Transport phenomena

When a voltage is applied to both ends of a metal rod, an electric field is generated within the metal, causing an electric current to flow (electrical conduction). When the temperature of an object is non-uniform, a flow of heat occurs within the object (thermal conduction). This phenomenon in which physical quantities such as electricity and heat flow within an object is generally called a transport phenomenon. It does not include the movement of the substance itself, such as the flow of liquid.

There are many other transport phenomena besides electrical conduction and thermal conduction. If ink is spilled into water, the ink spreads and stains the entire water. In this way, if the concentration of a dissolved substance (solute) in a solution is not uniform, the solute will flow (diffusion). If the flow speed in a flowing gas or liquid is not uniform, a kind of frictional force acts between fast and slow regions (viscosity). This force slows down the faster flow speed and speeds up the slower flow speed, so we can say that a flow of momentum is occurring there.

A characteristic of transport phenomena is that the direction of flow is fixed. Heat flows from high temperature regions to low temperature regions, and solutes flow from high concentration regions to low concentration regions. They do not flow in the opposite direction. When heat flows, the temperature difference decreases, and when solutes diffuse, the concentration difference decreases. A state in which the temperature and concentration are uniform and do not change further over time is called a thermal equilibrium state. If the state of a substance deviates from this thermal equilibrium, the flow occurs in the direction to restore it. This type of change is called an irreversible change, in the sense that it cannot occur in the opposite direction. Transport phenomena are typical examples of irreversible changes.

In order to create a "flow" in a material, some kind of "force" such as an electric field or temperature gradient must be applied. When the "force" is weak, the "flow" is also small. In such cases, the "flow" can be thought of as proportional to the "force." For example, if the current that flows when the electric field is E is I , then the relationship I = σ E holds. Also, if the heat flow that flows when the temperature gradient is dT / dx is J , then the relationship J = κ( dT / dx ) holds. The proportionality coefficients σ and κ are constants that depend on conditions such as the material and temperature, with σ being called electrical conductivity and κ being called thermal conductivity. In general, the proportionality coefficient that links "flow" and "force" is called the transport coefficient.

[Yosuke Nagaoka]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

金属の棒の両端に電圧をかけると、金属の中に電場が生じ、電流が流れる（電気伝導）。物体の温度が不均一なとき、物体中に熱の流れが生じる（熱伝導）。このように物体中を電気、熱などの物理量が流れる現象を、一般に輸送現象という。液体の流れのような、物質そのものの運動は含まない。

　輸送現象には、電気伝導や熱伝導のほかにもいろいろある。水の中にインクをこぼすと、インクは広がって水全体を染めるようになる。このように、溶液の中で溶けている物質（溶質）の濃度が不均一であれば、溶質の流れがおこる（拡散）。流れている気体や液体中で流れの速さが一様でないと、速い領域と遅い領域との間に一種の摩擦力が働く（粘性）。この力は速いほうの流速を遅くし、遅いほうの流速を速くするから、そこに運動量の流れがおきているとみてよい。

　輸送現象の特徴は、流れの向きが定まっていることである。熱の流れは高温の領域から低温の領域へ向けておこり、溶質は高濃度の領域から低濃度の領域へ向かって流れる。逆向きには流れない。熱が流れると温度差は減り、溶質が拡散すると濃度差が減る。温度や濃度が均一で、それ以上時間的に変化しない状態を、熱平衡状態という。物質の状態がこの熱平衡から外れると、流れはそれを回復する向きにおこるのである。このような変化を、逆向きにはおこりえないという意味で、不可逆変化という。輸送現象は代表的な不可逆変化である。

　物質中に「流れ」を生じさせるためには、電場、温度勾配(こうばい)のような、ある種の「力」を加えなければならない。「力」が弱いときには「流れ」も小さい。このような場合、「流れ」は「力」に比例すると考えてよい。たとえば、電場がEのとき流れる電流をIとすれば、I=σEの関係が成り立つ。また、温度勾配がdT/dxのとき流れる熱流をJとすれば、J=κ(dT/dx)の関係が成り立つ。比例係数σ、κは物質や温度などの条件による定数で、σを電気伝導度、κを熱伝導度という。一般に、「流れ」と「力」を結ぶ比例係数を輸送係数という。

［長岡洋介］

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