Forces Drive Molecules to Flow

Molecules diffuse, transporting matter and heat. For example, soft drinks lose their fizz when CO2 diffuses out of their plastic containers. Drugs diffuse out of clever encapsulation devices into the body according to tailored time schedules. Metabolites flow in and out of cells through biological membranes. External forces also drive particles to move and flow. The electrical currents that power household appliances, modern electronics, and neurons result from the flows of ions and electrons that are driven by gradients of electrical potentials. What arc the rates at which molecules flow from one place to another What forces drive them  

The flow of particles or fluids is described by their flux (see Chapter 17, page 308). To avoid vector arithmetic, let s just consider a flow along a single direction, which we ll choose to be the x-axis. The flux J is defined as the amount of material passing through a unit area per unit time. Sometimes the flux is defined as a quantity that is not divided by a unit area (see Chapter 17, for example), but we do divide by the unit area in this chapter. You are free to define the amount of material in terms of either the number of particles, or their mass, or their volume, whichever is most convenient for the problem at hand. 

This relationship will be useful throughout this chapter. 

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