Joule definition

Several additional terms related to the absorption of x-radiation require definition energy of a x-ray photon is properly represented in joules but more conveniently reported in eV fluence is the sum of the energy in a unit area intensity or flux is the fluence per unit time and the exposure is a measure of the number of ions produced in a mass of gas. The unit of exposure in medicine is the Rn ntgen, R, defined as the quantity of radiation required to produce 2.58 x C/kg of air. The absorbed dose for a tissue is a measure of energy dissipated per unit mass. The measure of absorbed dose most... 

But it was not until J. P. Joule published a definitive paper in 1847 that the ealorie idea was abandoned. Joule eonelusively showed that heat was a form of energy. As a result of the experiments of Rumford, Joule, and others, it was demonstrated (explieitly stated by Helmholtz in 1847), that the various forms of energy ean be transformed one into another. 

Measurements of energy are made in terms of absolute joules, but engineering practice has persistently retained the thermochemical calorie as the unit of energy. The two are related by the definition ... 

In the SI system, the unit of heat is taken as the same as that of mechanical energy and is therefore the Joule. For water at 298 K (the datum used for many definitions), the specific heat capacity Cp is 4186.8 J/kg K. 

Write the definition of a joule in terms of SI base units. 

A closely related quantity to the internal energy is the enthalpy, H. It, too, has SI units of joules and is defined as the internal energy plus the pressure-volume product, PV. As in most cases, we are concerned with changes in internal energy and enthalpy from one state to another, so that the definition of enthalpy for infinitesimal changes in state is... 

According to the modern convention, measurable quantities are expressed in SI (System Internationale) units and replace the centimetre-gram-second (cgs) system. In this system, the unit of length is a metre (m, the unit of mass is kilogram (kg) and the unit of time is second (s). All the other units are derived from these fundamental units. The unit of thermal energy, calorie, is replaced by joule (1 J = 107 erg) to rationalize the definition of thermal energy. Thus, Planck s constant... 

From a purely phenomenological point of view, this expression of the conservation of energy may be considered the definition ul the heat received by the system. The extension of the mechanical principle of conservation of energy to include the flow of heat is due mainly to Carnot. Joule. Helmholtz, and Clausius. 

Many of these effects of radioactive decay can be treated quantitatively using G values. Historically, the G value was defined as the number of molecules or species decomposed or formed per 100 eV of absorbed energy. A newer (SI) definition of the G value is the number of moles of molecules or species formed or decomposed per Joule of energy absorbed. (Note that 1 mol/J = 9.76 x 106 molecules/100 eV.) The G values depend on the radiation and the medium being irradiated and its physical state. Table 19.1 shows some typical G values for the irradiation of neutral liquid water. 

The quantity of energy transferred as heat is measured in joules, J. However, a unit of energy still widely used in biochemistry and related fields is the calorie (cal). The original definition of 1 cal was that it is the energy needed to raise the temperature of 1 g of water by 1°C. The modern definition is... 

The specific and molar heat capacities of some common substances are given in Table 6.1. Note that, although the values of the specific heat capacities are listed in joules per degree Celsius per gram (J-(°C) 1 -g 1), they could equally well be reported in joules per kelvin per gram (J-K 1-g ) with the same numerical values, because the size of the Celsius degree and the kelvin are the same. We can calculate the heat capacity of a substance from its mass and its specific heat capacity by rearranging the definition Cs = dm into C = mCs. Then we can use... 

The calorie was originally based on 1 g of water. For the purposes of a more exact definition it has been superseded by the joule, so that... 

The results of early experiments showed that the temperature did not change on the expansion of the gas, and consequently the value of the Joule coefficient was zero. The heat capacity of the gas is finite and nonzero. Therefore, it was concluded that (dE/dV)Tn was zero. Later and more-precise experiments have shown that the Joule coefficient is not zero for real gases, and therefore (dE/dV)Ttheoretical concepts of the ideal gas. 

The Joule effect is discussed in Section 2.8 in conjunction with the definition of an ideal gas. When Equations (2.40) and (4.64) are combined, the expression for the Joule coefficient becomes. 

In line with what was discussed in Chapter 6 with regard to the quality of the Joule, one can interpret Orwell [6], "All Joules are equal, but some Joules are more equal than others." This means that 1J of heat at 1000 K is more useful than, say, 1J of heat at 298 K. This is a direct consequence of the work available in these amounts of heat, as stated in Chapters 6 and 7, where precise definitions of physical and chemical exergy are given. A direct consequence of the second law of thermodynamics is that the available work (exergy) can never be utilized completely in real processes. Since all real processes are irreversible, every process step will produce a finite amount of lost work, thus diminishing the amount of useful work. 

For an experimental investigation, we could make use of a magic camera that could photograph individual argon atoms and take a million photographs of the contents of the box. The camera would be performing a time average of the motion of the atoms in the box.3 With a million photos and only four atoms in the box, it is likely that we would find some photos with all the atoms on the left side of the box. How would we interpret this result A reasonable interpretation would be in terms of probability. We could count the pictures that showed all four atoms on the left side of the box (consistent with our definition of the final state of the reverse Joule process) and divide it by the total number of pictures, where the four atoms could be anywhere in the box (consistent with the initial state). This ratio... 

It is clear that with the definition of the Ampere also the other electrical quantities are defined. Thermodynamics required the introduction of the base quantities temperature and amount of substance, with the Kelvin and the mol as units. The unit of energy is the Joule, so that no conversion factor is involved here either. 

Using the basic definitions of units and dimensions given in Sec. 1-5, arrive at expressions (a) to convert joules to British thermal units, (b) to convert dyne-centimeters to joules, (c) to convert British thermal units to calories. 

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