When an object (or system of objects) changes from one state to another, there is at least one way for both the object and the outside world to return to their original state. This is also called a reversible process. In , the coordinate axes x and y are in the horizontal plane, and the coordinate axis z is in the vertical (gravity) direction. Take two points P and Q in space, and consider any shape of path l 1 , l 2 , …, l n , … that connects these two points. When the position of a mass point changes from point P to point Q, no matter which path the change is made by, the potential energy of the mass point at point Q is greater by m g h ( m is the mass of the mass point, g is the gravitational constant, h is the difference between the z coordinate of Q and the z coordinate of P) than at point P. Conversely, when this mass point returns from Q to P, no matter which path it takes, the potential energy decreases by m g h . This means that the work done on the mass point by something (manpower, machine, etc.) that carried the mass point from P to Q against gravity on the way there (i.e. the increase in potential energy m g h ) is completely recovered on the way back. In other words, when the mass point moves from P to Q via any path, not only does the mass point return to its original state by returning from Q to P via any path, but the outside of the mass point can also return to exactly the same state as it was at the beginning. This type of change is called a reversible change. When the mass point returns to P, no matter which path it takes to go there and back, the outside has also returned to its original state. However, the original definition of a reversible change does not require "any path," but only "at least one path." In any case, there needs to be at least one way to return both the object and the outside to their original states.A change that is not reversible is called an irreversible change. Purely mechanical phenomena are reversible, and similarly purely electrical and magnetic phenomena are also reversible. However, all mechanical, electrical, and magnetic phenomena that actually occur are always accompanied by the generation of heat to a greater or lesser extent. Incidentally, the second law of thermodynamics stipulates that all actual heat transfer and generation is irreversible, so in the end, all natural phenomena are strictly irreversible, and reversible changes do not actually exist. However, just as we can better understand the nature of gases both macroscopically and microscopically by considering ideal gases (perfect gases) as the ideal limit of real gases, we can gain a deeper understanding of natural phenomena by considering reversible changes as the ideal limit of actual changes that occur. [Shozo Sawada] [Reference item] |To carry a mass point from P to higher Q against gravity, no matter what path it takes, work of is required. Conversely, when the mass point returns from Q to P, no matter what path it takes, work of is recovered. Such a purely mechanical phenomenon is a reversible change . Reversible change (Figure) Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend |
一つの物体(または物体系)が、一つの状態から他の状態へ変化するとき、物体、外部とも初めの状態へ戻る方法が少なくとも一つは存在するような変化のこと。可逆過程ともいう。 において、座標軸x、yを水平面内に、座標軸zを鉛直(重力)方向にとる。空間内に2点P、Qをとり、この2点間を結ぶ任意の形の道筋l1、l2、……、ln、……を考える。一つの質点の位置が点Pから点Qまで変化すると、その変化がどの道筋によって行われても、質点が点Qにおいてもつ位置エネルギーが点Pにおけるよりもmgh(mは質点の質量、gは重力の定数、hはQのz座標とPのz座標との差)だけ大きい。逆に、この質点がQからPへ帰るとき、どの道筋を通って帰っても、位置エネルギーはmghだけ減少する。このことは、往路で質点を重力に逆らってPからQまで運んだもの(人力、機械など)が質点になした仕事(つまり位置エネルギーの増加分mgh)は帰路において完全に回収されることを意味する。すなわち、質点の任意の道筋を通ってのPからQへの移動では、QからPへ任意の道筋を通って帰ることにより、質点自身が初めの状態に戻るだけでなく、質点の外部もまた初めとまったく同じ状態に戻ることができる。このような変化が可逆変化である。質点がPに戻ったとき、行きと戻りがどの道筋で行われても、外部もまた完全に元の状態に戻っている。しかし可逆変化の本来の定義では、「どの道筋ででも」の必要はなく、「少なくとも一つの道筋で」だけが必要である。とにかく物体、外部とも初めの状態に戻る方法が少なくとも一つあればよい。可逆でない変化を不可逆変化という。純粋な力学的現象は可逆的であり、同様に純粋な電気的、磁気的現象も可逆的である。しかし、実際におこる力学的、電気的、磁気的現象は、かならず大なり小なり熱の発生を伴う。ところで、熱力学第二法則は、現実的な熱の移動、発生はすべて不可逆であることを定めているから、結局、自然現象は厳密にはすべて不可逆であって、可逆変化は実際には存在しない。 しかし、実在する気体の理想的な極限として理想気体(完全気体)を考えることによって、気体の本性を巨視的にも微視的にもよく理解できると同様に、実際におこる変化の理想的極限として可逆変化を考えることによって、自然現象の理解をより深めることができる。 [沢田正三] [参照項目] |質量の質点を、Pの位置からさらに高いQまで、重力に逆らって運び上げるためには、、、……、、……のいかなる道筋をとろうとも、の仕事が必要となる。この逆に、質点がQからPに戻るとき、いかなる道筋をとろうとも、は回収される。このような純粋な力学的現象は可逆変化である©Shogakukan"> 可逆変化〔図〕 出典 小学館 日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について 情報 | 凡例 |
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