title
stringlengths 1
149
⌀ | section
stringlengths 1
1.9k
⌀ | text
stringlengths 13
73.5k
|
|---|---|---|
Bus garage
|
Description
|
Often garages will feature rest rooms for drivers assigned to 'as required' duties, whereby they may be required to drive relief or replacement buses in the event of breakdown. The garage may also have 'light duties' drivers, who merely move the buses internally around the garage, often called shunting. Shunter or light duty drivers are often employed in larger depot facilities and work night shifts in order to position buses in the correct order for morning departures from the depot with the first buses due to leave the depot parked logical order nearest the exit. Because they are driving on privately owned land in many jurisdictions a full bus licence may not be required to perform such tasks. In addition they may also perform other tasks such as cleaning buses, refuelling and light maintenance tasks.
|
Bus garage
|
United Kingdom
|
Several bus companies such as London Buses and Lothian Buses used to operate multiple storage garages around their operating area, supplemented by a central works facility. Central works have declined with increase in sub-contract engineering, and improvements in mechanical reliability of bus designs. Also, the practice of routine mid-life refurbishment of bus fleets has declined, which has resulted in generally shorter service lives.
|
Bus garage
|
United Kingdom
|
Bus garages will generally have large areas unobstructed by supporting columns as well as high roofs, especially for storage of double-decker buses. Recently in London, the transfer of routes from double-decker operation to articulated buses has caused problems at some garages that were found to be too small to accommodate all the replacement buses, requiring splitting of allocations, or the building of new garages.
|
Bus garage
|
United Kingdom
|
Some bus companies in the UK make use of outstations (or out-stations) as an additional bus storage facility. These are generally outdoor parking locations, where buses are stored overnight or between peaks, which are more conveniently located for operations, reducing dead mileage. There does not appear to be a universal definition of an outstation, but it seems agreed that there are no maintenance facilities at a bus outstation.
|
Bus garage
|
Largest
|
The largest bus depot in the world is Millennium Park Bus Depot In Delhi India, built for the Commonwealth Games in 2010.
|
Memorial bench
|
Memorial bench
|
A memorial bench, memorial seat or death bench is a piece of outdoor furniture which commemorates a dead person. Such benches are typically made of wood, but can also be made of metal, stone, or synthetic materials such as plastics. Typically memorial benches are placed in public places.
|
Solar eclipse of February 14, 1953
|
Solar eclipse of February 14, 1953
|
A partial solar eclipse occurred on February 14, 1953. A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby totally or partly obscuring the image of the Sun for a viewer on Earth. A partial solar eclipse occurs in the polar regions of the Earth when the center of the Moon's shadow misses the Earth.
|
Solar eclipse of February 14, 1953
|
Related eclipses
|
Solar eclipses of 1950–1953 This eclipse is a member of a semester series. An eclipse in a semester series of solar eclipses repeats approximately every 177 days and 4 hours (a semester) at alternating nodes of the Moon's orbit.
|
Bok Prize
|
Bok Prize
|
The Bok Prize is awarded annually by the Astronomical Society of Australia and the Australian Academy of Science to recognise outstanding research in astronomy by honoring a student at an Australian university. The prize consists of the Bok Medal together with an award of $1000 and ASA membership for the following year.
|
Bok Prize
|
History
|
The prize is named to commemorate the energetic work of Bart Bok in promoting the undergraduate and graduate study of astronomy in Australia, during his term (1957–1966) as Director of the Mount Stromlo Observatory.
|
Critical security parameter
|
Critical security parameter
|
In cryptography, a critical security parameter (CSP) is information that is either user or system defined and is used to operate a cryptography module in processing encryption functions including cryptographic keys and authentication data, such as passwords, the disclosure or modification of which can compromise the security of a cryptographic module or the security of the information protected by the module.
|
Potassium hydrosulfide
|
Potassium hydrosulfide
|
Potassium hydrosulfide is the inorganic compound with the formula KSH. This colourless salt consists of the cation K+ and the bisulfide anion [SH]−. It is the product of the half-neutralization of hydrogen sulfide with potassium hydroxide. The compound is used in the synthesis of some organosulfur compounds. Aqueous solutions of potassium sulfide consist of a mixture of potassium hydrosulfide and potassium hydroxide.
|
Potassium hydrosulfide
|
Potassium hydrosulfide
|
The structure of the potassium hydrosulfide resembles that for potassium chloride. Their structure is however complicated by the non-spherical symmetry of the SH− anions, but these tumble rapidly in the solid.Addition of sulfur gives dipotassium pentasulfide.
|
Potassium hydrosulfide
|
Synthesis
|
It is prepared by neutralizing aqueous KOH with H2S.
|
Knowledge integration
|
Knowledge integration
|
Knowledge integration is the process of synthesizing multiple knowledge models (or representations) into a common model (representation).
Compared to information integration, which involves merging information having different schemas and representation models, knowledge integration focuses more on synthesizing the understanding of a given subject from different perspectives.
For example, multiple interpretations are possible of a set of student grades, typically each from a certain perspective. An overall, integrated view and understanding of this information can be achieved if these interpretations can be put under a common model, say, a student performance index.
The Web-based Inquiry Science Environment (WISE), from the University of California at Berkeley has been developed along the lines of knowledge integration theory.
|
Knowledge integration
|
Knowledge integration
|
Knowledge integration has also been studied as the process of incorporating new information into a body of existing knowledge with an interdisciplinary approach. This process involves determining how the new information and the existing knowledge interact, how existing knowledge should be modified to accommodate the new information, and how the new information should be modified in light of the existing knowledge.
|
Knowledge integration
|
Knowledge integration
|
A learning agent that actively investigates the consequences of new information can detect and exploit a variety of learning opportunities; e.g., to resolve knowledge conflicts and to fill knowledge gaps. By exploiting these learning opportunities the learning agent is able to learn beyond the explicit content of the new information.
The machine learning program KI, developed by Murray and Porter at the University of Texas at Austin, was created to study the use of automated and semi-automated knowledge integration to assist knowledge engineers constructing a large knowledge base.
|
Knowledge integration
|
Knowledge integration
|
A possible technique which can be used is semantic matching. More recently, a technique useful to minimize the effort in mapping validation and visualization has been presented which is based on Minimal Mappings. Minimal mappings are high quality mappings such that i) all the other mappings can be computed from them in time linear in the size of the input graphs, and ii) none of them can be dropped without losing property i).
|
Knowledge integration
|
Knowledge integration
|
The University of Waterloo operates a Bachelor of Knowledge Integration undergraduate degree program as an academic major or minor. The program started in 2008.
|
Specific heat capacity
|
Specific heat capacity
|
In thermodynamics, the specific heat capacity (symbol c) of a substance is the heat capacity of a sample of the substance divided by the mass of the sample, also sometimes referred to as massic heat capacity. Informally, it is the amount of heat that must be added to one unit of mass of the substance in order to cause an increase of one unit in temperature. The SI unit of specific heat capacity is joule per kelvin per kilogram, J⋅kg−1⋅K−1. For example, the heat required to raise the temperature of 1 kg of water by 1 K is 4184 joules, so the specific heat capacity of water is 4184 J⋅kg−1⋅K−1.Specific heat capacity often varies with temperature, and is different for each state of matter. Liquid water has one of the highest specific heat capacities among common substances, about 4184 J⋅kg−1⋅K−1 at 20 °C; but that of ice, just below 0 °C, is only 2093 J⋅kg−1⋅K−1. The specific heat capacities of iron, granite, and hydrogen gas are about 449 J⋅kg−1⋅K−1, 790 J⋅kg−1⋅K−1, and 14300 J⋅kg−1⋅K−1, respectively. While the substance is undergoing a phase transition, such as melting or boiling, its specific heat capacity is technically undefined, because the heat goes into changing its state rather than raising its temperature.
|
Specific heat capacity
|
Specific heat capacity
|
The specific heat capacity of a substance, especially a gas, may be significantly higher when it is allowed to expand as it is heated (specific heat capacity at constant pressure) than when it is heated in a closed vessel that prevents expansion (specific heat capacity at constant volume). These two values are usually denoted by cp and cV , respectively; their quotient γ=cp/cV is the heat capacity ratio.
|
Specific heat capacity
|
Specific heat capacity
|
The term specific heat may also refer to the ratio between the specific heat capacities of a substance at a given temperature and of a reference substance at a reference temperature, such as water at 15 °C; much in the fashion of specific gravity. Specific heat capacity is also related to other intensive measures of heat capacity with other denominators. If the amount of substance is measured as a number of moles, one gets the molar heat capacity instead, whose SI unit is joule per kelvin per mole, J⋅mol−1⋅K−1. If the amount is taken to be the volume of the sample (as is sometimes done in engineering), one gets the volumetric heat capacity, whose SI unit is joule per kelvin per cubic meter, J⋅m−3⋅K−1.
|
Specific heat capacity
|
Specific heat capacity
|
One of the first scientists to use the concept was Joseph Black, an 18th-century medical doctor and professor of medicine at Glasgow University. He measured the specific heat capacities of many substances, using the term capacity for heat.
|
Specific heat capacity
|
Definition
|
The specific heat capacity of a substance, usually denoted by c or s, is the heat capacity C of a sample of the substance, divided by the mass M of the sample: where dQ represents the amount of heat needed to uniformly raise the temperature of the sample by a small increment dT Like the heat capacity of an object, the specific heat capacity of a substance may vary, sometimes substantially, depending on the starting temperature T of the sample and the pressure p applied to it. Therefore, it should be considered a function c(p,T) of those two variables.
|
Specific heat capacity
|
Definition
|
These parameters are usually specified when giving the specific heat capacity of a substance. For example, "Water (liquid): cp = 4187 J⋅kg−1⋅K−1 (15 °C)" When not specified, published values of the specific heat capacity c generally are valid for some standard conditions for temperature and pressure.
However, the dependency of c on starting temperature and pressure can often be ignored in practical contexts, e.g. when working in narrow ranges of those variables. In those contexts one usually omits the qualifier (p,T) , and approximates the specific heat capacity by a constant c suitable for those ranges.
|
Specific heat capacity
|
Definition
|
Specific heat capacity is an intensive property of a substance, an intrinsic characteristic that does not depend on the size or shape of the amount in consideration. (The qualifier "specific" in front of an extensive property often indicates an intensive property derived from it.) Variations The injection of heat energy into a substance, besides raising its temperature, usually causes an increase in its volume and/or its pressure, depending on how the sample is confined. The choice made about the latter affects the measured specific heat capacity, even for the same starting pressure p and starting temperature T . Two particular choices are widely used: If the pressure is kept constant (for instance, at the ambient atmospheric pressure), and the sample is allowed to expand, the expansion generates work as the force from the pressure displaces the enclosure or the surrounding fluid. That work must come from the heat energy provided. The specific heat capacity thus obtained is said to be measured at constant pressure (or isobaric), and is often denoted cp , cp , etc.
|
Specific heat capacity
|
Definition
|
On the other hand, if the expansion is prevented — for example by a sufficiently rigid enclosure, or by increasing the external pressure to counteract the internal one — no work is generated, and the heat energy that would have gone into it must instead contribute to the internal energy of the sample, including raising its temperature by an extra amount. The specific heat capacity obtained this way is said to be measured at constant volume (or isochoric) and denoted cV , cv , cv , etc.The value of cV is usually less than the value of cp . This difference is particularly notable in gases where values under constant pressure are typically 30% to 66.7% greater than those at constant volume. Hence the heat capacity ratio of gases is typically between 1.3 and 1.67.
|
Specific heat capacity
|
Definition
|
Applicability The specific heat capacity can be defined and measured for gases, liquids, and solids of fairly general composition and molecular structure. These include gas mixtures, solutions and alloys, or heterogenous materials such as milk, sand, granite, and concrete, if considered at a sufficiently large scale.
|
Specific heat capacity
|
Definition
|
The specific heat capacity can be defined also for materials that change state or composition as the temperature and pressure change, as long as the changes are reversible and gradual. Thus, for example, the concepts are definable for a gas or liquid that dissociates as the temperature increases, as long as the products of the dissociation promptly and completely recombine when it drops.
|
Specific heat capacity
|
Definition
|
The specific heat capacity is not meaningful if the substance undergoes irreversible chemical changes, or if there is a phase change, such as melting or boiling, at a sharp temperature within the range of temperatures spanned by the measurement.
|
Specific heat capacity
|
Measurement
|
The specific heat capacity of a substance is typically determined according to the definition; namely, by measuring the heat capacity of a sample of the substance, usually with a calorimeter, and dividing by the sample's mass. Several techniques can be applied for estimating the heat capacity of a substance, such as fast differential scanning calorimetry.
|
Specific heat capacity
|
Measurement
|
The specific heat capacities of gases can be measured at constant volume, by enclosing the sample in a rigid container. On the other hand, measuring the specific heat capacity at constant volume can be prohibitively difficult for liquids and solids, since one often would need impractical pressures in order to prevent the expansion that would be caused by even small increases in temperature. Instead, the common practice is to measure the specific heat capacity at constant pressure (allowing the material to expand or contract as it wishes), determine separately the coefficient of thermal expansion and the compressibility of the material, and compute the specific heat capacity at constant volume from these data according to the laws of thermodynamics.
|
Specific heat capacity
|
Units
|
International system The SI unit for specific heat capacity is joule per kelvin per kilogram J/kg⋅K, J⋅K−1⋅kg−1. Since an increment of temperature of one degree Celsius is the same as an increment of one kelvin, that is the same as joule per degree Celsius per kilogram: J/(kg⋅°C). Sometimes the gram is used instead of kilogram for the unit of mass: 1 J⋅g−1⋅K−1 = 1000 J⋅kg−1⋅K−1.
|
Specific heat capacity
|
Units
|
The specific heat capacity of a substance (per unit of mass) has dimension L2⋅Θ−1⋅T−2, or (L/T)2/Θ. Therefore, the SI unit J⋅kg−1⋅K−1 is equivalent to metre squared per second squared per kelvin (m2⋅K−1⋅s−2).
|
Specific heat capacity
|
Units
|
Imperial engineering units Professionals in construction, civil engineering, chemical engineering, and other technical disciplines, especially in the United States, may use English Engineering units including the pound (lb = 0.45359237 kg) as the unit of mass, the degree Fahrenheit or Rankine (°R = 5/9 K, about 0.555556 K) as the unit of temperature increment, and the British thermal unit (BTU ≈ 1055.056 J), as the unit of heat.
|
Specific heat capacity
|
Units
|
In those contexts, the unit of specific heat capacity is BTU/lb⋅°R, or 1 BTU/lb⋅°R = 4186.68J/kg⋅K. The BTU was originally defined so that the average specific heat capacity of water would be 1 BTU/lb⋅°F. Note the value's similarity to that of the calorie - 4187 J/kg⋅°C ≈ 4184 J/kg⋅°C (~.07%) - as they are essentially measuring the same energy, using water as a basis reference, scaled to their systems' respective lbs and °F, or kg and °C.
|
Specific heat capacity
|
Units
|
Calories In chemistry, heat amounts were often measured in calories. Confusingly, two units with that name, denoted "cal" or "Cal", have been commonly used to measure amounts of heat: the "small calorie" (or "gram-calorie", "cal") is 4.184 J, exactly. It was originally defined so that the specific heat capacity of liquid water would be 1 cal/(°C⋅g).
|
Specific heat capacity
|
Units
|
The "grand calorie" (also "kilocalorie", "kilogram-calorie", or "food calorie"; "kcal" or "Cal") is 1000 small calories, that is, 4184 J, exactly. It was defined so that the specific heat capacity of water would be 1 Cal/(°C⋅kg).While these units are still used in some contexts (such as kilogram calorie in nutrition), their use is now deprecated in technical and scientific fields. When heat is measured in these units, the unit of specific heat capacity is usually Note that while cal is 1⁄1000 of a Cal or kcal, it is also per gram instead of kilogram: ergo, in either unit, the specific heat capacity of water is approximately 1.
|
Specific heat capacity
|
Physical basis
|
The temperature of a sample of a substance reflects the average kinetic energy of its constituent particles (atoms or molecules) relative to its center of mass. However, not all energy provided to a sample of a substance will go into raising its temperature, exemplified via the equipartition theorem.
|
Specific heat capacity
|
Physical basis
|
Monatomic gases Quantum mechanics predicts that, at room temperature and ordinary pressures, an isolated atom in a gas cannot store any significant amount of energy except in the form of kinetic energy. Thus, heat capacity per mole is the same for all monatomic gases (such as the noble gases). More precisely, 12.5 J⋅K−1⋅mol−1 and 21 J⋅K−1⋅mol−1 , where 8.31446 J⋅K−1⋅mol−1 is the ideal gas unit (which is the product of Boltzmann conversion constant from kelvin microscopic energy unit to the macroscopic energy unit joule, and the Avogadro number).
|
Specific heat capacity
|
Physical basis
|
Therefore, the specific heat capacity (per unit of mass, not per mole) of a monatomic gas will be inversely proportional to its (adimensional) atomic weight A . That is, approximately, For the noble gases, from helium to xenon, these computed values are Polyatomic gases On the other hand, a polyatomic gas molecule (consisting of two or more atoms bound together) can store heat energy in other forms besides its kinetic energy. These forms include rotation of the molecule, and vibration of the atoms relative to its center of mass.
|
Specific heat capacity
|
Physical basis
|
These extra degrees of freedom or "modes" contribute to the specific heat capacity of the substance. Namely, when heat energy is injected into a gas with polyatomic molecules, only part of it will go into increasing their kinetic energy, and hence the temperature; the rest will go to into those other degrees of freedom. In order to achieve the same increase in temperature, more heat energy will have to be provided to a mol of that substance than to a mol of a monatomic gas. Therefore, the specific heat capacity of a polyatomic gas depends not only on its molecular mass, but also on the number of degrees of freedom that the molecules have.Quantum mechanics further says that each rotational or vibrational mode can only take or lose energy in certain discrete amount (quanta). Depending on the temperature, the average heat energy per molecule may be too small compared to the quanta needed to activate some of those degrees of freedom. Those modes are said to be "frozen out". In that case, the specific heat capacity of the substance is going to increase with temperature, sometimes in a step-like fashion, as more modes become unfrozen and start absorbing part of the input heat energy.
|
Specific heat capacity
|
Physical basis
|
For example, the molar heat capacity of nitrogen N2 at constant volume is 20.6 J⋅K−1⋅mol−1 (at 15 °C, 1 atm), which is 2.49 R . That is the value expected from theory if each molecule had 5 degrees of freedom. These turn out to be three degrees of the molecule's velocity vector, plus two degrees from its rotation about an axis through the center of mass and perpendicular to the line of the two atoms. Because of those two extra degrees of freedom, the specific heat capacity cV of N2 (736 J⋅K−1⋅kg−1) is greater than that of an hypothetical monatomic gas with the same molecular mass 28 (445 J⋅K−1⋅kg−1), by a factor of 5/3.
|
Specific heat capacity
|
Physical basis
|
This value for the specific heat capacity of nitrogen is practically constant from below −150 °C to about 300 °C. In that temperature range, the two additional degrees of freedom that correspond to vibrations of the atoms, stretching and compressing the bond, are still "frozen out". At about that temperature, those modes begin to "un-freeze", and as a result cV starts to increase rapidly at first, then slower as it tends to another constant value. It is 35.5 J⋅K−1⋅mol−1 at 1500 °C, 36.9 at 2500 °C, and 37.5 at 3500 °C. The last value corresponds almost exactly to the predicted value for 7 degrees of freedom per molecule.
|
Specific heat capacity
|
Derivations of heat capacity
|
Relation between specific heat capacities Starting from the fundamental thermodynamic relation one can show, where α is the coefficient of thermal expansion, βT is the isothermal compressibility, and ρ is density.A derivation is discussed in the article Relations between specific heats.
For an ideal gas, if ρ is expressed as molar density in the above equation, this equation reduces simply to Mayer's relation, where Cp,m and Cv,m are intensive property heat capacities expressed on a per mole basis at constant pressure and constant volume, respectively.
|
Specific heat capacity
|
Derivations of heat capacity
|
Specific heat capacity The specific heat capacity of a material on a per mass basis is which in the absence of phase transitions is equivalent to where C is the heat capacity of a body made of the material in question, m is the mass of the body, V is the volume of the body, and ρ=mV is the density of the material.For gases, and also for other materials under high pressures, there is need to distinguish between different boundary conditions for the processes under consideration (since values differ significantly between different conditions). Typical processes for which a heat capacity may be defined include isobaric (constant pressure, dp=0 ) or isochoric (constant volume, dV=0 ) processes. The corresponding specific heat capacities are expressed as A related parameter to c is CV−1 , the volumetric heat capacity. In engineering practice, cV for solids or liquids often signifies a volumetric heat capacity, rather than a constant-volume one. In such cases, the mass-specific heat capacity is often explicitly written with the subscript m , as cm . Of course, from the above relationships, for solids one writes For pure homogeneous chemical compounds with established molecular or molar mass or a molar quantity is established, heat capacity as an intensive property can be expressed on a per mole basis instead of a per mass basis by the following equations analogous to the per mass equations: where n = number of moles in the body or thermodynamic system. One may refer to such a per mole quantity as molar heat capacity to distinguish it from specific heat capacity on a per-mass basis.
|
Specific heat capacity
|
Derivations of heat capacity
|
Polytropic heat capacity The polytropic heat capacity is calculated at processes if all the thermodynamic properties (pressure, volume, temperature) change The most important polytropic processes run between the adiabatic and the isotherm functions, the polytropic index is between 1 and the adiabatic exponent (γ or κ) Dimensionless heat capacity The dimensionless heat capacity of a material is where C is the heat capacity of a body made of the material in question (J/K) n is the amount of substance in the body (mol) R is the gas constant (J⋅K−1⋅mol−1) N is the number of molecules in the body. (dimensionless) kB is the Boltzmann constant (J⋅K−1)Again, SI units shown for example.
|
Specific heat capacity
|
Derivations of heat capacity
|
Read more about the quantities of dimension one at BIPM In the Ideal gas article, dimensionless heat capacity C∗ is expressed as c^ Heat capacity at absolute zero From the definition of entropy the absolute entropy can be calculated by integrating from zero kelvins temperature to the final temperature Tf The heat capacity must be zero at zero temperature in order for the above integral not to yield an infinite absolute entropy, thus violating the third law of thermodynamics. One of the strengths of the Debye model is that (unlike the preceding Einstein model) it predicts the proper mathematical form of the approach of heat capacity toward zero, as absolute zero temperature is approached.
|
Specific heat capacity
|
Derivations of heat capacity
|
Solid phase The theoretical maximum heat capacity for larger and larger multi-atomic gases at higher temperatures, also approaches the Dulong–Petit limit of 3R, so long as this is calculated per mole of atoms, not molecules. The reason is that gases with very large molecules, in theory have almost the same high-temperature heat capacity as solids, lacking only the (small) heat capacity contribution that comes from potential energy that cannot be stored between separate molecules in a gas.
|
Specific heat capacity
|
Derivations of heat capacity
|
The Dulong–Petit limit results from the equipartition theorem, and as such is only valid in the classical limit of a microstate continuum, which is a high temperature limit. For light and non-metallic elements, as well as most of the common molecular solids based on carbon compounds at standard ambient temperature, quantum effects may also play an important role, as they do in multi-atomic gases. These effects usually combine to give heat capacities lower than 3R per mole of atoms in the solid, although in molecular solids, heat capacities calculated per mole of molecules in molecular solids may be more than 3R. For example, the heat capacity of water ice at the melting point is about 4.6R per mole of molecules, but only 1.5R per mole of atoms. The lower than 3R number "per atom" (as is the case with diamond and beryllium) results from the “freezing out” of possible vibration modes for light atoms at suitably low temperatures, just as in many low-mass-atom gases at room temperatures. Because of high crystal binding energies, these effects are seen in solids more often than liquids: for example the heat capacity of liquid water is twice that of ice at near the same temperature, and is again close to the 3R per mole of atoms of the Dulong–Petit theoretical maximum.
|
Specific heat capacity
|
Derivations of heat capacity
|
For a more modern and precise analysis of the heat capacities of solids, especially at low temperatures, it is useful to use the idea of phonons. See Debye model.
|
Specific heat capacity
|
Derivations of heat capacity
|
Theoretical estimation The path integral Monte Carlo method is a numerical approach for determining the values of heat capacity, based on quantum dynamical principles. However, good approximations can be made for gases in many states using simpler methods outlined below. For many solids composed of relatively heavy atoms (atomic number > iron), at non-cryogenic temperatures, the heat capacity at room temperature approaches 3R = 24.94 joules per kelvin per mole of atoms (Dulong–Petit law, R is the gas constant). Low temperature approximations for both gases and solids at temperatures less than their characteristic Einstein temperatures or Debye temperatures can be made by the methods of Einstein and Debye discussed below.
|
Specific heat capacity
|
Derivations of heat capacity
|
Water (liquid): CP = 4185.5 J⋅K−1⋅kg−1 (15 °C, 101.325 kPa) Water (liquid): CVH = 74.539 J⋅K−1⋅mol−1 (25 °C)For liquids and gases, it is important to know the pressure to which given heat capacity data refer. Most published data are given for standard pressure. However, different standard conditions for temperature and pressure have been defined by different organizations. The International Union of Pure and Applied Chemistry (IUPAC) changed its recommendation from one atmosphere to the round value 100 kPa (≈750.062 Torr).
|
Specific heat capacity
|
Derivations of heat capacity
|
Calculation from first principles The path integral Monte Carlo method is a numerical approach for determining the values of heat capacity, based on quantum dynamical principles. However, good approximations can be made for gases in many states using simpler methods outlined below. For many solids composed of relatively heavy atoms (atomic number > iron), at non-cryogenic temperatures, the heat capacity at room temperature approaches 3R = 24.94 joules per kelvin per mole of atoms (Dulong–Petit law, R is the gas constant). Low temperature approximations for both gases and solids at temperatures less than their characteristic Einstein temperatures or Debye temperatures can be made by the methods of Einstein and Debye discussed below.
|
Specific heat capacity
|
Derivations of heat capacity
|
Relation between heat capacities Measuring the specific heat capacity at constant volume can be prohibitively difficult for liquids and solids. That is, small temperature changes typically require large pressures to maintain a liquid or solid at constant volume, implying that the containing vessel must be nearly rigid or at least very strong (see coefficient of thermal expansion and compressibility). Instead, it is easier to measure the heat capacity at constant pressure (allowing the material to expand or contract freely) and solve for the heat capacity at constant volume using mathematical relationships derived from the basic thermodynamic laws.
|
Specific heat capacity
|
Derivations of heat capacity
|
The heat capacity ratio, or adiabatic index, is the ratio of the heat capacity at constant pressure to heat capacity at constant volume. It is sometimes also known as the isentropic expansion factor.
|
Specific heat capacity
|
Derivations of heat capacity
|
Ideal gas For an ideal gas, evaluating the partial derivatives above according to the equation of state, where R is the gas constant, for an ideal gas Substituting this equation reduces simply to Mayer's relation: The differences in heat capacities as defined by the above Mayer relation is only exact for an ideal gas and would be different for any real gas.
|
Specific heat capacity
|
Derivations of heat capacity
|
Specific heat capacity The specific heat capacity of a material on a per mass basis is which in the absence of phase transitions is equivalent to where C is the heat capacity of a body made of the material in question, m is the mass of the body, V is the volume of the body, ρ=mV is the density of the material.For gases, and also for other materials under high pressures, there is need to distinguish between different boundary conditions for the processes under consideration (since values differ significantly between different conditions). Typical processes for which a heat capacity may be defined include isobaric (constant pressure, dP=0 ) or isochoric (constant volume, dV=0 ) processes. The corresponding specific heat capacities are expressed as From the results of the previous section, dividing through by the mass gives the relation A related parameter to c is C/V , the volumetric heat capacity. In engineering practice, cV for solids or liquids often signifies a volumetric heat capacity, rather than a constant-volume one. In such cases, the specific heat capacity is often explicitly written with the subscript m , as cm . Of course, from the above relationships, for solids one writes For pure homogeneous chemical compounds with established molecular or molar mass, or a molar quantity, heat capacity as an intensive property can be expressed on a per-mole basis instead of a per-mass basis by the following equations analogous to the per mass equations: where n is the number of moles in the body or thermodynamic system. One may refer to such a per-mole quantity as molar heat capacity to distinguish it from specific heat capacity on a per-mass basis.
|
Specific heat capacity
|
Derivations of heat capacity
|
Polytropic heat capacity The polytropic heat capacity is calculated at processes if all the thermodynamic properties (pressure, volume, temperature) change: The most important polytropic processes run between the adiabatic and the isotherm functions, the polytropic index is between 1 and the adiabatic exponent (γ or κ).
|
Specific heat capacity
|
Derivations of heat capacity
|
Dimensionless heat capacity The dimensionless heat capacity of a material is where C is the heat capacity of a body made of the material in question (J/K), n is the amount of substance in the body (mol), R is the gas constant (J/(K⋅mol)), N is the number of molecules in the body (dimensionless), kB is the Boltzmann constant (J/(K⋅molecule)).In the ideal gas article, dimensionless heat capacity C∗ is expressed as c^ and is related there directly to half the number of degrees of freedom per particle. This holds true for quadratic degrees of freedom, a consequence of the equipartition theorem.
|
Specific heat capacity
|
Derivations of heat capacity
|
More generally, the dimensionless heat capacity relates the logarithmic increase in temperature to the increase in the dimensionless entropy per particle S∗=S/NkB , measured in nats.
Alternatively, using base-2 logarithms, C∗ relates the base-2 logarithmic increase in temperature to the increase in the dimensionless entropy measured in bits.
Heat capacity at absolute zero From the definition of entropy the absolute entropy can be calculated by integrating from zero to the final temperature Tf:
|
Specific heat capacity
|
Thermodynamic derivation
|
In theory, the specific heat capacity of a substance can also be derived from its abstract thermodynamic modeling by an equation of state and an internal energy function.
|
Specific heat capacity
|
Thermodynamic derivation
|
State of matter in a homogeneous sample To apply the theory, one considers the sample of the substance (solid, liquid, or gas) for which the specific heat capacity can be defined; in particular, that it has homogeneous composition and fixed mass M . Assume that the evolution of the system is always slow enough for the internal pressure P and temperature T be considered uniform throughout. The pressure P would be equal to the pressure applied to it by the enclosure or some surrounding fluid, such as air.
|
Specific heat capacity
|
Thermodynamic derivation
|
The state of the material can then be specified by three parameters: its temperature T , the pressure P , and its specific volume ν=V/M , where V is the volume of the sample. (This quantity is the reciprocal 1/ρ of the material's density ρ=M/V .) Like T and P , the specific volume ν is an intensive property of the material and its state, that does not depend on the amount of substance in the sample.
|
Specific heat capacity
|
Thermodynamic derivation
|
Those variables are not independent. The allowed states are defined by an equation of state relating those three variables: 0.
|
Specific heat capacity
|
Thermodynamic derivation
|
The function F depends on the material under consideration. The specific internal energy stored internally in the sample, per unit of mass, will then be another function U(T,P,ν) of these state variables, that is also specific of the material. The total internal energy in the sample then will be MU(T,P,ν) For some simple materials, like an ideal gas, one can derive from basic theory the equation of state F=0 and even the specific internal energy U In general, these functions must be determined experimentally for each substance.
|
Specific heat capacity
|
Thermodynamic derivation
|
Conservation of energy The absolute value of this quantity is undefined, and (for the purposes of thermodynamics) the state of "zero internal energy" can be chosen arbitrarily. However, by the law of conservation of energy, any infinitesimal increase MdU in the total internal energy MU must be matched by the net flow of heat energy dQ into the sample, plus any net mechanical energy provided to it by enclosure or surrounding medium on it. The latter is −PdV , where dV is the change in the sample's volume in that infinitesimal step. Therefore hence If the volume of the sample (hence the specific volume of the material) is kept constant during the injection of the heat amount dQ , then the term Pdν is zero (no mechanical work is done). Then, dividing by dT where dT is the change in temperature that resulted from the heat input. The left-hand side is the specific heat capacity at constant volume cV of the material.
|
Specific heat capacity
|
Thermodynamic derivation
|
For the heat capacity at constant pressure, it is useful to define the specific enthalpy of the system as the sum h(T,P,ν)=U(T,P,ν)+Pν . An infinitesimal change in the specific enthalpy will then be therefore If the pressure is kept constant, the second term on the left-hand side is zero, and The left-hand side is the specific heat capacity at constant pressure cP of the material.
|
Specific heat capacity
|
Thermodynamic derivation
|
Connection to equation of state In general, the infinitesimal quantities dT,dP,dV,dU are constrained by the equation of state and the specific internal energy function. Namely, Here (∂F/∂T)(T,P,V) denotes the (partial) derivative of the state equation F with respect to its T argument, keeping the other two arguments fixed, evaluated at the state (T,P,V) in question. The other partial derivatives are defined in the same way. These two equations on the four infinitesimal increments normally constrain them to a two-dimensional linear subspace space of possible infinitesimal state changes, that depends on the material and on the state. The constant-volume and constant-pressure changes are only two particular directions in this space.
|
Specific heat capacity
|
Thermodynamic derivation
|
This analysis also holds no matter how the energy increment dQ is injected into the sample, namely by heat conduction, irradiation, electromagnetic induction, radioactive decay, etc.
|
Specific heat capacity
|
Thermodynamic derivation
|
Relation between heat capacities For any specific volume ν , denote pν(T) the function that describes how the pressure varies with the temperature T , as allowed by the equation of state, when the specific volume of the material is forcefully kept constant at ν . Analogously, for any pressure P , let νP(T) be the function that describes how the specific volume varies with the temperature, when the pressure is kept constant at P . Namely, those functions are such that and for any values of T,P,ν . In other words, the graphs of pν(T) and νP(T) are slices of the surface defined by the state equation, cut by planes of constant ν and constant P , respectively.
|
Specific heat capacity
|
Thermodynamic derivation
|
Then, from the fundamental thermodynamic relation it follows that This equation can be rewritten as where α is the coefficient of thermal expansion, βT is the isothermal compressibility,both depending on the state (T,P,ν) The heat capacity ratio, or adiabatic index, is the ratio cP/cV of the heat capacity at constant pressure to heat capacity at constant volume. It is sometimes also known as the isentropic expansion factor.
|
Specific heat capacity
|
Thermodynamic derivation
|
Calculation from first principles The path integral Monte Carlo method is a numerical approach for determining the values of heat capacity, based on quantum dynamical principles. However, good approximations can be made for gases in many states using simpler methods outlined below. For many solids composed of relatively heavy atoms (atomic number > iron), at non-cryogenic temperatures, the heat capacity at room temperature approaches 3R = 24.94 joules per kelvin per mole of atoms (Dulong–Petit law, R is the gas constant). Low temperature approximations for both gases and solids at temperatures less than their characteristic Einstein temperatures or Debye temperatures can be made by the methods of Einstein and Debye discussed below. However, attention should be made for the consistency of such ab-initio considerations when used along with an equation of state for the considered material.
|
Specific heat capacity
|
Thermodynamic derivation
|
Ideal gas For an ideal gas, evaluating the partial derivatives above according to the equation of state, where R is the gas constant, for an ideal gas PV=nRT,CP−CV=T(∂P∂T)V,n(∂V∂T)P,n,P=nRTV⇒(∂P∂T)V,n=nRV,V=nRTP⇒(∂V∂T)P,n=nRP.
Substituting this equation reduces simply to Mayer's relation: The differences in heat capacities as defined by the above Mayer relation is only exact for an ideal gas and would be different for any real gas.
|
ChemCatChem
|
ChemCatChem
|
ChemCatChem is a biweekly peer-reviewed scientific journal covering heterogeneous, homogeneous, and biocatalysis. It is published by Wiley-VCH on behalf of Chemistry Europe.
According to the Journal Citation Reports, the journal has a 2021 impact factor of 5.497.
|
Coombs' method
|
Coombs' method
|
Coombs' method or the Coombs rule is a ranked voting system which uses a ballot counting method for ranked voting created by Clyde Coombs. The Coombs' method is the application of Coombs rule to single-winner elections, similarly to instant-runoff voting, it uses candidate elimination and redistribution of votes cast for that candidate until one candidate has a majority of votes.
|
Coombs' method
|
Procedures
|
Each voter rank-orders all of the candidates on their ballot. If at any time one candidate is ranked first (among non-eliminated candidates) by an absolute majority of the voters, that candidate wins. Otherwise, the candidate ranked last (again among non-eliminated candidates) by the largest number of (or a plurality of) voters is eliminated. Conversely, under instant-runoff voting, the candidate ranked first (among non-eliminated candidates) by the fewest voters is eliminated.
|
Coombs' method
|
Procedures
|
In some sources, the elimination proceeds regardless of whether any candidate is ranked first by a majority of voters, and the last candidate to be eliminated is the winner. This variant of the method can result in a different winner than the former one (unlike in instant-runoff voting, where checking to see if any candidate is ranked first by a majority of voters is only a shortcut that does not affect the outcome).
|
Coombs' method
|
An example
|
Imagine that Tennessee is having an election on the location of its capital. The population of Tennessee is concentrated around its four major cities, which are spread throughout the state. For this example, suppose that the entire electorate lives in these four cities and that everyone wants to live as near to the capital as possible.
|
Coombs' method
|
An example
|
The candidates for the capital are: Memphis, the state's largest city, with 42% of the voters, but located far from the other cities Nashville, with 26% of the voters, near the center of the state Knoxville, with 17% of the voters Chattanooga, with 15% of the votersThe preferences of the voters would be divided like this: Assuming all of the voters vote sincerely (strategic voting is discussed below), the results would be as follows, by percentage: In the first round, no candidate has an absolute majority of first-place votes (51).
|
Coombs' method
|
An example
|
Memphis, having the most last-place votes (26+15+17=58), is therefore eliminated.
In the second round, Memphis is out of the running, and so must be factored out. Memphis was ranked first on Group A's ballots, so the second choice of Group A, Nashville, gets an additional 42 first-place votes, giving it an absolute majority of first-place votes (68 versus 15+17=32), and making it the winner.
Note that the last-place votes are only used to eliminate a candidate in a voting round where no candidate achieves an absolute majority; they are disregarded in a round where any candidate has 51% or more. Thus last-place votes play no role in the final round.
|
Coombs' method
|
Use
|
The voting rounds used in the reality television program Survivor could be considered a variation of Coombs' method, with sequential voting rounds. Everyone votes for one candidate they support for elimination each round, and the candidate with a plurality of that vote is eliminated. A strategy difference is that sequential rounds of voting means the elimination choice is fixed in a ranked ballot Coombs' method until that candidate is eliminated.
|
Coombs' method
|
Potential for strategic voting
|
The Coombs' method is vulnerable to three tactical voting strategies: compromising, push-over, and teaming. Coombs is sensitive to incomplete ballots, and how voters fill in the bottom of their ballots makes a big difference.
|
Wechsler Memory Scale
|
Wechsler Memory Scale
|
The Wechsler Memory Scale (WMS) is a neuropsychological test designed to measure different memory functions in a person. Anyone ages 16 to 90 is eligible to take this test. The current version is the fourth edition (WMS-IV) which was published in 2009 and which was designed to be used with the WAIS-IV. A person's performance is reported as five Index Scores: Auditory Memory, Visual Memory, Visual Working Memory, Immediate Memory, and Delayed Memory. The WMS-IV also incorporates an optional cognitive exam (Brief Cognitive Status Exam) that helps to assess global cognitive functioning in people with suspected memory deficits or those who have been diagnosed with a various neural, psychiatric and/or developmental disorders. This may include conditions such as dementias or mild learning difficulties.There is clear evidence that the WMS differentiates clinical groups (such as those with dementias or neurological disorders) from those with normal memory functioning and that the primary index scores can distinguish among the memory-impaired clinical groups.
|
Wechsler Memory Scale
|
History
|
The original WMS was published by The Psychological Corporation (later acquired by Pearson), first in 1945, with revisions in 1987, 1997, and 2009.The WMS-IV was normed with the WAIS-IV in the United States. This resulted in a representative normative sample of 1,400 adults (between the ages of 16 and 90) who completed both scales.
|
C3-Benzenes
|
C3-Benzenes
|
The C3-benzenes are a class of organic aromatic compounds which contain a benzene ring and three other carbon atoms.
For the hydrocarbons with no further unsaturation, there are four isomers. The chemical formula for all the saturated isomers is C9H12.
There are three trimethylbenzenes, three ethylmetylbenzenes, and two propylbenzene isomers. Petrol (gasoline) can contain 3-4% C3-benzenes.
|
C3-Benzenes
|
Other
|
Saturated 1,2-Ethylmethylbenzene 1,3-Ethylmethylbenzene 1,4-Ethylmethylbenzene Cumene n-Propylbenzene Unsaturated trans-Propenylbenzene 4-Vinyltoluene
|
Operational definition
|
Operational definition
|
An operational definition specifies concrete, replicable procedures designed to represent a construct. In the words of American psychologist S.S. Stevens (1935), "An operation is the performance which we execute in order to make known a concept." For example, an operational definition of "fear" (the construct) often includes measurable physiologic responses that occur in response to a perceived threat. Thus, "fear" might be operationally defined as specified changes in heart rate, galvanic skin response, pupil dilation, and blood pressure.
|
Operational definition
|
Overview
|
An operational definition is designed to model or represent a concept or theoretical definition, also known as a construct. Scientists should describe the operations (procedures, actions, or processes) that define the concept with enough specificity such that other investigators can replicate their research.Operational definitions are also used to define system states in terms of a specific, publicly accessible process of preparation or validation testing. For example, 100 degrees Celsius may be operationally defined as the process of heating water at sea level until it is observed to boil.
|
Operational definition
|
Overview
|
A cake can be operationally defined by a cake recipe.
|
Operational definition
|
Application
|
Despite the controversial philosophical origins of the concept, particularly its close association with logical positivism, operational definitions have undisputed practical applications. This is especially so in the social and medical sciences, where operational definitions of key terms are used to preserve the unambiguous empirical testability of hypothesis and theory. Operational definitions are also important in the physical sciences.
|
Operational definition
|
Application
|
Philosophy The Stanford Encyclopedia of Philosophy entry on scientific realism, written by Richard Boyd, indicates that the modern concept owes its origin in part to Percy Williams Bridgman, who felt that the expression of scientific concepts was often abstract and unclear. Inspired by Ernst Mach, in 1914 Bridgman attempted to redefine unobservable entities concretely in terms of the physical and mental operations used to measure them. Accordingly, the definition of each unobservable entity was uniquely identified with the instrumentation used to define it. From the beginning objections were raised to this approach, in large part around the inflexibility. As Boyd notes, "In actual, and apparently reliable, scientific practice, changes in the instrumentation associated with theoretical terms are routine. and apparently crucial to the progress of science. According to a 'pure' operationalist conception, these sorts of modifications would not be methodologically acceptable, since each definition must be considered to identify a unique 'object' (or class of objects)." However, this rejection of operationalism as a general project destined ultimately to define all experiential phenomena uniquely did not mean that operational definitions ceased to have any practical use or that they could not be applied in particular cases.
|
Operational definition
|
Application
|
Science The special theory of relativity can be viewed as the introduction of operational definitions for simultaneity of events and of distance, that is, as providing the operations needed to define these terms.In quantum mechanics the notion of operational definitions is closely related to the idea of observables, that is, definitions based upon what can be measured.Operational definitions are often most challenging in the fields of psychology and psychiatry, where intuitive concepts, such as intelligence need to be operationally defined before they become amenable to scientific investigation, for example, through processes such as IQ tests.
|
Operational definition
|
Application
|
Business On October 15, 1970, the West Gate Bridge in Melbourne, Australia collapsed, killing 35 construction workers. The subsequent enquiry found that the failure arose because engineers had specified the supply of a quantity of flat steel plate. The word flat in this context lacked an operational definition, so there was no test for accepting or rejecting a particular shipment or for controlling quality.
|
Operational definition
|
Application
|
In his managerial and statistical writings, W. Edwards Deming placed great importance on the value of using operational definitions in all agreements in business. As he said: "An operational definition is a procedure agreed upon for translation of a concept into measurement of some kind." – W. Edwards Deming"There is no true value of any characteristic, state, or condition that is defined in terms of measurement or observation. Change of procedure for measurement (change of operational definition) or observation produces a new number." – W. Edwards Deming General process Operational, in a process context, also can denote a working method or a philosophy that focuses principally on cause and effect relationships (or stimulus/response, behavior, etc.) of specific interest to a particular domain at a particular point in time. As a working method, it does not consider issues related to a domain that are more general, such as the ontological, etc.
|
Operational definition
|
Application
|
In computing Science uses computing. Computing uses science. We have seen the development of computer science. There are not many who can bridge all three of these. One effect is that, when results are obtained using a computer, the results can be impossible to replicate if the code is poorly documented, contains errors, or if parts are omitted entirely.Many times, issues are related to persistence and clarity of use of variables, functions, and so forth. Also, systems dependence is an issue. In brief, length (as a standard) has matter as its definitional basis. What pray tell can be used when standards are to be computationally framed? Hence, operational definition can be used within the realm of the interactions of humans with advanced computational systems. In this sense, one area of discourse deals with computational thinking in, and with how it might influence, the sciences. To quote the American Scientist: The computer revolution has profoundly affected how we think about science, experimentation, and research.One referenced project pulled together fluid experts, including some who were expert in the numeric modeling related to computational fluid dynamics, in a team with computer scientists. Essentially, it turned out that the computer guys did not know enough to weigh in as much as they would have liked. Thus, their role, to their chagrin, many times was "mere" programmer.
|
Operational definition
|
Application
|
Some knowledge-based engineering projects experienced similarly that there is a trade-off between trying to teach programming to a domain expert versus getting a programmer to understand the intricacies of a domain. That, of course, depends upon the domain. In short, any team member has to decide which side of the coin to spend one's time.
|
Operational definition
|
Application
|
The International Society for Technology in Education has a brochure detailing an "operational definition" of computational thinking. At the same time, the ISTE made an attempt at defining related skills.A recognized skill is tolerance for ambiguity and being able to handle open-ended problems. For instance, a knowledge-based engineering system can enhance its operational aspect and thereby its stability through more involvement by the subject-matter expert, thereby opening up issues of limits that are related to being human. As in, many times, computational results have to be taken at face value due to several factors (hence the duck test's necessity arises) that even an expert cannot overcome. The end proof may be the final results (reasonable facsimile by simulation or artifact, working design, etc.) that are not guaranteed to be repeatable, may have been costly to attain (time and money), and so forth.
|
Operational definition
|
Application
|
In advanced modeling, with the requisite computational support such as knowledge-based engineering, mappings must be maintained between a real-world object, its abstracted counterparts as defined by the domain and its experts, and the computer models. Mismatches between domain models and their computational mirrors can raise issues apropos this topic. Techniques that allow the flexible modeling required for many hard problems must resolve issues of identity, type, etc. which then lead to methods, such as duck typing. Many domains, with a numerics focus, use limit theory, of various sorts, to overcome the duck test necessity with varying degrees of success. Yet, with that, issues still remain as representational frameworks bear heavily on what we can know.
|
Operational definition
|
Application
|
In arguing for an object-based methodology, Peter Wegner suggested that "positivist scientific philosophies, such as operationalism in physics and behaviorism in psychology" were powerfully applied in the early part of the 20th century. However, computation has changed the landscape. He notes that we need to distinguish four levels of "irreversible physical and computational abstraction" (Platonic abstraction, computational approximation, functional abstraction, and value computation). Then, we must rely on interactive methods, that have behavior as their focus (see duck test).
|
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.