Thermodynamics is the branch of physics that deals with heat and temperature, their relation to energy, work and properties of bodies of matter. The behavior of these quantities is governed by the four laws of thermodynamics, irrespective of the specific composition of the material or system in question; the laws of thermodynamics are explained in terms of microscopic constituents by statistical mechanics. Thermodynamics applies to a wide variety of topics in science and engineering physical chemistry, chemical engineering and mechanical engineering. Thermodynamics developed out of a desire to increase the efficiency of early steam engines through the work of French physicist Nicolas Léonard Sadi Carnot who believed that engine efficiency was the key that could help France win the Napoleonic Wars. Scots-Irish physicist Lord Kelvin was the first to formulate a concise definition of thermodynamics in 1854 which stated, "Thermo-dynamics is the subject of the relation of heat to forces acting between contiguous parts of bodies, the relation of heat to electrical agency."
The initial application of thermodynamics to mechanical heat engines was extended early on to the study of chemical compounds and chemical reactions. Chemical thermodynamics studies the nature of the role of entropy in the process of chemical reactions and has provided the bulk of expansion and knowledge of the field. Other formulations of thermodynamics emerged in the following decades. Statistical thermodynamics, or statistical mechanics, concerned itself with statistical predictions of the collective motion of particles from their microscopic behavior. In 1909, Constantin Carathéodory presented a purely mathematical approach to the field in his axiomatic formulation of thermodynamics, a description referred to as geometrical thermodynamics. A description of any thermodynamic system employs the four laws of thermodynamics that form an axiomatic basis; the first law specifies that energy can be exchanged between physical systems as work. The second law defines the existence of a quantity called entropy, that describes the direction, thermodynamically, that a system can evolve and quantifies the state of order of a system and that can be used to quantify the useful work that can be extracted from the system.
In thermodynamics, interactions between large ensembles of objects are categorized. Central to this are the concepts of its surroundings. A system is composed of particles, whose average motions define its properties, those properties are in turn related to one another through equations of state. Properties can be combined to express internal energy and thermodynamic potentials, which are useful for determining conditions for equilibrium and spontaneous processes. With these tools, thermodynamics can be used to describe how systems respond to changes in their environment; this can be applied to a wide variety of topics in science and engineering, such as engines, phase transitions, chemical reactions, transport phenomena, black holes. The results of thermodynamics are essential for other fields of physics and for chemistry, chemical engineering, corrosion engineering, aerospace engineering, mechanical engineering, cell biology, biomedical engineering, materials science, economics, to name a few.
This article is focused on classical thermodynamics which studies systems in thermodynamic equilibrium. Non-equilibrium thermodynamics is treated as an extension of the classical treatment, but statistical mechanics has brought many advances to that field; the history of thermodynamics as a scientific discipline begins with Otto von Guericke who, in 1650, built and designed the world's first vacuum pump and demonstrated a vacuum using his Magdeburg hemispheres. Guericke was driven to make a vacuum in order to disprove Aristotle's long-held supposition that'nature abhors a vacuum'. Shortly after Guericke, the English physicist and chemist Robert Boyle had learned of Guericke's designs and, in 1656, in coordination with English scientist Robert Hooke, built an air pump. Using this pump and Hooke noticed a correlation between pressure and volume. In time, Boyle's Law was formulated, which states that pressure and volume are inversely proportional. In 1679, based on these concepts, an associate of Boyle's named Denis Papin built a steam digester, a closed vessel with a fitting lid that confined steam until a high pressure was generated.
Designs implemented a steam release valve that kept the machine from exploding. By watching the valve rhythmically move up and down, Papin conceived of the idea of a piston and a cylinder engine, he did not, follow through with his design. In 1697, based on Papin's designs, engineer Thomas Savery built the first engine, followed by Thomas Newcomen in 1712. Although these early engines were crude and inefficient, they attracted the attention of the leading scientists of the time; the fundamental concepts of heat capacity and latent heat, which were necessary for the development of thermodynamics, were developed by Professor Joseph Black at the University of Glasgow, where James Watt was employed as an instrument maker. Black and Watt performed experiments together, but it was Watt who conceived the idea of the external condenser which resulted in a large increase in steam engine efficiency. Drawing on all the previous work led Sadi Carnot, the "father of thermodynamics", to publish Reflections on the Motive Power of Fire, a discourse on heat, power and engine efficiency.
The book outlined the basic energetic relations between the Carnot engine, the Carnot cycle, motive power. It marked the start of thermodynamics as a modern scien
Solar physics is the branch of astrophysics that specializes in the study of the Sun. It deals with detailed measurements, it intersects with many disciplines of pure physics and computer science, including fluid dynamics, plasma physics including magnetohydrodynamics, particle physics, atomic physics, nuclear physics, stellar evolution, space physics, radiative transfer, applied optics, signal processing, computer vision, computational physics, stellar physics and solar astronomy. Because the Sun is uniquely situated for close-range observing, there is a split between the related discipline of observational astrophysics and observational solar physics; the study of solar physics is important as it provides a "physical laboratory" for the study of plasma physics. Babylonians were keeping a record of solar eclipses, with the oldest record originating from the ancient city of Ugarit, in modern-day Syria; this record dates to about 1300 BC. Ancient Chinese astronomers were observing solar phenomena with the purpose of keeping track of calendars, which were based on lunar and solar cycles.
Records kept before 720 BC are vague and offer no useful information. However, after 720 BC, 37 solar eclipses were noted over the course of 240 years. Astronomical knowledge flourished in the Islamic world during medieval times. Many observatories were built in cities from Damascus to Baghdad, where detailed astronomical observations were taken. A few solar parameters were measured and detailed observations of the Sun were taken. Solar observations were taken with the purpose of navigation, but for timekeeping. Islam requires its followers to pray five times a day, at specific position of the Sun in the sky; as such, accurate observations of the Sun and its trajectory on the sky were needed. In the late 10th century, Iranian astronomer Abu-Mahmud Khojandi built a massive observatory near Tehran. There, he took accurate measurements of a series of meridian transits of the Sun, which he used to calculate the obliquity of the ecliptic. Following the fall of the Western Roman Empire, Western Europe was cut from all sources of ancient scientific knowledge those written in Greek.
This, plus de-urbanisation and diseases such as the Black Death led to a decline in scientific knowledge in Medieval Europe in the early Middle Ages. During this period, observations of the Sun were taken either in relation to the zodiac, or to assist in building places of worship such as churches and cathedrals. In astronomy, the renaissance period started with the work of Nicolaus Copernicus, he proposed that planets revolve around the Sun and not around the Earth, as it was believed at the time. This model is known as the heliocentric model, his work was expanded by Johannes Kepler and Galileo Galilei. Galilei used his new telescope to look at the Sun. In 1610, he discovered sunspots on its surface. In the autumn of 1611, Johannes Fabricius wrote the first book on sunspots, De Maculis in Sole Observatis. Modern day solar physics is focused towards understanding the many phenomena observed with the help of modern telescopes and satellites. Of particular interest are the structure of the solar photosphere, the coronal heat problem and sunspots.
The Solar Physics Division of the American Astronomical Society boasts 555 members, compared to several thousand in the parent organization. A major thrust of current effort in the field of solar physics is integrated understanding of the entire Solar System including the Sun and its effects throughout interplanetary space within the heliosphere and on planets and planetary atmospheres. Studies of phenomena that affect multiple systems in the heliosphere, or that are considered to fit within a heliospheric context, are called heliophysics, a new coinage that entered usage in the early years of the current millennium; the Solar Dynamics Observatory was launched by NASA in February 2010 from Cape Canaveral. The main goals of the mission are understanding how solar activity arises and how it affects life on Earth by determining how the Sun's magnetic field is generated and structured and how the stored magnetic energy is converted and released into space; the Solar and Heliospheric Observatory, SOHO, is a joint project between NASA and ESA, launched in December 1995.
It was launched to probe the interior of the Sun, make observations of the solar wind and phenomena associated with it and investigate the outer layers of the Sun. A publicly funded mission led by the Japanese Aerospace Exploration Agency, the HINODE satellite, launched in 2006, consists of a coordinated set of optical, extreme ultraviolet and X-ray instruments; these investigate the interaction between the Sun's magnetic field. The Advanced Technology Solar Telescope is a solar telescope facility, under construction in Maui. Twenty-two institutions are collaborating on the ATST project, with the main funding agency being the National Science Foundation. Sunspot Solar Observatory operates the Richard B. Dunn Solar Telescope on behalf of the NSF; the Big Bear Solar Observatory in California houses several telescopes including the New Solar Telescope, a 1.6 meter, clear-aperture, off-axis Gregorian telescope. The NTS saw first light in December 2008; until the ATST comes on line, the NTS remains the largest solar telescope in the world.
The Big Bear Observatory is one of several facilities operated by the Center for Solar-Terrestrial Research at New Jerse
Condensed matter physics
Condensed matter physics is the field of physics that deals with the macroscopic and microscopic physical properties of matter. In particular it is concerned with the "condensed" phases that appear whenever the number of constituents in a system is large and the interactions between the constituents are strong; the most familiar examples of condensed phases are solids and liquids, which arise from the electromagnetic forces between atoms. Condensed matter physicists seek to understand the behavior of these phases by using physical laws. In particular, they include the laws of quantum mechanics and statistical mechanics; the most familiar condensed phases are solids and liquids while more exotic condensed phases include the superconducting phase exhibited by certain materials at low temperature, the ferromagnetic and antiferromagnetic phases of spins on crystal lattices of atoms, the Bose–Einstein condensate found in ultracold atomic systems. The study of condensed matter physics involves measuring various material properties via experimental probes along with using methods of theoretical physics to develop mathematical models that help in understanding physical behavior.
The diversity of systems and phenomena available for study makes condensed matter physics the most active field of contemporary physics: one third of all American physicists self-identify as condensed matter physicists, the Division of Condensed Matter Physics is the largest division at the American Physical Society. The field overlaps with chemistry, materials science, nanotechnology, relates to atomic physics and biophysics; the theoretical physics of condensed matter shares important concepts and methods with that of particle physics and nuclear physics. A variety of topics in physics such as crystallography, elasticity, etc. were treated as distinct areas until the 1940s, when they were grouped together as solid state physics. Around the 1960s, the study of physical properties of liquids was added to this list, forming the basis for the new, related specialty of condensed matter physics. According to physicist Philip Warren Anderson, the term was coined by him and Volker Heine, when they changed the name of their group at the Cavendish Laboratories, Cambridge from Solid state theory to Theory of Condensed Matter in 1967, as they felt it did not exclude their interests in the study of liquids, nuclear matter, so on.
Although Anderson and Heine helped popularize the name "condensed matter", it had been present in Europe for some years, most prominently in the form of a journal published in English and German by Springer-Verlag titled Physics of Condensed Matter, launched in 1963. The funding environment and Cold War politics of the 1960s and 1970s were factors that lead some physicists to prefer the name "condensed matter physics", which emphasized the commonality of scientific problems encountered by physicists working on solids, liquids and other complex matter, over "solid state physics", associated with the industrial applications of metals and semiconductors; the Bell Telephone Laboratories was one of the first institutes to conduct a research program in condensed matter physics. References to "condensed" state can be traced to earlier sources. For example, in the introduction to his 1947 book Kinetic Theory of Liquids, Yakov Frenkel proposed that "The kinetic theory of liquids must accordingly be developed as a generalization and extension of the kinetic theory of solid bodies.
As a matter of fact, it would be more correct to unify them under the title of'condensed bodies'". One of the first studies of condensed states of matter was by English chemist Humphry Davy, in the first decades of the nineteenth century. Davy observed that of the forty chemical elements known at the time, twenty-six had metallic properties such as lustre and high electrical and thermal conductivity; this indicated that the atoms in John Dalton's atomic theory were not indivisible as Dalton claimed, but had inner structure. Davy further claimed that elements that were believed to be gases, such as nitrogen and hydrogen could be liquefied under the right conditions and would behave as metals. In 1823, Michael Faraday an assistant in Davy's lab liquefied chlorine and went on to liquefy all known gaseous elements, except for nitrogen and oxygen. Shortly after, in 1869, Irish chemist Thomas Andrews studied the phase transition from a liquid to a gas and coined the term critical point to describe the condition where a gas and a liquid were indistinguishable as phases, Dutch physicist Johannes van der Waals supplied the theoretical framework which allowed the prediction of critical behavior based on measurements at much higher temperatures.
By 1908, James Dewar and Heike Kamerlingh Onnes were able to liquefy hydrogen and newly discovered helium, respectively. Paul Drude in 1900 proposed the first theoretical model for a classical electron moving through a metallic solid. Drude's model described properties of metals in terms of a gas of free electrons, was the first microscopic model to explain empirical observations such as the Wiedemann–Franz law. However, despite the success of Drude's free electron model, it had one notable problem: it was unable to explain the electronic contribution to the specific heat and magnetic properties of metals, the temperature dependence of resistivity at low temperatures. In 1911, three years after helium was first liquefied, Onnes working at University of Leiden discovered superconductivity in mercury, when he observed the electrical resistivity of mercury to vanish at temperatures below a certain value; the phenomenon surprised the best theoretical physicists of the time, it remain
General relativity is the geometric theory of gravitation published by Albert Einstein in 1915 and the current description of gravitation in modern physics. General relativity generalizes special relativity and Newton's law of universal gravitation, providing a unified description of gravity as a geometric property of space and time, or spacetime. In particular, the curvature of spacetime is directly related to the energy and momentum of whatever matter and radiation are present; the relation is specified by the Einstein field equations, a system of partial differential equations. Some predictions of general relativity differ from those of classical physics concerning the passage of time, the geometry of space, the motion of bodies in free fall, the propagation of light. Examples of such differences include gravitational time dilation, gravitational lensing, the gravitational redshift of light, the gravitational time delay; the predictions of general relativity in relation to classical physics have been confirmed in all observations and experiments to date.
Although general relativity is not the only relativistic theory of gravity, it is the simplest theory, consistent with experimental data. However, unanswered questions remain, the most fundamental being how general relativity can be reconciled with the laws of quantum physics to produce a complete and self-consistent theory of quantum gravity. Einstein's theory has important astrophysical implications. For example, it implies the existence of black holes—regions of space in which space and time are distorted in such a way that nothing, not light, can escape—as an end-state for massive stars. There is ample evidence that the intense radiation emitted by certain kinds of astronomical objects is due to black holes; the bending of light by gravity can lead to the phenomenon of gravitational lensing, in which multiple images of the same distant astronomical object are visible in the sky. General relativity predicts the existence of gravitational waves, which have since been observed directly by the physics collaboration LIGO.
In addition, general relativity is the basis of current cosmological models of a expanding universe. Acknowledged as a theory of extraordinary beauty, general relativity has been described as the most beautiful of all existing physical theories. Soon after publishing the special theory of relativity in 1905, Einstein started thinking about how to incorporate gravity into his new relativistic framework. In 1907, beginning with a simple thought experiment involving an observer in free fall, he embarked on what would be an eight-year search for a relativistic theory of gravity. After numerous detours and false starts, his work culminated in the presentation to the Prussian Academy of Science in November 1915 of what are now known as the Einstein field equations; these equations specify how the geometry of space and time is influenced by whatever matter and radiation are present, form the core of Einstein's general theory of relativity. The Einstein field equations are nonlinear and difficult to solve.
Einstein used approximation methods in working out initial predictions of the theory. But as early as 1916, the astrophysicist Karl Schwarzschild found the first non-trivial exact solution to the Einstein field equations, the Schwarzschild metric; this solution laid the groundwork for the description of the final stages of gravitational collapse, the objects known today as black holes. In the same year, the first steps towards generalizing Schwarzschild's solution to electrically charged objects were taken, which resulted in the Reissner–Nordström solution, now associated with electrically charged black holes. In 1917, Einstein applied his theory to the universe as a whole, initiating the field of relativistic cosmology. In line with contemporary thinking, he assumed a static universe, adding a new parameter to his original field equations—the cosmological constant—to match that observational presumption. By 1929, the work of Hubble and others had shown that our universe is expanding; this is described by the expanding cosmological solutions found by Friedmann in 1922, which do not require a cosmological constant.
Lemaître used these solutions to formulate the earliest version of the Big Bang models, in which our universe has evolved from an hot and dense earlier state. Einstein declared the cosmological constant the biggest blunder of his life. During that period, general relativity remained something of a curiosity among physical theories, it was superior to Newtonian gravity, being consistent with special relativity and accounting for several effects unexplained by the Newtonian theory. Einstein himself had shown in 1915 how his theory explained the anomalous perihelion advance of the planet Mercury without any arbitrary parameters. A 1919 expedition led by Eddington confirmed general relativity's prediction for the deflection of starlight by the Sun during the total solar eclipse of May 29, 1919, making Einstein famous, yet the theory entered the mainstream of theoretical physics and astrophysics only with the developments between 1960 and 1975, now known as the golden age of general relativity. Physicists began to understand the concept of a black hole, to identify quasars as one of these objects' astrophysical manifestations.
More precise solar system tests confirmed the theory's predictive power, relativistic cosmology, became amenable to direct observational tests. Over the years, general relativity has acqui
Modern physics is an effort to understand the underlying processes of the interactions of matter utilizing the tools of science & engineering. In general, the term is used to refer to any branch of physics either developed in the early 20th century and onwards, or branches influenced by early 20th century physics. Small velocities and large distances is the realm of classical physics. Modern physics, however involves extreme conditions: quantum effects involve distances comparable to atoms, while relativistic effects involve velocities comparable to the speed of light. In general and relativistic effects exist across all scales, although these effects can be small in everyday life. In a literal sense, the term modern physics, means up-to-date physics. In this sense, a significant portion of so-called classical physics is modern. However, since 1890, new discoveries have caused significant paradigm shifts: the advent of quantum mechanics and of Einsteinian relativity. Physics that incorporates elements of either QM or ER is said to be modern physics.
It is in this latter sense that the term is used. Modern physics is encountered when dealing with extreme conditions. Quantum mechanical effects tend to appear when dealing with "lows", while relativistic effects tend to appear when dealing with "highs", the "middles" being classical behaviour. For example, when analysing the behaviour of a gas at room temperature, most phenomena will involve the Maxwell–Boltzmann distribution; however near absolute zero, the Maxwell–Boltzmann distribution fails to account for the observed behaviour of the gas, the Fermi–Dirac or Bose–Einstein distributions have to be used instead. It is possible to find – or "retrieve" – the classical behaviour from the modern description by analysing the modern description at low speeds and large distances; when doing so, the result is called the classical limit. These are considered to be the topics regarded as the "core" of the foundation of modern physics: History of physics Classical physics Quantum mechanics Theory of relativity Quantum field theory Unified field Theory A. Beiser.
Concepts of Modern Physics. McGraw-Hill. ISBN 978-0-07-123460-3. P. Tipler, R. Llewellyn. Modern Physics. W. H. Freeman. ISBN 978-0-7167-4345-3
Physical cosmology is a branch of cosmology concerned with the studies of the largest-scale structures and dynamics of the Universe and with fundamental questions about its origin, structure and ultimate fate. Cosmology as a science originated with the Copernican principle, which implies that celestial bodies obey identical physical laws to those on Earth, Newtonian mechanics, which first allowed us to understand those physical laws. Physical cosmology, as it is now understood, began with the development in 1915 of Albert Einstein's general theory of relativity, followed by major observational discoveries in the 1920s: first, Edwin Hubble discovered that the universe contains a huge number of external galaxies beyond our own Milky Way; these advances made it possible to speculate about the origin of the universe, allowed the establishment of the Big Bang Theory, by Georges Lemaître, as the leading cosmological model. A few researchers still advocate a handful of alternative cosmologies. Dramatic advances in observational cosmology since the 1990s, including the cosmic microwave background, distant supernovae and galaxy redshift surveys, have led to the development of a standard model of cosmology.
This model requires the universe to contain large amounts of dark matter and dark energy whose nature is not well understood, but the model gives detailed predictions that are in excellent agreement with many diverse observations. Cosmology draws on the work of many disparate areas of research in theoretical and applied physics. Areas relevant to cosmology include particle physics experiments and theory and observational astrophysics, general relativity, quantum mechanics, plasma physics. Modern cosmology developed along tandem tracks of observation. In 1916, Albert Einstein published his theory of general relativity, which provided a unified description of gravity as a geometric property of space and time. At the time, Einstein believed in a static universe, but found that his original formulation of the theory did not permit it; this is because masses distributed throughout the universe gravitationally attract, move toward each other over time. However, he realized that his equations permitted the introduction of a constant term which could counteract the attractive force of gravity on the cosmic scale.
Einstein published his first paper on relativistic cosmology in 1917, in which he added this cosmological constant to his field equations in order to force them to model a static universe. The Einstein model describes a static universe. However, this so-called Einstein model is unstable to small perturbations—it will start to expand or contract, it was realized that Einstein's model was just one of a larger set of possibilities, all of which were consistent with general relativity and the cosmological principle. The cosmological solutions of general relativity were found by Alexander Friedmann in the early 1920s, his equations describe the Friedmann–Lemaître–Robertson–Walker universe, which may expand or contract, whose geometry may be open, flat, or closed. In the 1910s, Vesto Slipher interpreted the red shift of spiral nebulae as a Doppler shift that indicated they were receding from Earth. However, it is difficult to determine the distance to astronomical objects. One way is to compare the physical size of an object to its angular size, but a physical size must be assumed to do this.
Another method is to measure the brightness of an object and assume an intrinsic luminosity, from which the distance may be determined using the inverse square law. Due to the difficulty of using these methods, they did not realize that the nebulae were galaxies outside our own Milky Way, nor did they speculate about the cosmological implications. In 1927, the Belgian Roman Catholic priest Georges Lemaître independently derived the Friedmann–Lemaître–Robertson–Walker equations and proposed, on the basis of the recession of spiral nebulae, that the universe began with the "explosion" of a "primeval atom"—which was called the Big Bang. In 1929, Edwin Hubble provided an observational basis for Lemaître's theory. Hubble showed that the spiral nebulae were galaxies by determining their distances using measurements of the brightness of Cepheid variable stars, he discovered a relationship between the redshift of its distance. He interpreted this as evidence that the galaxies are receding from Earth in every direction at speeds proportional to their distance.
This fact is now known as Hubble's law, though the numerical factor Hubble found relating recessional velocity and distance was off by a factor of ten, due to not knowing about the types of Cepheid variables. Given the cosmological principle, Hubble's law suggested. Two primary explanations were proposed for the expansion. One was Lemaître's Big Bang theory and developed by George Gamow; the other explanation was Fred Hoyle's steady state model in which new matter is created as the galaxies move away from each other. In this model, the universe is the same at any point in time. For a number of years, support for these theories was evenly divided. However, the observational evidence began to support the idea that the universe evolved from a hot dense state; the discovery of the cosmic microwave background in 1965 lent strong support to the Big Bang model, since the precise measurements of the cosmic microwave background by the Cosmic Background Explorer in the early 1990s, few cosmologists ha
Geophysics is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, the use of quantitative methods for their analysis. The term geophysics sometimes refers only to the geological applications: Earth's shape. However, modern geophysics organizations use a broader definition that includes the water cycle including snow and ice. Although geophysics was only recognized as a separate discipline in the 19th century, its origins date back to ancient times; the first magnetic compasses were made from lodestones, while more modern magnetic compasses played an important role in the history of navigation. The first seismic instrument was built in 132 AD. Isaac Newton applied his theory of mechanics to the precession of the equinox. In the 20th century, geophysical methods were developed for remote exploration of the solid Earth and the ocean, geophysics played an essential role in the development of the theory of plate tectonics.
Geophysics is applied to societal needs, such as mineral resources, mitigation of natural hazards and environmental protection. In Exploration Geophysics, Geophysical survey data are used to analyze potential petroleum reservoirs and mineral deposits, locate groundwater, find archaeological relics, determine the thickness of glaciers and soils, assess sites for environmental remediation. Geophysics is a interdisciplinary subject, geophysicists contribute to every area of the Earth sciences. To provide a clearer idea of what constitutes geophysics, this section describes phenomena that are studied in physics and how they relate to the Earth and its surroundings; the gravitational pull of the Moon and Sun give rise to two high tides and two low tides every lunar day, or every 24 hours and 50 minutes. Therefore, there is a gap of 12 hours and 25 minutes between every high tide and between every low tide. Gravitational forces make rocks press down on deeper rocks, increasing their density as the depth increases.
Measurements of gravitational acceleration and gravitational potential at the Earth's surface and above it can be used to look for mineral deposits. The surface gravitational field provides information on the dynamics of tectonic plates; the geopotential surface called. The geoid would be the global mean sea level if the oceans were in equilibrium and could be extended through the continents; the Earth is cooling, the resulting heat flow generates the Earth's magnetic field through the geodynamo and plate tectonics through mantle convection. The main sources of heat are the primordial heat and radioactivity, although there are contributions from phase transitions. Heat is carried to the surface by thermal convection, although there are two thermal boundary layers – the core-mantle boundary and the lithosphere – in which heat is transported by conduction; some heat is carried up from the bottom of the mantle by mantle plumes. The heat flow at the Earth's surface is about 4.2 × 1013 W, it is a potential source of geothermal energy.
Seismic waves are vibrations that travel along its surface. The entire Earth can oscillate in forms that are called normal modes or free oscillations of the Earth. Ground motions from waves or normal modes are measured using seismographs. If the waves come from a localized source such as an earthquake or explosion, measurements at more than one location can be used to locate the source; the locations of earthquakes provide information on mantle convection. Recording of seismic waves from controlled sources provide information on the region that the waves travel through. If the density or composition of the rock changes, waves are reflected. Reflections recorded using Reflection Seismology can provide a wealth of information on the structure of the earth up to several kilometers deep and are used to increase our understanding of the geology as well as to explore for oil and gas. Changes in the travel direction, called refraction, can be used to infer the deep structure of the Earth. Earthquakes pose a risk to humans.
Understanding their mechanisms, which depend on the type of earthquake, can lead to better estimates of earthquake risk and improvements in earthquake engineering. Although we notice electricity during thunderstorms, there is always a downward electric field near the surface that averages 120 volts per meter. Relative to the solid Earth, the atmosphere has a net positive charge due to bombardment by cosmic rays. A current of about 1800 amperes flows in the global circuit, it flows downward from the ionosphere over most of the Earth and back upwards through thunderstorms. The flow is manifested by lightning below the sprites above. A variety of electric methods are used in geophysical survey; some measure spontaneous potential, a potential that arises in the ground because of man-made or natural disturbances. Telluric currents flow in the oceans, they have two causes: electromagnetic induction by the time-varying, external-origin geomagnetic field and motion of conducting bodies across the Earth's per