Energy, Work, and Heat

The first law of thermodynamics goes a step further. Taking account of the fact that there are two kinds of energy, heat and work, the first law states ... 

Very closely interrelated concepts in thermodynamics are those of energy, work and heat. Energy is generally perceived as the capacity to do work. Mechanical work is performed whenever the point of application of a force is displaced in the direction of the applied force. Heat is a form of energy. Heat and work are interconvertible. The interconversion of heat and work is one of the prime concerns of thermodynamics. 

The differences between the two forms of energy, heat and work, provide some insight into the second law. In an energy balance, both work and heat are included as simple additive terms, implying that one unit of heat, a joule, is equivalent to the same unit of work. Although this is true with respect to an energy balance, experience teaches that there is a difference in quality between heat and work. This experience is summarized by the following facts. 

This equation is merely an expression of the law of conservation of energy. By convention, energy, heat, and work added to a system are regarded as positive, but any of these removed from a system is regarded as negative. Be sure to get the sign right. 

Experimental observations (>Sec. 1.5). In any change of state the total energy—which includes internal, potential, and kinetic energy, heat, and work—is a conserved quantity. ... 

Experimental observation 4 (Sec. 1.5). A flow of heat and a flow of work are equivalent in that supplying a given amount of energy to a system in either of these forms can be made to result in the same increase ip its internal energy. Heat and work, or niore generally, thermal-and mechanical energy, are not equivalent in the sense... 

The SI unit of heat, as well as of work and energy is the joule, J as summarized in Fig. 2.3. Its dimension is expressed in [kg m s" ]. Heat and work describe the energy exchanged between thermodynamic systems, as discussed in Sect. 2.1.5 with the equation 6q -i- 8w = dU. An earlier, empirical unit, the calorie, was based on the specific heat capacity of water (1 calthemochemicai = 4.184 J). Since the early 2(f century, however, energy, heat, and work are more precisely determined in joules, making the calorie a superfluous unit. The calorie is not part of the SI units and should be abandoned. AU modem calorimetry is ultimately based on a comparison with heat generated by electrical work. 

Defining Energy Forms of Energy Heat and Work Energy Units... 

Unlike internal energy, heat and work are properties only of the process, not of the state, so they are not state functions. In Example 7.1, for instance, the initial and final states are the same in parts (a) and (b), but the amount of woik done is different because the external, opposing pressures are different. We cannot write Aw = Wf - W for a change. Work done depends not only on the initial state and final state, but also on how the process is carried out, that is, the amount of work done depends on the path. 

In thermodynamics, one deals with closed and open systems, the difference between the two being that the latter involves the exchange of matter in addition to energy (heat and work), Clearly, a cell is an open system. Similarly, organelles such as chloroplasts and mitochondria are also open systems. Other energy transducing systems of interest are found in bacteria, in visual receptors. We shall mainly focus out attention on the thylakoid membrane of chloroplasts and the cristae membrane of rnito-chondria. It is particularly noteworthy that energy transduction and material transport in these two systems are coupled the products of photosynthesis are utilized as the reactants in respiration, and vice versa. 

Differentiate between open, ciosed, and isoiated systems and between kinetic energy, potential energy, heat, and work. 

TABLE 2.2 Summary of Expressions for Change in Internal Energy, Heat, and Work for an Ideal Gas Undergoing a Reversible Process... 

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