The Many Faces of Transport
Nature exhibits a bewildering variety of different transport phenomena. We all are aware that walking from one place to another demands the use of muscular strength to realize the activity. It requires an effort to perform the motion. This simple example shows that transport is related to energy conversion. We must convert potential energy into kinetic energy. However, this is not the whole story. We must also overcome friction during the walk in order to sustain the motion. These two aspects will play a crucial role in identifying motion as transport. In general we could say that motion in a physical system is a transport phenomenon, if the motion is correlated with dissipation. Dissipation is conversion of energy into chaotic motion or heat. The motion of the planets around the sun is usually not considered as transport, because there is no accompanying, or negligible, friction. Other examples of transport are the flow of water through a narrow tube, where friction is caused by the wall of the tube on the water flow: the transport of electrons in semiconductors, where dissipation or friction is caused by collisions of electrons on impurities, lattice defects and lattice vibrations, and possible other mechanisms. The latter example is already somewhat more subtle, because in a perfect lattice there will be no frictional forces, at least at zero temperature.
Unable to display preview. Download preview PDF.