Mechanical Heat Equivalent

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. 

That such a mechanical equivalent exists is a consequence of the fact that heat and work are interconvertible. The further fact that it is a fixed constant and independent of the process of conversion was proved by the experiments of Joule (1848-1880), referred to below. In the determination of this constant two measurements are required ... 

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. 

Rowland first observed that the value of J varies with the. temperature, and if the mechanical equivalent of the 15° calorie be taken as standard, the ratio of the value of J at any temperature to this gives the specific heat of water at that temperature. He found the curious result that the specific heat of water had a maximum value at about 30° Callendar and Barnes located this at 37° 5. 

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 maximum amount of work obtainable from a given quantity of heat, called its motivity by Lord Kelvin (1852), is thus always less than the mechanical equivalent of the quantity of heat, except in the limiting case when the refrigerator is at absolute zero (T2 = 0). It cannot be specified in terms of the condition of the body from which the heat is taken, or into which the heat passes, but requires in addition a knowdedge of the lowest available temperature, T2. For if we had another body at temperature T0, where T0 < T2, which could be used as a refrigerator, the amount of work ... 

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... 

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