, see Energy conservation.

The law of conservation of energy, first formulated in the nineteenth century, is a law of physics. It states that the total amount of energy in an isolated system remains constant over time. The total energy is said to be conserved over time. For an isolated system, this law means that energy can change its location within the system, and that it can change form within the system, for instance chemical energy can become kinetic energy, but that energy can be neither created nor destroyed.

In the twentieth century, the definition of energy was broadened. It was found that particles that have rest mass, and those that do not, are subject to interconversions. There can occurcreation and annihilation of (ponderable) matter particles, and imponderable non-matter particles. Matter is then not conserved. Matter particles (such as electrons) can be converted to non-matter (such as photons), or even into potential or kinetic energy. In such a transformation process of an isolated system that is alternatively described by these apparently distinct quantities, neither the mass nor the energy changes over time. Conservation of total energy, and conservation of total mass, each still holds as a law in its own right. When stated alternatively, in terms of mass and of energy, they appear as the apparently distinct laws of the nineteenth century.

A consequence of the law of conservation of energy is that no intended "perpetual motion machine" can perpetually deliver energy to its surroundings.^[2] Any delivery of energy by such a device would result in delivery of mass also, and the machine would lose mass continually until it eventually disappeared.

History

Gottfried Leibniz

Ancient philosophers as far back as Thales of Miletus c.~550 BCE had inklings of the conservation of some underlying substance of which everything is made. However, there is no particular reason to identify this with what we know today as "mass-energy" (for example, Thales thought it was water). In 1638, Galileo published his analysis of several situations—including the celebrated "interrupted pendulum"—which can be described (in modern language) as conservatively converting potential energy to kinetic energy and back again. However, Galileo did not state the process in modern terms and again cannot be credited with the crucial insight. It was Gottfried Wilhelm Leibniz during 1676–1689 who first attempted a mathematical formulation of the kind of energy which is connected with motion(kinetic energy). Leibniz noticed that in many mechanical systems (of several masses, m_i each with velocity v_i ),

was conserved so long as the masses did not interact. He called this quantity the vis viva or living force of the system. The principle represents an accurate statement of the approximate conservation ofkinetic energy in situations where there is no friction. Many physicists at that time held that the conservation of momentum, which holds even in systems with friction, as defined by the momentum:

was the conserved vis viva. It was later shown that, under the proper conditions, both quantities are conserved simultaneously such as in elastic collisions.

It was largely engineers such as John Smeaton, Peter Ewart, Carl Holtzmann, Gustave-Adolphe Hirn and Marc Seguin who objected that conservation of momentum alone was not adequate for practical calculation and made use ofLeibniz's principle. The principle was also championed by some chemists such as William Hyde Wollaston. Academics such as John Playfair were quick to point out that kinetic energy is clearly not conserved. This is obvious to a modern analysis based on the second law of thermodynamics but in the 18th and 19th centuries, the fate of the lost energy was still unknown. Gradually it came to be suspected that the heat inevitably generated by motion under friction, was another form of vis viva. In 1783, Antoine Lavoisier and Pierre-Simon Laplace reviewed the two competing theories of vis viva and caloric theory.^[3] Count Rumford's 1798 observations of heat generation during the boring ofcannons added more weight to the view that mechanical motion could be converted into heat, and (as importantly) that the conversion was quantitative and could be predicted (allowing for a universal conversion constant between kinetic energy and heat). Vis viva now started to be known as energy, after the term was first used in that sense by Thomas Young in 1807.

The recalibration of vis viva to

which can be understood as finding the exact value for the kinetic energy to work conversion constant, was largely the result of the work of Gaspard-Gustave Coriolis and Jean-Victor Poncelet over the period 1819–1839. The former called the quantity quantité de travail (quantity of work) and the latter, travail mécanique (mechanical work), and both championed its use in engineering calculation.

In a paper Über die Natur der Wärme, published in the Zeitschrift für Physik in 1837, Karl Friedrich Mohr gave one of the earliest general statements of the doctrine of the conservation of energy in the words: "besides the 54 known chemical elements there is in the physical world one agent only, and this is called Kraft [energy or work]. It may appear, according to circumstances, as motion, chemical affinity, cohesion, electricity, light and magnetism; and from any one of these forms it can be transformed into any of the others."

[edit]Mechanical equivalent of heat

A key stage in the development of the modern conservation principle was the demonstration of the mechanical equivalent of heat. The caloric theory maintained that heat could neither be created nor destroyed but conservation of energy entails the contrary principle that heat and mechanical work are interchangeable.

In 1798 Count Rumford (Benjamin Thompson) performed measurements of the frictional heat generated in boring cannons and developed the idea that heat is a form of kinetic energy; his measurements refuted caloric theory, but were imprecise enough to leave room for doubt.

James Prescott Joule

The mechanical equivalence principle was first stated in its modern form by the German surgeon Julius Robert von Mayer in 1842.^[4] Mayer reached his conclusion on a voyage to the Dutch East Indies, where he found that his patients' blood was a deeper red because they were consuming less oxygen, and therefore less energy, to maintain their body temperature in the hotter climate. He had discovered that heatand mechanical work were both forms of energy, and later, after improving his knowledge of physics, he calculated a quantitative relationship between them (pub' 1845).

Meanwhile, in 1843 James Prescott Joule independently discovered the mechanical equivalent in a series of experiments. In the most famous, now called the "Joule apparatus", a descending weight attached to a string caused a paddle immersed in water to rotate. He showed that the gravitational potential energy lost by the weight in descending was equal to the thermal energy (heat) gained by the water by friction with the paddle.

Over the period 1840–1843, similar work was carried out by engineer Ludwig A. Colding though it was little known outside his native Denmark.

Both Joule's and Mayer's work suffered from resistance and neglect but it was Joule's that, perhaps unjustly, eventually drew the wider recognition.

For the dispute between Joule and Mayer over priority, see Mechanical equivalent of heat: Priority

In 1844, William Robert Grove postulated a relationship between mechanics, heat, light, electricity and magnetism by treating them all as manifestations of a single "force" (energy in modern terms). In 1874 Grove published his theories in his book The Correlation of Physical Forces.^[5] In 1847, drawing on the earlier work of Joule,Sadi Carnot and Émile Clapeyron, Hermann von Helmholtz arrived at conclusions similar to Grove's and published his theories in his book Über die Erhaltung der Kraft (On the Conservation of Force, 1847). The general modern acceptance of the principle stems from this publication.

In 1877, Peter Guthrie Tait claimed that the principle originated with Sir Isaac Newton, based on a creative reading of propositions 40 and 41 of the Philosophiae Naturalis Principia Mathematica. This is now regarded as an example of Whig history.^[6]

[edit]Mass–energy equivalence

In the nineteenth century, mass and energy were considered to be of quite different natures. Then Albert Einstein's theory of special relativity showed that mass and energy are related by an equivalence. Energy has an equivalent mass, and mass has an equivalent energy. Physicists now speak of a unified law of conservation of mass-energy. This is a recognition that the two nineteenth century conservation laws are restricted versions of one and the same more general law. While matter can be actually converted into non-matter, the relation between mass and energy is a simply theoretical equivalence, so that it makes no sense to think of their "actual interconversion". Thus, the modern view is that conservation of energy and conservation of mass are simply the same conservation law, stated differently, in different units. Einstein's E = mc² and other equations serve to convert one unit to the other.

[edit]First law of thermodynamics

Main article: First law of thermodynamics

For a closed thermodynamic system, the first law of thermodynamics may be stated as:

where  is the amount of energy added to the system by a heating process,  is the amount of energy lost by the system due to work done by the system on its surroundings and  is the change in the internal energy of the system.

The δ's before the heat and work terms are used to indicate that they describe an increment of energy which is to be interpreted somewhat differently than the  increment of internal energy (see Inexact differential). Work and heat areprocesses which add or subtract energy, while the internal energy  is a particular form of energy associated with the system. Thus the term "heat energy" for  means "that amount of energy added as the result of heating" rather than referring to a particular form of energy. Likewise, the term "work energy" for  means "that amount of energy lost as the result of work". The most significant result of this distinction is the fact that one can clearly state the amount of internal energy possessed by a thermodynamic system, but one cannot tell how much energy has flowed into or out of the system as a result of its being heated or cooled, nor as the result of work being performed on or by the system. In simple terms, this means that energy cannot be created or destroyed, only converted from one form to another.

Entropy is a function of the state of a system which tells of the possibility of conversion of heat into work.

For a simple compressible system, the work performed by the system may be written

where  is the pressure and  is a small change in the volume of the system, each of which are system variables. The heat energy may be written

where  is the temperature and  is a small change in the entropy of the system. Temperature and entropy are variables of state of a system.

[edit]Mechanics

In mechanics, conservation of energy is usually stated as

where T is kinetic and V potential energy.

For this particular form to be valid, the following must be true:

• The system is scleronomous (neither kinetic nor potential energy are explicit functions of time)
• The potential energy doesn't depend on velocities.
• The kinetic energy is a quadratic form with regard to velocities.
• The total energy E depends on the motion of the frame of reference (and it turns out that it is minimum for the center of mass frame).

[edit]Noether's theorem

Main article: Noether's theorem

The conservation of energy is a common feature in many physical theories. From a mathematical point of view it is understood as a consequence of Noether's theorem, which states every continuous symmetry of a physical theory has an associated conserved quantity; if the theory's symmetry is time invariance then the conserved quantity is called "energy". The energy conservation law is a consequence of the shift symmetry of time; energy conservation is implied by the empirical fact that the laws of physics do not change with time itself. Philosophically this can be stated as "nothing depends on time per se". In other words, if the physical system is invariant under the continuous symmetry of timetranslation then its energy (which is canonical conjugate quantity to time) is conserved. Conversely, systems which are not invariant under shifts in time (for example, systems with time dependent potential energy) do not exhibit conservation of energy – unless we consider them to exchange energy with another, external system so that the theory of the enlarged system becomes time invariant again. Since any time-varying system can be embedded within a larger time-invariant system, conservation can always be recovered by a suitable re-definition of what energy is. Conservation of energy for finite systems is valid in such physical theories as special relativity and quantum theory (including QED) in the flat space-time.

[edit]Relativity

With the discovery of special relativity by Albert Einstein, energy was proposed to be one component of an energy-momentum 4-vector. Each of the four components (one of energy and three of momentum) of this vector is separately conserved across time, in any closed system, as seen from any given inertial reference frame. Also conserved is the vector length (Minkowski norm), which is the rest mass for single particles, and the invariant mass for systems of particles (where momenta and energy are separately summed before the length is calculated—see the article on invariant mass).

The relativistic energy of a single massive particle contains a term related to its rest mass in addition to its kinetic energy of motion. In the limit of zero kinetic energy (or equivalently in the rest frame) of a massive particle; or else in thecenter of momentum frame for objects or systems which retain kinetic energy, the total energy of particle or object (including internal kinetic energy in systems) is related to its rest mass or its invariant mass via the famous equation .

Thus, the rule of conservation of energy over time in special relativity continues to hold, so long as the reference frame of the observer is unchanged. This applies to the total energy of systems, although different observers disagree as to the energy value. Also conserved, and invariant to all observers, is the invariant mass, which is the minimal system mass and energy that can be seen by any observer, and which is defined by the energy–momentum relation.

In general relativity conservation of energy-momentum is expressed with the aid of a stress-energy-momentum pseudotensor. The theory of general relativity leaves open the question of whether there is a conservation of energy for the entire universe.

[edit]Quantum theory

In quantum mechanics, energy of a quantum system is described by a self-adjoint (Hermite) operator called Hamiltonian, which acts on the Hilbert space (or a space of wave functions ) of the system. If the Hamiltonian is a time independent operator, emergence probability of the measurement result does not change in time over the evolution of the system. Thus the expectation value of energy is also time independent. The local energy conservation in quantum field theory is ensured by the quantum Noether's theorem for energy-momentum tensor operator. Note that due to the lack of the (universal) time operator in quantum theory, the uncertainty relations for time and energy are not fundamental in contrast to the position momentum uncertainty principle, and merely holds in specific cases (See Uncertainty principle). Energy at each fixed time can be precisely measured in principle without any problem caused by the time energy uncertainty relations. Thus the conservation of energy in time is a well defined concept even in quantum mechanics.

[edit]See also

• Conservation law
• Conservation of mass
• Energy quality
• Energy transformation
• Groundwater energy balance
• Laws of thermodynamics
• Lagrangian
• Principles of energetics

[edit]Notes

1. ^ Walter Lewin (October 4, 1999) (in English) (ogg). Work, Kinetic Energy, and Universal Gravitation. MIT Course 8.01: Classical Mechanics, Lecture 11. (videotape). Cambridge, MA USA: MIT OCW. Event occurs at 45:35-49:11. Retrieved December 23, 2010. ""150 Joules is enough to kill you.""
2. ^ Planck, M. (1923/1927). Treatise on Thermodynamics, third English edition translated by A. Ogg from the seventh German edition, Longmans, Green & Co., London, page 40.
3. ^ Lavoisier, A.L. & Laplace, P.S. (1780) "Memoir on Heat", Académie Royale des Sciences pp4-355
4. ^ von Mayer, J.R. (1842) "Remarks on the forces of inorganic nature" in Annalen der Chemie und Pharmacie, 43, 233
5. ^ Grove, W. R. (1874). The Correlation of Physical Forces (6th ed.). London: Longmans, Green.
6. ^ Hadden, Richard W. (1994). On the shoulders of merchants: exchange and the mathematical conception of nature in early modern Europe. SUNY Press. p. 13. ISBN 0-7914-2011-6., Chapter 1, p. 13

[edit]References

[edit]Modern accounts

• Goldstein, Martin, and Inge F., 1993. The Refrigerator and the Universe. Harvard Univ. Press. A gentle introduction.
• Kroemer, Herbert; Kittel, Charles (1980). Thermal Physics (2nd ed.). W. H. Freeman Company. ISBN 0-7167-1088-9.
• Nolan, Peter J. (1996). Fundamentals of College Physics, 2nd ed.. William C. Brown Publishers.
• Oxtoby & Nachtrieb (1996). Principles of Modern Chemistry, 3rd ed.. Saunders College Publishing.
• Papineau, D. (2002). Thinking about Consciousness. Oxford: Oxford University Press.
• Serway, Raymond A.; Jewett, John W. (2004). Physics for Scientists and Engineers (6th ed.). Brooks/Cole. ISBN 0-534-40842-7.
• Stenger, Victor J. (2000). Timeless Reality. Prometheus Books. Especially chpt. 12. Nontechnical.
• Tipler, Paul (2004). Physics for Scientists and Engineers: Mechanics, Oscillations and Waves, Thermodynamics (5th ed.). W. H. Freeman. ISBN 0-7167-0809-4.
• Lanczos, Cornelius (1970). The Variational Principles of Mechanics. Toronto: University of Toronto Press. ISBN 0-8020-1743-6.

[edit]History of ideas

• Brown, T.M. (1965). "Resource letter EEC-1 on the evolution of energy concepts from Galileo to Helmholtz". American Journal of Physics 33 (10): 759–765. Bibcode 1965AmJPh..33..759B. doi:10.1119/1.1970980.
• Cardwell, D.S.L. (1971). From Watt to Clausius: The Rise of Thermodynamics in the Early Industrial Age. London: Heinemann. ISBN 0-435-54150-1.
• Guillen, M. (1999). Five Equations That Changed the World. New York: Abacus. ISBN 0-349-11064-6.
• Hiebert, E.N. (1981). Historical Roots of the Principle of Conservation of Energy. Madison, Wis.: Ayer Co Pub. ISBN 0-405-13880-6.
• Kuhn, T.S. (1957) “Energy conservation as an example of simultaneous discovery”, in M. Clagett (ed.) Critical Problems in the History of Science pp.321–56
• Sarton, G.; Joule, J. P.; Carnot, Sadi (1929). "The discovery of the law of conservation of energy". Isis 13: 18–49. doi:10.1086/346430.
• Smith, C. (1998). The Science of Energy: Cultural History of Energy Physics in Victorian Britain. London: Heinemann. ISBN 0-485-11431-3.
• Mach, E. (1872). History and Root of the Principles of the Conservation of Energy. Open Court Pub. Co., IL.
• Poincaré, H. (1905). Science and Hypothesis. Walter Scott Publishing Co. Ltd; Dover reprint, 1952. ISBN 0-486-60221-4., Chapter 8, "Energy and Thermo-dynamics"