[Physics] What does the second law of thermodynamics really mean

Question

I started reading about entropy and the second law of thermodynamics.

Different sites give different definitions and meanings of this law. A few of them:

1. Disorder always increases

2. heat always flows from a hot to a cold object

3. Unusable energy of a system always increases.

In the first one,what is really meant by disorder? What is the true meaning of disorder? Does disorder depend upon the observation of a conscious mind, or does it have a meaning even without an observer?

Also, are all these three definitions actually interconnected?

The third definition made the most sense to me;but since it is a law,can it actually be proven or is it just accepted because of experimental evidence?Can human find ways to utilize this unusable energy in an efficient way and still not increase the entropy of the system? And what does "unusable" energy mean?Aren't usable and unusable energy only significant for humans?Why does the universe even care if energy can be used or not?

I have just started understanding this topic.An answer without maths and one which gives conceptual clarity would be appreciated.

Answer 1

The second law of thermodynamics states, that the total entropy of an isolated system never decreases with time. There are a couple of details in this sentence, which are worth pointing out:

• You can decrease the entropy locally, but not globally: E.g. consider a refrigerator. Here we decrease the temperature locally (=inside the refrigerator), which leads to a local decrease in entropy. However, the entropy of the complete system is increased, by transferring heat from the colder to the warmer place. Hence the words total (entropy), and isolated (system) are important.
• It's possible to have a process which keeps the total entropy constant. Therefore, the entropy does not have to strictly increase with time. Therefore, in the second law "never decreases with time" is often replaced by "always increases with time". However, none physicists often mistake increasing with strictly increasing.

• As entropy is linked to disorder by the law of Statistical Physics (see below), the first statement of yours

  Disorder always increases

  is equivalent to the upper definition.

• Your second definition

  heat always flows from a hot to a cold object

  is a consequence of the definition given above (the idea is presented below). By doing the calculation we find, that the disorder increases, if heat is transferred from the warm to the cold and that the heat transfer stops in thermal equilibrium.

• Your third definition

  Unusable energy of a system always increases.

  is as well a restatement of the upper definition. However, one has to ask, how we measure the "usability" of energy. This would lead to the entropy.

In Stat. Physics entropy is defined as $$ S = k_B \cdot \ln{\Omega} $$ where $\Omega$ is the number of accessible micro-states (link to a video). In order to understand this concept think of a system with two compartments (left / right), which can exchange energy. Each compartment has 10 distinguishable particles in it and each particles has two states:

• The first state is the ground state. No energy is needed to place a particle in this state.
• The second state is an excited state. We need one unit of energy to place a particle in this state.

Now suppose, you are given 10 units of energy and the particles are not allowed to change the compartment. However, the energy is allowed to switch the compartment. What are possible configurations:

1. One config is that all particles in the left compartment are in the excited state. Then no energy units are left for the right compartment. Hence, all particles in the right compartment are in the ground state. So we have only one possible realization in which the 10 units of energy are in the left compartment. Hence, the number of accessible micro-states, where 10 units of energy are on the left and none is on the right, is 1.
2. An other configuration is where we have 9 energy units in the left compartment and 1 in the right. How many different micro-states do we have? Well, in the left compartment each particle could be the one which is in the ground state. Hence, here we have 9 different state. In the right compartment each of the particles could be in the excited state. Hence, again we have 9 possible states. In total we have $9\cdot 9 = 81$ different micro-states. Hence, the entropy of this state is much larger than the entropy of the state, where all 10 energy units are in the left compartment.
3. If you keep going like this, you will find, that the state where the number of accessible micro-states is the larges is "5 energy units are in the left compartment and 5 are in the right". This is the equilibrium state, where both compartments have the same "temperature".

The basic of this logic is the law of "a priori equal probability".