Principles of Heat Flow

When the system (50.0 g of HzO) absorbs heat from the surroundings (hot plate), Its temperature increases from 50.0°C to 80.0°C. 

When the hot plate Is turned off, the system gives off heat to the surrounding air, and Its temperature drops. 

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When a chemical reaction takes place, we consider the substances involved, reactants and products, to be the system. The surroundings include the vessel in which the reaction takes place (test tube, beaker, and so on) and the air or other material in thermal contact with the reaction system. 

When this system is heated, its state changes, perhaps to one described as 

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The distance between two cities depends on path, so it isn t a state 

Certain quantities, called state properties, depend only on the state of the system, not on the way the system reached that state. Putting it another way, if X is a state property, then 

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The basic principle of heat-flow calorimetry is certainly to be found in the linear equations of Onsager which relate the temperature or potential gradients across the thermoelements to the resulting flux of heat or electricity (16). Experimental verifications have been made (89-41) and they have shown that the Calvet microcalorimeter, for instance, behaves, within 0.2%, as a linear system at 25°C (41)-A. heat-flow calorimeter may be therefore considered as a transducer which produces the linear transformation of any function of time f(t), the input, i.e., the thermal phenomenon under investigation]] into another function of time ig(t), the response, i.e., the thermogram]. The problem is evidently to define the corresponding linear operator. 

Although the principles of heat flow have lieen understood and treated mathematically since the early 19th century (Fourier, LaPlace. Poisson, Peclel. Lord Kelvin. Riemann. and many otherst. it was not until nearly... 

Knowledge Required (1) The principles of heat flow. (2) The meaning of the term specific heat. (3) The methods for carrying out calorimetry calculations. 

The guarded hot-plate test is based on the principle of heat flow across a large flat sample at steady state, in which the heat flow is unidirectional through the thickness. A temperature diflerential across the sample provides the driving force for heat transfer, following the Fourier equation ... 

The early chapters (Part I) include a brief review of the principles of heat flow, a review of the basic principles of IR radiometry and thermography, and a discussion of how these principles are reflected in instrument design and performance. Later chapters (Part II) illustrate how these performance parameters are utilized to solve a wide variety of applications problems. 

Thermochemistry refers to the study of the heat flow that accompanies chemical reactions. Our discussion of this subject will focus on the basic principles of heat flow (Section 8.1). 

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