# PesWiki.com

## PowerPedia:Heat

Lasted edited by Andrew Munsey, updated on June 14, 2016 at 10:04 pm.

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In There was an error working with the wiki: Code[14]), There was an error working with the wiki: Code[15] taking place inside the There was an error working with the wiki: Code[16] dissipation (such as There was an error working with the wiki: Code[17], There was an error working with the wiki: Code[18] and There was an error working with the wiki: Code[48]. Temperature, defined as the measure of an object to spontaneously give up energy, is used as a measure of the internal energy or There was an error working with the wiki: Code[49], 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 There was an error working with the wiki: Code[50]), and then, in the absence of work, only in the direction of the colder body (as per the Second law of thermodynamics).

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#### History

The first to have put forward a semblance of a theory on heat was the There was an error working with the wiki: Code[19] There was an error working with the wiki: Code[20] 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 There was an error working with the wiki: Code[52], the father of medicine, postulated that:

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The hypothesis that heat is a form of motion was proposed initially in the 12th century. Around 1600, the English philosopher and scientist There was an error working with the wiki: Code[53] surmised that:

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This echoed the mid-17th century view of English scientist There was an error working with the wiki: Code[54], who stated:

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In 1761, Scottish chemist There was an error working with the wiki: Code[21]. Between 1759 and 1763 he evolved that theory of "There was an error working with the wiki: Code[55]" on which his scientific fame chiefly rests, and also showed that different substances have different specific heats. There was an error working with the wiki: Code[56], who later invented the There was an error working with the wiki: Code[57], 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 There was an error working with the wiki: Code[58] and There was an error working with the wiki: Code[59]. In addition, in 1797 a cannon manufacturer Sir There was an error working with the wiki: Code[60], 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, There was an error working with the wiki: Code[22] proposed that heat was associated with an undetectable material called There was an error working with the wiki: Code[23] demonstrating the importance of oxygen in burning in 1783. He proposed instead the There was an error working with the wiki: Code[24] 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 There was an error working with the wiki: Code[61], the basis of which was laid out in 1738 by the Swiss physician and mathematician There was an error working with the wiki: Code[62] 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 There was an error working with the wiki: Code[25] and There was an error working with the wiki: Code[26] demonstrated that heat and work were interchangeable, and led to the statement of the principle of the There was an error working with the wiki: Code[27] 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

The First Law of Thermodynamics states that in the absence of nuclear reactions, the energy of a closed system (There was an error working with the wiki: Code[28] is the There was an error working with the wiki: Code[63] (as it is a form of energy), though the There was an error working with the wiki: Code[64] is still occasionally used in the United States.

Heat can only be identified as it is being transferred from one system to another. Heat transfer is a There was an error working with the wiki: Code[65] (There was an error working with the wiki: Code[66]), as opposed to a There was an error working with the wiki: Code[67] (There was an error working with the wiki: Code[68]). Heat flows between systems that are not in thermal equilibrium with each other it spontaneously flows from the areas of high Temperature to areas of low temperature. All systems (There was an error working with the wiki: Code[69]) have a certain amount of There was an error working with the wiki: Code[70]. Internal energy is a There was an error working with the wiki: Code[71] that is a measure of all microscopic ways by which a system can possess energy, for example the random motion of its Atoms or There was an error working with the wiki: Code[72]s. When two bodies of different temperature come into thermal contact, they will exchange internal energy until their temperatures are equalized that is, until they reach Thermal equilibrium. The amount of heat transferred is equal to the amount of energy exchanged between the two bodies. It is a common misconception to confuse heat with internal energy. A hot object doesn’t contain heat it contains internal energy. The adjective hot is used as a relative term to compare the object’s temperature to that of the surroundings (or that of the person using the term). The term heat is used to describe the flow of energy. In the absence of work interactions, the heat that is transferred to an object ends up getting stored in the object in form internal energy.

There was an error working with the wiki: Code[29]. Specific heat is a property, which means that it depends on the substance under consideration and its state as specified by its properties. There was an error working with the wiki: Code[73]s when burned, release much of the energy in the chemical bonds of their molecules. Upon changing from one phase to another, a pure substance releases or absorbs heat without its temperature changing. The amount of heat transfer during a phase change is known as There was an error working with the wiki: Code[74] and depends primarily on the substance and its state.

#### 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&nbsp&lt&nbsp0 (-) when a body absorbs heat from its surroundings, Q&nbsp&gt&nbsp0 (+). 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 There was an error working with the wiki: Code[30] 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 of thermodynamics 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 \Delta U + W_{boundary} is There was an error working with the wiki: Code[75], H, one of the There was an error working with the wiki: Code[76]. Both enthalpy, H , and internal energy, U are There was an error working with the wiki: Code[77]s. 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, \delta Q , forms an There was an error working with the wiki: Code[78] for processes involving work. However, for processes involving no change in volume, applied magnetic field, or other external parameters, \delta Q , forms an There was an error working with the wiki: Code[79]. Likewise, for adiabatic processes (no heat transfer), the expression for work forms an There was an error working with the wiki: Code[79], but for processes involving transfer of heat it forms an There was an error working with the wiki: Code[78] .

The changes in enthalpy and internal energy can be related to the There was an error working with the wiki: Code[82] 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, T0, to a final temperature, Tf 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 There was an error working with the wiki: Code[83]s and There was an error working with the wiki: Code[84]s, 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 There was an error working with the wiki: Code[85], c_s \,\! according to:

:C_p = mc_s \,\!

or is dependent on the number of There was an error working with the wiki: Code[31]s 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 behavior of There was an error working with the wiki: Code[86] such as helium-4. For such substances, the behavior of heat capacity with temperature is discontinuous at the There was an error working with the wiki: Code[87] point.

The quantum behavior of solids is adequately characterized by the There was an error working with the wiki: Code[32].

#### Changes of phase

The boiling point of Water, at There was an error working with the wiki: Code[88] 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 There was an error working with the wiki: Code[89]. The heat added to change the phase of a substance in this way is said to be "hidden," and thus it is called There was an error working with the wiki: Code[90] (from the There was an error working with the wiki: Code[91] 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

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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 There was an error working with the wiki: Code[33] and There was an error working with the wiki: Code[34]. In There was an error working with the wiki: Code[35] is used to describe the combined effects of conduction and fluid flow and is regarded as a third mechanism of heat transfer.

##### Conduction

There was an error working with the wiki: Code[36] 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 There was an error working with the wiki: Code[37] the heat flux is carried almost entirely by There was an error working with the wiki: Code[92] vibrations.

The "electron fluid" of a There was an error working with the wiki: Code[38] 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 There was an error working with the wiki: Code[39] and There was an error working with the wiki: Code[40] of most There was an error working with the wiki: Code[93]s have about the same ratio. A good electrical conductor, such as There was an error working with the wiki: Code[94], usually also conducts heat well. The There was an error working with the wiki: Code[95] exhibits the propensity of electrons to conduct heat through an electrically conductive solid. There was an error working with the wiki: Code[96] is caused by the relationship between electrons, heat fluxes and electrical currents.

##### Convection

There was an error working with the wiki: Code[41] convection is because of the effects of gravity, and hence does not occur in There was an error working with the wiki: Code[97] environments.

There was an error working with the wiki: Code[42] 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 There was an error working with the wiki: Code[98]. 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 (There was an error working with the wiki: Code[99]s and Electrons), their movements result in the emission of There was an error working with the wiki: Code[100], 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 There was an error working with the wiki: Code[43]. 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 There was an error working with the wiki: Code[44]. 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 There was an error working with the wiki: Code[101].

The behavior 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 There was an error working with the wiki: Code[45] 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 There was an error working with the wiki: Code[46].

##### Other heat transfer mechanisms

There was an error working with the wiki: Code[102]: 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, There was an error working with the wiki: Code[103] etc. (see There was an error working with the wiki: Code[104])

There was an error working with the wiki: Code[105]: 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 There was an error working with the wiki: Code[106], they are starting to have applications in There was an error working with the wiki: Code[107]s.

#### 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 There was an error working with the wiki: Code[108] 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 There was an error working with the wiki: Code[109] 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.

#### Related

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Temperature

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#### External artiles and references

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Plasma heat at 2 gigakelvins - Article about extremely high temperature generated by scientists (Foxnews.com)

Heat and Thermodynamics - Georgia State University

Correlations for Convective Heat Transfer - ChE Online Resources

There was an error working with the wiki: Code[1], Wikipedia: The Free Encyclopedia. Wikimedia Foundation.

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Discourse on Heat and Work - Department of Physics and Astronomy, Georgia State University: Hyperphysics (online)

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