Mechanical equivalent of heat

Enthalpy. Enthalpy is the thermodynamic property of a substance defined as the sum of its internal energy plus the quantity Pv//, where P = pressure of the substance, v = its specific volume, and J = the mechanical equivalent of heat. Enthalpy is also known as total heat and heat content. 

Mechanical equivalent of heat Correction factors for baffle by )assing, baffle configuration, baffle 1.0(N-m)/J 778(ft-lkf)/Btii... 

Arbeitswert, m. value in work. — der Warme, mechanical equivalent of heat. 

Definition.—The Mechanical Equivalent of Heat (J) is the number of ergs of work which, if completely converted into heat, would give rise to one calorie. 

J. R. Mayer (1842) made the first calculation of the mechanical equivalent of heat by comparing the work done on expansion of air with the heat absorbed. 

The entire agreement between the values of the mechanical equivalent of heat obtained by many different methods establishes the proposition that it is independent of the process in which the conversion of work into heat occurs, and depends solely on the choice of the units of these two magnitudes. This result was first established by Joule. 

The following statement is a consequence of the experimental results of Joule, that is, of the existence of a unique mechanical equivalent of heat, and is known as the First Lair of Thermo-dynamics ... 

The correctness of this statement is to be inferred from the exact agreement between the values of the mechanical equivalent of heat obtained by different methods. Thus, in Joule s second series of experiments, mechanical work is directly converted into heat in the first and third series, it is indirectly transformed through the medium of electro-magnetic energy in the fourth series, the energy of an electric current is converted into heat the identity of the values of J so obtained implies a complete conversion of the initial forms of energy into heat energy. 

The calculation of Mayer was thrown into a different form by Rankine (1850), who showed that, instead of estimating the mechanical equivalent of heat from the difference of the specific heats of air, one could take Joule s value of the mechanical equivalent and the known ratio of the specific heats, and thence determine the specific heats themselves. 

In all of these systems, by definition, the specific heat capacity of water is unity. It may be noted that, by comparing the definitions used in the SI and the mks systems, the kilocalorie is equivalent to 4186.8 J/kg K. This quantity has often been referred to as the mechanical equivalent of heat J. 

The subject of physics may be characterized as that Branch of Philosophy to which men look for exact information [my emphasis] . .. the difficulty of physical investigation can be realized when we reflect that an accurate determination, for instance, of the mechanical equivalent of heat would take all the time of the most competent physicist for at least a year.93... 

It is important to bear in mind that an electrophoresis gel is an element in an electrical circuit and as such obeys the fundamental laws of electricity. Each gel has an intrinsic resistance, R, determined by the ionic strength of its buffer (R changes with time in discontinuous systems). When a voltage V is impressed across the gel, a current I flows through the gel and the external circuitry. Ohm s law relates these three quantities V = IR, where V is expressed in volts, I in amperes, and R in ohms. In addition, power P, in watts, is given by P = IV. The generation of Joule heat, H, is related to power by the mechanical equivalent of heat, 4.18 J/cal, so that H = (PI4.18) cal/sec. 

Q is the heat of explosion and 425 mechanical equivalent of heat). Some examples are given in Table E12... 

Potential (Potential or Effet utile, in Ft). According to definition given in Refs 1 2 it is equal to QX425 kg-m/kg, where Q is heat of ezpln in Kcal/kg and 425 is mechanical equivalent of heat. This unit is identical with W which is the maximum quantity of work that can possihly be done by a unit weight of the explosive A slightly different definition is given by Muraour(Ref 3) the potential de V exp Iosif is equal to QX428, where Q is the heat evolved on decomposition of 1 kg of explosive and 428 is the mech equlv of heat. 

Credit for the first recognizable statement of the principle of conservation of energy (heat plus work) apparently belongs to J. Robert Mayer (Sidebar 3.2), who published such a statement in 1842. Mayer also obtained a (slightly) improved estimate, approximately 3.56 J cal-1, for the mechanical equivalent of heat. Mayer had actually submitted his first paper on the energy-conservation principle two years earlier, but his treatment of the concepts of force, momentum, work, and energy was so confused that the paper was rejected. By 1842, Mayer had sufficiently straightened out his ideas to win publication,... 

I joule = l volt coulomb (the Si-approved unit of energy) l calorie = 4.184 joules (the so-called mechanical equivalent of heat)... 

He is remembered for Joule s Law lhat describes the rale at which heal is produced by an electric current. Joule s work showed there were different kinds or energy, which can be changed into each other. He established the mechanical equivalence of heat. His work led to the law of conservation of energy. Alsu, he collaborated with William Thomson (Lord Kelvin) and verified experimentally the Joule-Thomson refrigeration effect. 

Berthelot considered the characteristic product as a measure of the mechanical work per-formed by so osplosioo. This work) csliod pc lentiel de 1 explosif or action explosive in Fr, can also be calcd from the expression QeE, where E is the mechanical equivalent of heat. This is >iven hv MuraourfRef 8.0 76) as 428... 

One of the most intriguing achievements of the 19th century was James Joule s (1818-1889) determination of the mechanical equivalent of heat and, therefore, the first law of thermodynamics. There is a study of the background to this paper, through the analysis of Joule s work in electrochemistry.88... 

Heat transfer processes are described by physical properties and process-related parameters, the dimensions of which not only include the base dimensions of Mass, Length and Time but also Temperature, , as the fourth one. In the discussion of the heat transfer characteristic of a mixing vessel (Example 20) it was shown that, in the dimensional analysis of thermal problems, it is advantageous to expand the dimensional system to include the amount of heat, H [kcal], as the fifth base dimension. Joule s mechanical equivalent of heat, J, must then be introduced as the corresponding dimensional constant in the relevance list. Although this procedure does not change the pi-space, a dimensionless number is formed which contains J and, as such, frequently proves to be irrelevant. As a result, the pi-set is finally reduced by one dimensionless number. 

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