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Tham khảo tài liệu 'passive sampling techniques episode 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Solid phase microextraction properties of the diffusing chemical as well as on those of the medium through which it moves. Equation 1.10 is often referred to as Fick s second law of diffusion and can be readily derived from Fick s first law of diffusion Fick s second law states that the local concentration change with time due to a diffusive transport process is proportional to the second spatial derivative of the concentration. As a special case consider a linear concentration profile along the z-direction C z a0 a1z. Since the second derivative of C z of such a profile is zero diffusion leaves the concentrations along the z-direction unchanged. In other words a linear profile is a steady-state solution of Eq. 1.10 . The relationship between the flux of a property and the spatial gradient of a related property called a gradient-flux law is typical for an entire class of physical processes in which some physical quantity such as mass or energy or momentum or electrical charge is transported from one region of a system to another. For example heat flows through the bar from the high-temperature reservoir to the low-temperature reservoir. Another example is the transport of the electrical charge through a conductor by the application of an electrical potential difference between the ends of the conductor. Table 1.1 lists some physical processes obeying the gradient-flux law 23 . TABLE 1.1 Physical processes that obey the gradient-flux law Physical process Law Equation Variables Molecular diffusion Fick Jz d dz J Mass flux C Concentration D Diffusion coefficient Conduction of heat Fourier T - dT Jz - k-t-dz J Heat flux T Temperature k Thermal conductivity Electric conductivity Ohm T - fcdV Jz -k-T-dz J Electrical current flux V Voltage K Electric conductivity 13 Chapter 2 The use of different designs of passive samplers for air monitoring of persistent organic pollutants Rosalinda Gioia Kevin C. Jones and Tom Harner 2.1 INTRODUCTION The United Nations Economic .
