   #copyright

Heat

2007 Schools Wikipedia Selection. Related subjects: General Physics

   In physics, heat, symbolized by Q, is defined as energy in transit.
   Generally, heat is a form of energy transfer associated with the
   different motions of atoms, molecules and other particles that comprise
   matter when it is hot and when it is cold. High temperature bodies,
   which often result in high heat transfer, can be created by chemical
   reactions (such as burning), nuclear reactions (such as fusion taking
   place inside the Sun), electromagnetic dissipation (as in electric
   stoves), or mechanical dissipation (such as friction). Heat can be
   transferred between objects by radiation, conduction and convection.
   Temperature, defined as the measure of an object to spontaneously give
   up energy, is used as a measure of the internal energy or enthalpy,
   that is the level of elementary motion giving rise to heat transfer.
   Heat can only be transferred between objects, or areas within an
   object, with different temperatures (as given by the zeroth law of
   thermodynamics), and then, in the absence of work, only in the
   direction of the colder body (as per the second law of thermodynamics).

History

   The first to have put forward a semblance of a theory on heat was the
   Greek philosopher Heraclitus who lived around 500 BC in the city of
   Ephesus in Ionia, Asia Minor. He became famous as the "flux and fire"
   philosopher for his proverbial utterance: "All things are flowing."
   Heraclitus argued that the three principle elements in nature were
   fire, earth, and water. Of these three, however, fire is assigned as
   the central element controlling and modifying the other two. The
   universe was postulated to be in a continuous state of flux or
   permanent condition of change as a result of transformations of fire.
   Heraclitus summarized his philosophy as: "All things are an exchange
   for fire."

   As early as 460 BC Hippocrates, the father of medicine, postulated
   that:


Heat

    Heat, a quantity which functions to animate, derives from an internal
                     fire located in the left ventricle.


                                                                         Heat

   The hypothesis that heat is a form of motion was proposed initially in
   the 12th century. Around 1600, the English philosopher and scientist
   Francis Bacon surmised that:


   Heat

       Heat itself, its essence and quiddity is motion and nothing else.


                                                                         Heat

   This echoed the mid-17th century view of English scientist Robert
   Hooke, who stated:


Heat

    heat being nothing else but a brisk and vehement agitation of the parts
                                  of a body.


                                                                           Heat

   In 1761, Scottish chemist Joseph Black discovered that ice absorbs heat
   without changing temperature when melting. From this he concluded that
   the heat must have combined with the ice particles and become latent.
   Between 1759 and 1763 he evolved that theory of " latent heat" on which
   his scientific fame chiefly rests, and also showed that different
   substances have different specific heats. James Watt, who later
   invented the Watt engine, was Black's pupil and assistant.

   In this direction, the ability to be able to use heat transfer to
   generate work allowed the invention and development of the steam engine
   by people such as Thomas Newcomen and James Watt. In addition, in 1797
   a cannon manufacturer Sir Benjamin Thompson, Count Rumford,
   demonstrated through the use of friction it was possible to convert
   work to heat. To do this, he designed a specially shaped cannon barrel,
   thoroughly insulated against heat loss, then replaced the sharp boring
   tool with a dull drill bit, and immersed the front part of the gun in a
   tank full of water. Using this setup, to the amazement of his
   onlookers, he made cold water boil in two-and-half-hours time, without
   the use of fire.

   Several theories on the nature of heat were developed. In the 17th
   century, Johann Becher proposed that heat was associated with an
   undetectable material called phlogiston that was driven out of a
   substance when it was burnt. This was finally refuted by Lavosier
   demonstrating the importance of oxygen in burning in 1783. He proposed
   instead the caloric theory which saw heat as a type of weightless,
   invisible fluid that moved when out of equilibrium. It was this theory
   used in 1824 by the French engineer Sadi Carnot when he published
   Reflections on the Motive Power of Fire. He set forth the importance of
   heat transfer: "production of motive power is due not to an actual
   consumption of caloric, but to its transportation form a warm body to a
   cold body, i.e. to its re-establishment of equilibrium." According to
   Carnot, this principle applies to any machine set in motion by heat.

   Another theory was the kinetic theory of gases, the basis of which was
   laid out in 1738 by the Swiss physician and mathematician Daniel
   Bernoulli in his Hydrodynamica. In this work, Bernoulli first proposed
   that gases consist of great numbers of molecules moving in all
   directions, that their impact on a surface causes the gas pressure that
   we feel. The internal energy of a substance is then the sum of the
   kinetic energy associated with each molecule, and heat transfer occurs
   from regions with energetic molecules, and so high internal energy, to
   those with less energetic molecules, and so lower internal energy.

   The work of Joule and Mayer demonstrated that heat and work were
   interchangeable, and led to the statement of the principle of the
   conservation of energy by Hermann von Helmholtz in 1847. Clausius
   demonstrated in 1850 that caloric theory could be reconciled with
   kinetic theory provided that the conservation of energy was employed
   rather than the movement of a substance, and stated the First Law of
   Thermodynamics.

Overview

   Under the First Law of Thermodynamics, heat (and work) are processes
   that change the internal energy of a substance or object. Heat is the
   transfer of energy over the boundary of a system owing to a temperature
   gradient. The SI unit for heat is the joule (as it is a form of
   energy), though the British Thermal Unit is still occasionally used in
   the United States.
   Heat emanating from a red-hot iron rod.
   Enlarge
   Heat emanating from a red-hot iron rod.

   Heat is a process quantity, as opposed to being a state quantity, and
   is to thermal energy as work is to mechanical energy. Heat flows
   between regions that are not in thermal equilibrium with each other; it
   spontaneously flows from areas of high temperature to areas of low
   temperature. All objects (matter) have a certain amount of internal
   energy, a state quantity that is related to the random motion of their
   atoms or molecules. When two bodies of different temperature come into
   thermal contact, they will exchange internal energy until the
   temperature is equalized; that is, until they reach thermal
   equilibrium. The amount of energy transferred is the amount of heat
   exchanged. It is a common misconception to confuse heat with internal
   energy: heat is related to the change in internal energy and the work
   performed by the system. The term heat is used to describe the flow of
   energy, while the term internal energy is used to describe the energy
   itself.

   In common usage the term heat denotes the warmth, or hotness, of
   surrounding objects and is used to mean that an object has a high
   temperature. The concept that warm objects "contain heat" is not
   uncommon, but hot is nearly always used as a relative term (an object
   is hot compared with its surroundings or those of the person using the
   term) so that high temperature is directly associated with high heat
   transfer.

   The amount of heat that has to be transferred to or from an object when
   its temperature varies by one degree is called heat capacity. Heat
   capacity is specific to each and every object or substance. When
   referred to a quantity unit (such as mass or moles), the heat exchanged
   per degree is termed specific heat, and depends primarily on the
   composition and physical state (phase) of an object. Fuels generate
   predictable amounts of heat when burned; this heat is known as heating
   value and is expressed per unit of quantity. Upon changing from one
   phase to another, pure substances can exchange heat without their
   temperature suffering any change. The amount of heat exchanged during a
   phase change is known as latent heat and depends primarily on the
   substance and the initial and final phase.

Notation

   The total amount of energy transferred through heat transfer is
   conventionally abbreviated as Q. The conventional sign convention is
   that when a body releases heat into its surroundings, Q < 0 (-); when a
   body absorbs heat from its surroundings, Q > 0 (+). Heat transfer rate,
   or heat flow per unit time, is denoted by:

          \dot{Q} = {dQ\over dt} \,\! .

   It is measured in watts. Heat flux is defined as rate of heat transfer
   per unit cross-sectional area, and is denoted q, resulting in units of
   watts per metre squared. Slightly different notation conventions can be
   used, which may denote heat flux as, for example, \dot{Q}'' .

Thermodynamics

   Heat is related to the internal energy U of the system and work W done
   by the system by the first law of thermodynamics:

          \Delta U = Q - W \

   which means that the energy of the system can change either via work or
   via heat. The transfer of heat to an ideal gas at constant pressure
   increases the internal energy and performs boundary work (i.e. allows a
   control volume of gas to become larger or smaller), provided the volume
   is not constrained. Returning to the first law equation and separating
   the work term into two types, "boundary work" and "other" (e.g. shaft
   work performed by a compressor fan), yields the following:

          \Delta U + W_{boundary} = Q - W_{other}\

   This combined quantity ΔU + W[boundary] is enthalpy, H, one of the
   thermodynamic potentials. Both enthalpy, H, and internal energy, U are
   state functions. State functions return to their initial values upon
   completion of each cycle in cyclic processes such as that of a heat
   engine. In contrast, neither Q nor W are properties of a system and
   need not sum to zero over the steps of a cycle. The infinitesimal
   expression for heat, δQ, forms an inexact differential for processes
   involving work. However, for processes involving no change in volume,
   applied magnetic field, or other external parameters, δQ, forms an
   exact differential. Likewise, for adiabatic processes (no heat
   transfer), the expression for work forms an exact differential, but for
   processes involving transfer of heat it forms an inexact differential .

   The changes in enthalpy and internal energy can be related to the heat
   capacity of a gas at constant pressure and volume respectively. When
   there is no work, the heat , Q, required to change the temperature of a
   gas from an initial temperature, T[0], to a final temperature, T[f]
   depends on the relationship:

          Q = \int_{T_0}^{T_f}C_p\,dT \,\!

   for constant pressure, whereas at constant volume:

          Q = \int_{T_0}^{T_f}C_v\,dT \,\!

   For incompressible substances, such as solids and liquids, there is no
   distinction among the two expressions as they are nearly
   incompressible. Heat capacity is an extensive quantity and as such is
   dependent on the number of molecules in the system. It can be
   represented as the product of mass, m , and specific heat capacity, c_s
   \,\! according to:

          C_p = mc_s \,\!

   or is dependent on the number of moles and the molar heat capacity, c_n
   \,\! according to:

          C_p = nc_n \,\!

   The molar and specific heat capacities are dependent upon the internal
   degrees of freedom of the system and not on any external properties
   such as volume and number of molecules.

   The specific heats of monatomic gases (e.g., helium) are nearly
   constant with temperature. Diatomic gases such as hydrogen display some
   temperature dependence, and triatomic gases (e.g., carbon dioxide)
   still more.

   In liquids at sufficiently low temperatures, quantum effects become
   significant. An example is the behaviour of bosons such as helium-4.
   For such substances, the behaviour of heat capacity with temperature is
   discontinuous at the Bose-Einstein condensation point.

   The quantum behaviour of solids is adequately characterized by the
   Debye model. At temperatures well below the characteristic Debye
   temperature of a solid lattice, its specific heat will be proportional
   to the cube of absolute temperature. A second, smaller term is needed
   to complete the expresssion for low-temperature metals having
   conduction electrons, an example of Fermi-Dirac statistics.

Changes of phase

   The boiling point of water, at sea level and normal atmospheric
   pressure and temperature will always be at nearly 100 °C no matter how
   much heat is added. The extra heat changes the phase of the water from
   liquid into water vapor. The heat added to change the phase of a
   substance in this way is said to be "hidden," and thus it is called
   latent heat (from the Latin latere meaning "to lie hidden"). Latent
   heat is the heat per unit mass necessary to change the state of a given
   substance, or:

          L = \frac{Q}{\Delta m} \,\!

   and

          Q = \int_{M_0}^{M} L\,dm \,\!

   Note that as pressure increases, the L rises slightly. Here, M[o] is
   the amount of mass initially in the new phase, and M is the amount of
   mass that ends up in the new phase. Also,L generally does not depend on
   the amount of mass that changes phase, so the equation can normally be
   written:

          Q = L\Delta m \,\!

   Sometimes L can be time-dependent if pressure and volume are changing
   with time, so that the integral can be written as:

          Q = \int L\frac{dm}{dt}dt \,\!

Heat transfer mechanisms

   As mentioned previously, heat tends to move from a high temperature
   region to a low temperature region. This heat transfer may occur by the
   mechanisms conduction and radiation. In engineering, the term
   convective heat transfer is used to describe the combined effects of
   conduction and fluid flow and is regarded as a third mechanism of heat
   transfer.

Conduction

   Conduction is the most significant means of heat transfer in a solid.
   On a microscopic scale, conduction occurs as hot, rapidly moving or
   vibrating atoms and molecules interact with neighboring atoms and
   molecules, transferring some of their energy (heat) to these
   neighboring atoms. In insulators the heat flux is carried almost
   entirely by phonon vibrations.

   The "electron fluid" of a conductive metallic solid conducts nearly all
   of the heat flux through the solid. Phonon flux is still present, but
   carries less than 1% of the energy. Electrons also conduct electric
   current through conductive solids, and the thermal and electrical
   conductivities of most metals have about the same ratio. A good
   electrical conductor, such as copper, usually also conducts heat well.
   The Peltier-Seebeck effect exhibits the propensity of electrons to
   conduct heat through an electrically conductive solid.
   Thermoelectricity is caused by the relationship between electrons, heat
   fluxes and electrical currents.

Convection

   Convection is usually the dominant form of heat transfer in liquids and
   gases. This is a term used to characterize the combined effects of
   conduction and fluid flow. In convection, enthalpy transfer occurs by
   the movement of hot or cold portions of the fluid together with heat
   transfer by conduction. For example, when water is heated on a stove,
   hot water from the bottom of the pan rises, heating the water at the
   top of the pan. Two types of convection are commonly distinguished,
   free convection, in which gravity and buoyancy forces drive the fluid
   movement, and forced convection, where a fan, stirrer, or other means
   is used to move the fluid. Buoyant convection is because of the effects
   of gravity, and hence does not occur in microgravity environments.

Radiation

   Radiation is the only form of heat transfer that can occur in the
   absence of any form of medium and as such is the only means of heat
   transfer through a vacuum. Thermal radiation is a direct result of the
   movements of atoms and molecules in a material. Since these atoms and
   molecules are composed of charged particles (protons and electrons),
   their movements result in the emission of electromagnetic radiation,
   which carries energy away from the surface. At the same time, the
   surface is constantly bombarded by radiation from the surroundings,
   resulting in the transfer of energy to the surface. Since the amount of
   emitted radiation increases with increasing temperature, a net transfer
   of energy from higher temperatures to lower temperatures results.

   The frequencies of the emitted photons are described by the Planck
   distribution. A black body at higher temperature will emit photons
   having a distributional peak at a higher frequency than will a colder
   object, and their respective spectral peaks will be separated according
   to Wien's displacement law. The photosphere of the Sun, at a
   temperature of approximately 6000 K, emits radiation principally in the
   visible portion of the spectrum. The solar radiation incident upon the
   earth's atmosphere is largely passed through to the surface. The
   atmosphere is largely transparent in the visible spectrum. However, in
   the infrared spectrum that is characteristic of a blackbody at 300K,
   the temperature of the earth, the atmosphere is largely opaque. The
   blackbody radiation from earth's surface is absorbed or scattered by
   the atmosphere. Though some radiation escapes into space, it is the
   radiation absorbed and subsequently emitted by atmospheric gases. It is
   this spectral selectivity of the atmosphere that is responsible for the
   planetary greenhouse effect.

   The behaviour of a common household lightbulb has a spectrum
   overlapping the blackbody spectra of the sun and the earth. A portion
   of the photons emitted by a tungsten light bulb filament at 3000K lie
   in the visible spectrum. However, the majority of the photonic energy
   is associated with longer wavelengths and will transfer heat to the
   environment, as can be deduced empirically by observing a household
   incandescent lightbulb. Whenever EM radiation is emitted and then
   absorbed, heat is transferred. This principle is used in microwave
   ovens, laser cutting, and RF hair removal.

Other heat transfer mechanisms

     * Latent heat: Transfer of heat through a physical change in the
       medium such as water-to-ice or water-to-steam involves significant
       energy and is exploited in many ways: steam engine, refrigerator
       etc. (see latent heat of fusion)
     * Heat pipe: Using latent heat and capillary action to move heat, it
       can carry many times as much heat as a similar sized copper rod.
       Originally invented for use in satellites, they are starting to
       have applications in personal computers.

Heat dissipation

   In cold climates, houses with their heating systems form dissipative
   systems. In spite of efforts to insulate such houses, to reduce heat
   losses to their exteriors, considerable heat is lost, or dissipated,
   from them which can make their interiors uncomfortably cool or cold.
   Furthermore, the interior of the house must be maintained out of
   thermal equilibrium with its external surroundings for the comfort of
   its inhabitants. In effect domestic residences are oases of warmth in a
   sea of cold and the thermal gradient between the inside and outside is
   often quite steep. This can lead to problems such as condensation and
   uncomfortable draughts which, if left unaddressed, can cause structural
   damage to the property. This is why modern insulation techniques are
   required to reduce heat loss.

   In such a house, a thermostat is a device capable of starting the
   heating system when the house's interior falls below a set temperature,
   and of stopping that same system when another (higher) set temperature
   has been achieved. Thus the thermostat controls the flow of energy into
   the house, that energy eventually being dissipated to the exterior.
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