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理解热力学第二定律

Understanding Second Law of Thermodynamics !

热力学第二定律是自然界的一个基本定律
The second law of thermodynamics is a fundamental law of nature,
毫无疑问是人类最有价值的发现之一
unarguably one of the most valuable discoveries of mankind.
然而 对大多数工程师或学生来讲
However, this law is slightly confusing
这条定律有点难懂
for most engineers or students.
难懂的主要原因是
The main reason for this is
这条定律里面有太多的复杂术语
because it has so many complex terms in it,
还有很多种不同的陈述
and that there are many ways that this second law
但最重要的一点是
can be stated, but most importantly,
大多数人不理解
the majority do not understand
这条定律有什么应用
what are the applications of this law.
在这个视频里 我们将会
In this video we will create a real,
用真实的物理学视角剖析该定律
physical insight into this law,
并且尽可能少涉及数学的东西
with a minimum use of mathematics.
最重要的问题是
The million dollar question is,
这个定律有什么用
for what purpose is this law used.
热力学第二定律的主要用途之一
One of the main uses of the second law of thermodynamics
是去决定一个过程是不是自发进行的
is to determine whether a process is spontaneous or not.
我们来考虑几个例子
Let’s consider a few examples.
两种气体混合在一起 气球正在漏气
Here two gases are mixing together, air is being leaked
一个物体正在下落
from a balloon, and the mass is falling down,
一杯热茶正在变凉
and the hot tea is losing its heat.
这些例子中系统从一个状态变为
You can see that the process of moving from state 1
另一个状态的过程是自发进行的
to state 2, will happen spontaneously,
也就意味着不需要任何外部帮助
which means without any external aid.
但是如果过程反过来
But what about the opposite process?
反向过程可以自发进行么?
Will that happen spontaneously?
混合气体可以自发地
Would this mixed gas become unmixed
不需要任何外部帮助分开吗?
spontaneously without any external aid?
根据你自己的经验
From your own experience,
你知道这是不可能的
you know this will not happen.
所以这个反向过程不是自发的
So this process is not spontaneous.
然而 根据能量守恒定律
However, according to the rules of energy conservation,
即热力学第一定律
or the first law of thermodynamics,
即使是反向过程也是有可能的
even the reverse process is possible.
因为两个状态的能量是相同的
because in both states, the energy is the same.
所以这里缺少了什么?
So what is missing here?
一定有另外一条定律
There must be one more law
控制着过程进行的方向
which governs the direction of a process,
这就是热力学第二定律
and that law is second law of thermodynamics.
现在 或许你脑中有个问题
Now probably you have a question in your mind.
我真的需要一条定律
Do I really require a law just to
只是用来预测过程进行的方向吗
predict the direction of a process?
我可以根据我的直觉来预测所有
I can predict the direction of all these processes
过程的进行方向
just from my intuition.
如果你真的有这样的疑问
If you do have such questions in your mind,
就让我们来分析下一个例子
let us analyze one more example.
这是个化学反应的例子
This is a chemical reaction.
我把两种化学物质放到了一个容器里
Here I am putting two chemicals together in a chamber,
等待化学反应发生
and I am waiting for a reaction to happen.
我想看一下这两个蓝色原子能否
I want to check if two blue atoms react
与一个黄色原子反应 生成新的分子
with one yellow atom to form a new molecule.
你知道答案吗?
Do you have an answer for this?
这可不能通过直觉来预测
This cannot be predicted through intuition.
这就是热力学第二定律的用处所在了
This is exactly what the second law is used for.
在这个视频里 我们要回顾下第二定律
In this video, we will review the second law.
我们会理解好它
We will learn it well,
现在回到这个问题上
and we will come back to this same problem,
再来看下这个化学反应的例子
to this chemical reaction.
这是热力学第二定律的两种标准定义
Here are the two standard definitions of the second law
你可能已经听说过了
You may have already heard of them
这两种说法含义一致
Both these statements mean the same
但是它们的说法对工程师来讲没法直接用
but they are not in the state directly useful to engineers.
这里有一个第二定律的有用形式
Here is a useful form of the second law,
这对工程师来讲有用
that is useful for engineers.
即克劳不等式
The Clausius Inequality.
这个积分式虽然简单
This is a small integral equation,
但是有深刻含义
but with deep inner meanings
请不要被这个式子吓住
please don’t caryatid about this equation
我们会借助一个物理实验来理解它
We will conduct a physical experiment to understand it.
克劳不等式的意思是
the Clausius inequality means that
如果你进行一个循环过程 比如在冰箱里
if you take a cyclic process, such as in a refrigerator,
所有在边界处发生的热量转换过程
and that all the heating action happening at the boundary
除以边界处的温度
divided by the temperature of the boundary
结果值会小于或等于零
the resulting value will be less than, or equal to zero
这是个有趣的不等式
This is an interesting inequality,
对所有的循环过程都成立
which is true for all cyclic processes.
为了让这个式子更面向应用
To make this equation more application oriented,
我们引入一个新术语 即著名的“熵”
let us introduce a new term: the famous term of entropy.
熵有两部分组成 第一部分表示混乱度
Entropy has two parts, one to represent disorder,
另一部分描述热量转换的影响
and the other to describe the heat transfer effect.
这里辟一下谣
Here’s a common mythbuster.
熵不仅仅指混乱度
Entropy is not only disorder, but it has
它还包括另一部分 即热量转换
one more part within it, that of heat transfer.
简而言之 一个过程的熵变可以定义为
In short, entropy change of process can be defined as
这个过程中产生和转换的熵之和
the sum of the change in the entropy production and entropy transfer.
对于一个完全可逆的过程来说
For a perfectly reversible process,
当这个过程中没有摩擦和混合
when the process has no friction and mixing,
那么它产生的熵等于零
the entropy production becomes zero.
如果你采用如上的熵的定义
If you use this definition of entropy
以及克劳不等式
and the Clausius inequality,
我们可以在数学上证明
We can prove mathematically that
在自发过程中
during this spontaneous process,
体系的熵始终是增加的
the entropy of the universe always increases.
这被称为“熵增原理”
This is known as the “increase in entropy principle”.
这是第二定律的一个很有用的形式
This is a very useful form of the second law.
现在我们举个实际的例子
Now let’s take a practical case
来更好地理解熵增原理
to understand this principle better.
考虑这个热茶问题
Consider this hot tea problem.
我们想要知道这杯热茶
We want to find out whether the hot tea
会吸热还是放热
will absorb or release heat.
这杯茶是个热力学系统
The tea is the system,
除此之外的其他东西是环境
and everything except the tea is the surroundings.
假设这杯茶是要吸收10焦耳的热量
Assuming the tea is absorbing the heat, a heat of 10J,
那么这杯茶的熵的变化
so entropy change of hot tea
等于10除以这杯热茶的温度
is 10 divided by temperature of the hot tea.
由此周围环境损失了相同数量的热量
The same heat amount is lost by the surroundings,
那么环境熵的变化等于
so the entropy change of the surroundings is
负10除以环境的温度
minus 10 divided by the temperature of surroundings.
如果你把这两项加起来
If you add these two quantities,
就能得到整个体系的熵变
you will get the entropy change of the universe.
在这个例子中是一个负值
It’s a negtive quantity in this case.
而根据第二定律 这是不可能的
This is impossible due to the second law.
现在 假设这杯热茶是要放热
Now, assume the hot tea is losing heat.
那么Q在这里就等于负10
So Q will be minus 10 in this case.
可以看到体系的熵变是正值了
You can see that the entropy change of the universe
这个过程就是可能的
is positive here, and this is possible.
所以应用热力学第二定律
So using the second law of thermal dynamics.
我们就证明了这杯热茶
We are proved that the hot tea can only release heat,
只能放热 不能吸热
and it cannot absorb heat.
现在 我们回到之前的化学反应问题
Now let’s get back to the chemical reaction problem.
假设这个反应已经发生
Assume the reaction has happened,
并且系统的熵增加了ΔS
and the system’s entropy is increased by delta S.
系统还吸收了些热量
And it has absorbed some heat.
我们称之为ΔH
We call it enthalpy.
我们要计算的是体系的熵变
What we have to calculate is the entropy change of the universe.
这里我们已经有了系统的熵变了
We already have the entropy change of the system here.
那么环境熵的变化值是多少呢?
What will be the value of entropy change of the surroundings?
这里有个小线索
Here is a small clue for that.
如果这个系统吸收了些热量
If the system has absorbed some amount of heat,
那周围的环境一定丢失了相同数量的热量
the surroundings have lost the same amount of heat.
所以环境熵的变化值为
So the entropy change of the surroundings
负ΔH除以T
is negtive delta H divided by T.
因此 你就可以像这样很容易地
Thus you can easily represent the entropy change
表示出体系的熵变
of the universe like this.
如果这个数量大于等于零
If this quantity is greater than zero,
那这个反应就是可以发生的
then this reaction is feasible.
现在 我们来改写下这个式子
Now, let’s have a rearrangement of this equation.
因为热力学温度T总是正值
Since the temperature T is always posotive,
如果把这个式子乘以负T
if I multiply it by minus T,
那这个不等式会变成这样
the inequality equation will become like this.
当左边的项小于等于零时
When this term is less than or equal to zero,
这个过程是可以发生的
this process is possible.
这个新项叫做“吉布斯自由能”
We call this new term “Gibbs free energy”.
或简单地说 系统吉布斯自由能的
Or in simple words, the change of Gibbs free energy
变化如果小于等于零
of the system is less than or equal to zero,
那该过程就是可以发生的
then that process is possible.
这就是用吉布斯自由能描述的好处
This is the advantage of using Gibbs free energy.
它不像“熵增原理”
Unlike the “increase in entropy ptinciple”, you need not
你无需担心环境发生了什么
to worry about what is happening in the surroundings.
你只需要将研究重点放在系统上
You can concentrate your study only on the system.
这样 你就可以预测一个过程
In this way, you can predict whether a process
是自发的还是非自发的
is spontaneous or not.
我们希望通过这个视频
We hope from this video you have developed good
让你对热力学第二定律有个深入的认识
insight into the second law of thermal dynamics.
感谢观看!
Thank you for watching the video.

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视频概述

本视频用借助几个物理学中的实例,用简洁的语言介绍了热力学第二定律的定义,以及其最主要的应用,即判断一个过程能否自发进行,使观众对于热力学第二定律有更为深入的认识。

听录译者

收集自网络

翻译译者

Imagist

审核员

审核员_DB

视频来源

https://www.youtube.com/watch?v=WTtxlaeC9PY

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