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Heat
*** Shopping-Tip: Heat
{{dablink|For other senses of this word, see
heat (disambiguation).}}
In
physics, '''heat''' is defined as ''energy in transit''.{{ref.html">energy associated with the motion of
atoms,
molecules and other particles which comprise
matter. Heat can be created by
chemical reactions (such as
burning.html" title="Meaning of energy.html" title="Meaning of combustion burning">combustion|burning),_
nuclear reactions (such as
Nuclear fusion fusion taking place inside the
Sun),
electromagnetic dissipation (as in
electric stoves), or
mechanics mechanical dissipation (such as
friction). Heat can be transferred between objects by
thermal radiation radiation,
conduction and
convection.
Temperature, defined as the measure of an object to spontaneously give up energy, is used to indicate the level of elementary motion associated with heat. Heat can only be transferred between objects, or areas within an object, with different temperatures.
Image:SOHO solar flare sun large 20031026 0119 eit 304.png Solar and Heliospheric Observatory thumb|right|250px|Solar-thermal '''"heat"''' emissions recorded via the [[Solar and Heliospheric Observatory|SOHO/EIT
telescope..html" title="Meaning of SOHO.html" title="Meaning of thumb|right|250px|Solar-thermal '''"heat"''' emissions recorded via the [[Solar and Heliospheric Observatory|SOHO">thumb|right|250px|Solar-thermal '''"heat"''' emissions recorded via the [[Solar and Heliospheric Observatory|SOHO/EIT
telescope.">SOHO.html" title="Meaning of thumb|right|250px|Solar-thermal '''"heat"''' emissions recorded via the [[Solar and Heliospheric Observatory|SOHO">thumb|right|250px|Solar-thermal '''"heat"''' emissions recorded via the [[Solar and Heliospheric Observatory|SOHO/EIT
telescope.
History
As early as 460 BC
Hippocrates, the father of medicine, postulated that “'''heat''', a quantity which functions to animate, derives from an internal fire located in the left ventricle.� 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 itself, its essence and quiddity is motion and nothing else.� Similarly, in the mid 17th century, English scientist
Robert Hooke states: “…heat being nothing else but a brisk and vehement agitation of the parts of a body…�
The modern history of heat, however, begins in 1797 when cannon manufacturer
Benjamin Thompson methodically first set out to quantify the well-known phenomenon of frictional heat, i.e. to find out how much heat is produced by metal rubbing against metal. To do this, he designed a specially shaped cannon barrel, thoroughly insulate 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!{{ref|baeyer}}
Rumford summarizes this phenomena as follows: “It is hardly necessary to add, that anything which any insulated body … can continue to furnish without limitation, cannot possibly be a material substance; and it appears to me to be extremely difficult, if not quite impossible, to form any distinct idea of anything capable of being excited and communicated in the manner the Heat was excited and communicated in these experiments, except it be Motion.� As far as what of this "heat" is moving, where it is moving, and how it is moving, Rumford was at a relative standstill. As he states: “I am very far from pretending to know how … that particular kind of motion in bodies which has been supposed to constitute heat is excited, continued, and propagated...�
It would not be until 20th century, with confirmation of the theory that all matter is composed of atoms, that these questions could be answered. Other important historical postulates of heat include the
phlogiston (1733),
fire air (1775), and the
caloric (1787).
Overview
By common knowledge, the term ''heat'' has been used in connection with the
warmth, or
hotness, of surrounding objects. The concept that warm objects "contain heat" is not uncommon. During its 350 year development, the science of
thermodynamics had established a physical quantity named ''temperature'' to quantify the level of "warmth", whereas ''heat'' (also improperly called ''heat change'') was defined as a
transient form of
energy that quantifies the
spontaneous transfer of
thermal energy due to a temperature difference (or
gradient.) The
International System of Units SI unit for heat is the
joule; an alternative unit still in use in the U.S. and other countries is the
British thermal unit.
Image:Hot metalwork.jpg thumb|left|250px|'''"Heat"''' emanating from a red-hot iron rod.
The amount of heat exchanged by an object when its temperature varies by one
degree (temperature) degree is called '''
heat capacity'''. Heat capacity is specific to each and every object. 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 (matter) phase) of objects.
Fuels generate predictable amounts of heat when burned; this heat is known as '''
heating value''' and is expressed per unit of quantity. Upon transitioning 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.
Heat is a
process quantity—as opposed to being a
state quantity—and is to thermal energy as
mechanical work 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. Understanding this difference is a necessary part of understanding the
first law of thermodynamics.
Infrared radiation is often linked to heat, since objects at room temperature or above will
spontaneous emission emit radiation mostly concentrated in the mid-infrared band (see
black body).
Notation
'''Total heat''' is traditionally abbreviated as ''Q'', and is measured in
joules in
SI units. Total heat, heat transfer rate, and heat flux are often abbreviated with different cases of the letter ''Q''. They are often switched in different contexts. Regarding sign convention, 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 labeled:
:
to indicate a change per unit time. It is measured in
watts. '''Heat flux''' is defined as amount of heat per unit time per unit cross-sectional area, is abbreviated '''''q''''', and is measured in watts per meter squared. It is also sometimes notated as ''Q''″ or ''q''″ or
.
Thermodynamics
The amount of heat energy,
, required to change the temperature of a material from an initial temperature, ''T''
0, to a final temperature, ''T
f'' depends on the
heat capacity of that material according to the relationship:
:
The heat capacity is dependent on both the amount of material that is exchanging heat and its properties. The heat capacity can be broken up in several different ways. First of all, it can be represented as a product of mass and
specific heat capacity (more commonly called specific heat):
:
or the number of
mole (unit) moles and the molar heat capacity:
:
Both the molar and specific heat capacities only depend upon the physical properties of the substance being heated, not on any specific properties of the sample. The above definitions of heat capacity only work approximately for
solids and
liquids, but for
gases they don't work at all most of the time. The molar heat capacity can be "patched up" if the changes of temperature occur at either a constant volume or constant
pressure.
Heat can be derived from the equation for
internal energy by rearranging:
:
.
where
is
work. It is important to note that although
is a
state function and therefore constant after each cycle of a
heat engine, neither
nor
is conserved.
Changes of phase
A boiling pot of
water, at
sea level and normal atmospheric pressure, will always be at 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:
:
and
:
For example, turning 1 pound of water into one pound of steam at 100 °C and at normal atmospheric pressure would be: 1000 BTU = (1000 BTU/lb)(1 lb). Note that as pressure increases, the ''L'' rises slightly. Here,
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 doesn't depend on the amount of mass that changes phase, so the equation can normally be written:
:
Sometimes ''L'' can be time-dependent if pressure and volume are time-varying, so that the integral can be handled:
:
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
heat conduction conduction and
Thermal radiation radiation. In
engineering, the term ''
convection 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
heat conduction Conduction is the most common 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
thermal insulation insulators the heat flux is carried almost entirely by
phonon vibrations.
The "electron fluid" of a
conductor (material) 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 conductivity thermal and
electrical conductivity 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.
buoyancy Buoyant convection is due to the effects of gravity, and absent in
microgravity environments.
Radiation
Thermal 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.
For room temperature objects (~300 K), the majority of photons emitted (and involved in radiative heat transfer) are in the
infrared spectrum, but this is by no means the only frequency range involved in radiation. The frequencies emitted are partially related to
black-body radiation. Hotter objects—a campfire is around 700 K, for instance—transfer heat in the visible spectrum or beyond. Whenever EM radiation is emitted and then absorbed, heat is transferred. This principle is used in
microwave ovens,
laser cutting, and
Electrolysis (cosmetology) 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 capilliary 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 sake 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.
References
#{{note|one}} Summation of the definitions give in the following six sources; see:
Talk:Heat.
#{{note|smith}}{{cite book|author= Smith, J.M., Van Ness, H.C., Abbot, M.M.|title=Introduction to Chemical Engineering Thermodynamics|publisher=McGraw-Hill|year=2005|id=ISBN 0073104450}}
#{{note|baierlein}}{{cite book|author= Baierlein, Ralph|title=Thermal Physics|publisher=Cambridge University Press|year=2003|id=ISBN 0521658381}}
#{{note|schroeder}}{{cite book|author= Schroeder, Daniel, R.|title=Thermal Physics|publisher=New York: Addison Wesley Longman|year=2000|id=ISBN 0201380277}}
#{{note|4}}[http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heat.html Discourse on Heat and Work] - Department of Physics and Astronomy, Georgia State University: Hyperphysics (online)
#{{note|perrot}}{{cite book | author=Perrot, Pierre | title=A to Z of Thermodynamics | publisher=Oxford University Press | year=1998 | id=ISBN 0198565526}}
#{{note|clark}}{{cite book | author=Clark, John, O.E. | title=The Essential Dictionary of Science | publisher=Barnes & Noble Books | year=2004 | id=ISBN 0760746168}}
#{{note|baeyer}}{{cite book|author=Baeyer, H.C. von |title=Warmth Disperses and Time Passes – the History of Heat|publisher=New York: The Modern Library|year=1998|id=ISBN 0375753729}}
*
Heat death of the Universe
*
Heat equation
*
Heat exchanger
*
Heat pump
*
Heat transfer coefficient
*
Effect of sun angle on climate
*
Internal energy
*
Shock heating
External links
*[http://www.foxnews.com/story/0,2933,187464,00.html] - Article about extremely high heat generated by scientists.
-
Heat and Thermodynamics - Georgia State University
-
Correlations for Convective Heat Transfer - ChE Online Resources
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see :
high explosive anti-tank
'''
Heat''' (abbreviated '''''Q''''', also called '''heat change''') is the transfer of
thermal energy between two bodies which are at different
temperatures. The
SI unit for heat is the
Joule.
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