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Showing posts from July, 2020


根据热力学第二定律的开尔文表述,不可能从单一热源吸热并全部用来做功而不引起其他变化,从这个角度完全可以说明为什么不能以海水为单一热源,从它获取源源不断的能量并用来做功。到这里似乎问题就已经结束了,但是如果海水并不是体系当中唯一的热源,除此之外还存在另外一个高温热源,如下图所示, 在这种情况下是否能实现通过让海水不断降温将热量传向高温热源从而源源不断的向外输出功?同理可以问类似的一个问题,能否将冰箱在制冷的过程中放出的热量收集起来然后带动热机对外做功,或者干脆带动冰箱自己来实现下一轮的制冷过程,从而实现冰箱自己带动自己制冷进而无限循环下去而不需要外界提供任何能量?两个问题的答案显然都是,不能。如图所示,左半部分代表的是冰箱制冷的过程(也可以代表海水降温向外放热的过程),这里实际上就是热力学第二定律的克劳修斯表述,要想实现热量从低温热源传向高温热源,外界是必须要做功的(图中\(W_1\)),右半部分对应的是从高温热源收集热量再用来驱动热机对外做功的过程,这里对应的是热力学第二定律的开尔文表述,在驱动热机做功的过程中,是必须要向低温热源(冰箱,或者海水)放出一定的热量的。如果想要实现上面所说的两个永动机,整个图示的过程综合起来最终要满足一个关系:\(W_2 > W_1\),因为只有这样才有有效的对外净输出。换句话说,如果这里提到的两个永动机能够对外输出的功还不够让低温热源降温所需要做的功(或者刚好能够提供低温热源降温所需要做的功),那么这个时候此类永动机根本不是“永动”的(因为它无法向外界输出有用功)。针对\(W_1=W_2\)的情况需要一点特殊说明,这个时候,上面提到的两个永动机理论上是可以“永动”的,只是无法向外界输出功。但是实际上这也是不可能的,因为热机在运转的过程中总是存在各种能量损耗,如果\(W_1 = W_2\),那么体系是没有多余的输出来抵消哪怕是丝毫的损耗的,这时永动机也就无法“永动”了。这有点类似于说物体在水平方向不受外力的情况下可以在水平绝对光滑的平面上一直运动下去的,但是实际上绝对光滑的平面是不存在的。同理,没有损耗的热机也是不存在的,因而\(W_1=W_2\)也是无法保证永动机“永动”的。 要满足\(W_2 > W_1\),总体上看,如图所示,这就相当于从高温热源放出了\((Q_2^{'} - Q_2) - (Q_1^{'

Notes on temperature

📌 Introduction - What is temperature? By temperature (\(T\)), we could mean simply the 'hotness' of something, i. e. as always the case in our daily life, the higher \(T\) of an object, the hotter it is. But in physics, when we say temperature, we could also mean a parameter that is shared between systems when they reach equilibrium, in terms of heat flow. Quite often, these two specific meanings of temperature agree with each other. For example, in terms of heat flow, the natural  (by which we mean no extra work needs to be done for it to happen, naturally ) direction of heat flow will always be from hotter object (thus with higher \(T\)) to colder (thus with lower \(T\)) one. As heat flows out of the hotter object, its temperature (now, we mean 'temperature' by its first definition as given above) will decrease, and vice versa for the colder object. This process will continue until these two objects in question reach equilibrium ‒ they now share the same temperature

On occupational short-range ordering in crystal

When multiple atomic species (including vacancies) coexist on the same crystallographic site, it immediately brings up a question - say, we have species A and B sharing the same site, then do we have clustering of A and B in separate domains, or do we have A and B preferring to stay together, or do we have random distribution of A and B? This is usually what we mean by short-range order (SRO) and what we have mentioned here is specifically the occupational SRO (one can find introduction about more types of SRO in Chapter-10 in Ref. [1]). Detailed theoretical description about SRO can be found in Refs. [1-3] - the early paper by J. M. Cowley [2] gives the definition of SRO for binary systems; the one by D.De Fontaine [3] extends the definition to multi-component system (to give the so-called pairwise multi-component SRO, i. e. PM-SRO); the book by R. B. Neder and T. Proffen presents in details practical implementation and calculation of SRO, in DISCUS framework. Here I am not going to

Sublime Text 3 - Key binding configuration issue

For many packages installed in Sublime Text 3, either we can configure the key bindings for executing available commands by going to 'Preferences' →'Key Bindings', or we can go to the package specific key binding setting section. However, sometimes, we don't have a dedicated key binding setting section for some of the installed package, e. g. 'Google Search' package. In this case, it becomes a headache to set the key binding since we don't even know what commands are actually available there. To get an idea about what commands are available for such types of packages, we can go to '%APPDATA%\Sublime Text 3\Installed Packages' directory (taking Windows OS as an example) and find the package file, e. g. 'Google Search.sublime-package'. This file is just a zip file which contains all the necessary stuff of the installed package. Therefore, we can copy it to somewhere and change the file name extension to '.zip' and open the package w