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Setting Up an Integral for Diffusion of a Chemical Using Shells in Calculus

This concept covers the shell method for setting up a definite integral to compute a total quantity (here, mass) from a radially varying density/concentration function over a region with rotational symmetry. It relies on approximating the region as a union of thin concentric cylindrical shells, computing each shell's volume and its associated quantity via volume times local concentration, summing via a Riemann-sum-like construction, and taking the limit as the number of shells goes to infinity and shell thickness goes to zero to obtain an integral. This is a topic within integral calculus, specifically the application of Riemann sums and the limit definition of the definite integral to physical/applied problems involving accumulation over a continuously varying quantity.