Isothermal specific entropy change

Table 7.3. Selected materials exhibiting a magnetocaloric effect Material Magnetocaloric Relative Isothermal specific entropy peak temperature cooling power change per unit (K) (K ) of magnetic induction (As /R)inJm T Adiabatic temperature drop per unit of magnetic induction (Ar/B)inKr 7 Magnetic Materials...

The entropy change accompanying the isothermal compression or expansion of an ideal gas can be expressed in terms of its initial and final pressures. To do so, we use the ideal gas law—specifically, Boyle s law—to express the ratio of volumes in Eq. 3 in terms of the ratio of the initial and final pressures. Because pressure is inversely proportional to volume (Boyle s law), we know that at constant temperature V2/Vj = E /E2 where l is the initial pressure and P2 is the final pressure. Therefore,... 

Thermodynamic methods, which have been those most widely used in the past, utilize isotherms and heats of adsorption as their foundations. Entropy changes calculated from such data are not easy to transform unambiguously into specific descriptions of the adsorbed phase. 

It is easy to calculate entropy changes for isothermal processes, because T is constant and comes outside the integral to give AS = q ev/T. A specific example is the isothermal compression or expansion of an ideal gas, for which AS = nR InlVf/V ). A second example is any phase transition at constant pressure for which q, y = The entropy change is then AS ang =... 

For an ideal gas, adiabatic expansion is isothermal (Section 3.1.2), that is, pV = mRT/M = constant. Hence, the specific work for volume change (J kg ) equals the term representing the change of entropy [Eq. (3.1.42)] ... 

Some of polymer properties change at the glass transition in discontinuous way (the coefficient of isobaric thermal expansion a, coefficient of isothermal compressibility kj, specific heat, etc), whereas the other ones change continuously (volume V, enthalpy H, and entropy S). As it is schematically shown in Figure 3, the onset of solidlike rigidity in an amorphous polymer at Tg is accompanied by sharp reductions in heat capacity Cp, thermal expansion coefficient up, and compressibility coefficient /cj (22). 

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