ABSTRACT
In previous chapters we dealt with background information. Beginning here we enter into the main subject of study, the rate of transport processes. We will be considering the simplest form of transport „rst-the 1-D, steady-state, diffusive process. Fortunately, most aspects of transport processes can be understood using 1-D systems. Our starting point is the general balance equation or conservation law:
In Out Gen Acc− + = (4.1)
This equation applies to a predetermined system of interest and accounts for all mechanisms that might produce a change in the state of the system. The balance states that the accumulation of our quantity of interest within the system is increased by the amount of that quantity entering the system, decreased by the amount that leaves the system, and increased (decreased) by the generation (consumption) of our quantity via sources (sinks) within the system. This balance equation will be applied to all our transport processes and can be applied to all processes where we can identify a speci„c component that is conserved. A personal example might include the accumulation of knowledge in our brains. The In terms represent all information coming into our brains from our senses; the Out terms represent everything we forget; the Gen terms account for all our original thoughts and ideas; and the Acc term is the net knowledge we retain, or wisdom. A more general example includes the national budget where In terms represent taxes and tariffs, Out terms represent government spending, Gen terms represent the printing of currency, and the Acc term, usually negative, implies de„cit spending. We can dream up many more examples from energy to entropy to body weight and even to the accumulation of neuroses or other forms of psychological baggage. Equation 4.1 is the most important equation in this text.
