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Second law:  This law is also known as the "Law of Increasing Entropy". A French physicist named Nicolas Léonard Sadi Carnot, also known as the "father of thermodynamics", basically laid down the Second Law of Thermodynamics. However, according to his statement, he emphasized the use of caloric theory for the definition of the law. Caloric (fluid that propels itself) is related to heat, and Carnot observed that some calories are lost in the cycle of motion.

All naturally occurring processes proceed in the direction of increased entropy. Entropy, in its most general sense, is a measure of disorder. For example, this law states that heat flows from hot objects to cold objects. The entropy of the universe only increases and never decreases. Many people take this statement lightly, but it has far-reaching implications and consequences.

If a room is not organized or cleaned, it always gets more messy and untidy over time. When the room is cleaned, its entropy decreases, but the effort to clean it causes the entropy outside the room to increase, exceeding the entropy lost.

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The second law of thermodynamics imposes restrictions on the direction of heat transfer and the achievable efficiency of heat engines. The first law of thermodynamics states that the energy of the universe remains constant but cannot be created or destroyed, although energy can be exchanged between the system and the environment.

thermodynamics  first law  While it gives information about the amount of energy transfer as a process, it does not give any idea about the direction of energy transfer and the quality of energy. The first law cannot show whether a metallic rod of uniform temperature will spontaneously be hotter at one end and colder at the other. All the law can say is that there will always be energy balance if the process takes place. It is the second law of thermodynamics that provides the criterion for the feasibility of any operation. A process cannot occur unless it satisfies both the first and second laws of thermodynamics.

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The second law of thermodynamics states that any spontaneous process always takes place in the universe.  indicates that it will lead to an increase in entropy (S).  In simple words, the law explains that the entropy of an isolated system will never decrease with time.

However, in some cases when the system is in thermodynamic equilibrium or undergoing a reversible process, the total entropy of the system and its surroundings remains constant.

The second law clearly states that it is impossible to convert heat energy into mechanical energy with 100 percent efficiency. For example, if we look at a piston in an engine, the gas is heated to increase its pressure and drive a piston. However, even if the piston moves, there is always some residual heat in the gas that cannot be used to do any other work. The heat is wasted and must be discarded. In this case it is done by transferring to a cooler, or in the case of a car engine, waste heat is removed by expelling the spent fuel and air mixture to the atmosphere. In addition, heat from friction, which is often unusable, must also be removed from the system.

Mathematically, the second law of thermodynamics is represented as:

ΔWater > 0

ΔWater: is the change in the entropy of the universe.

Entropy is a measure of the randomness of the system, or a measure of the energy or chaos within an isolated system. It can be thought of as a quantitative index describing the quality of energy.

Meanwhile, there are several factors that cause an increase in the entropy of the closed system. First, in a closed system, heat is exchanged with the surroundings while the mass remains constant. This change in heat content creates a disturbance in the system and thus increases the entropy of the system.

Second, internal changes can occur in the movements of the molecules of the system. This leads to disturbances that cause irreversibility within the system and result in increased entropy.

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There are two statements about the second law of thermodynamics: 

1. Kelvin-Plank Description

2. Clausius Explanation

Both Clausius's and Kelvin's statements are equivalent, so a device that violates Clausius' statement will also violate Kelvin's statement and vice versa.

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Kelvin-Plank Description

If a heat engine only exchanges heat with bodies of one constant temperature,

it is impossible for it to form a network in a complete cycle.

Exceptions:

If Q 2 =0 (ie W net = Q 1 or efficiency=1.00)

The heat engine produces work in a full cycle by exchanging heat with only one reservoir.

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Clausius Explanation

It is impossible to make a device that operates on a cycle that can transfer heat from a colder object to a warmer one without spending any work. Also, energy does not spontaneously flow from an object at a lower temperature to an object at a higher temperature. It is important to note that we are talking about net energy transfer. Energy transfer can occur from a cold object to a hot object by the transfer of energetic particles or electromagnetic radiation. However, there will be net transfer from the hot object to the cold object in any spontaneous process. And some kind of work is required to transfer the net energy to the hot body. In other words, the refrigerator will not operate unless the compressor is powered by an external source.

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Third law: As the temperature approaches absolute zero, the entropy approaches zero. Absolute zero temperature corresponds to -273.15 degrees Celsius on the Celsius scale and 0 degrees on the Kelvin scale. 

This law was developed by the German chemist Walther Nernst between 1906 and 1912.

A pure crystalline substance (perfect order) has zero entropy at absolute zero temperature. This statement is valid if the perfect crystal has a single state with minimum energy.

Let's take steam as an example to understand the third law of thermodynamics step by step:

1. The molecules in it move freely and have high entropy.

2. If the temperature is lowered below 100 °C, the steam turns into water where the movement of molecules is restricted, reducing the entropy of the water.

3. When water is cooled below 0 °C, it turns into solid ice. In this case, the movement of molecules is further restricted and the entropy of the system decreases further.

4. As the temperature of the ice decreases further, the movement of the molecules in it is further restricted and the entropy of the matter continues to decrease.

5. Ideally, entropy should be zero when ice is cooled to absolute zero. But in reality, it is impossible to cool any substance to zero.

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Entropy, denoted by 'S', is a measure of disorder/randomness in a closed system. It is directly related to the number of microstates accessible by the system (a fixed microscopic state that can be occupied by a system), ie the more microstates the closed system can occupy, the greater its entropy. The microstate where the energy of the system is at its minimum is called the ground state of the system.

At zero Kelvin, the following phenomena can be observed in a closed system:

• The system does not contain any heat.

• All atoms and molecules in the system are at their lowest energy points.

Therefore, a system at absolute zero has only one accessible microstate - this is the ground state. According to the third law of thermodynamics, such a system  entropy  is exactly zero.

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