The second law of thermodynamics

Introduction

Thermodynamics is a branch of physics  that explains the relationship between heat, work, temperature, and energy. 

The main principle of thermodynamics is that heat is a form of energy that can be utilized to get a definite amount of mechanical work done.

A thermodynamic setup has mainly 3 things:

1.System: A system is defined as the space chosen to study.

• Closed System: Both heat and matter exchange occurs through the boundary.
• Open system: the only heat exchange takes place through the boundary.
• Isolated system: a thermally insulated system, with no exchange of heat in the surrounding. Though, the work is done within the system.

2. Surrounding: The space surrounding the system.

3. Boundary: A boundary can be considered a “wall” that allows in exchange of heat and matter between the system and the surrounding.

important equilibriums are observed in a thermodynamic system

1. Thermal equilibrium: A system is said to be in thermal equilibrium when the temperature is constant in the system, and no flow of heat takes place within the system.
2. Mechanical equilibrium: A system is said to be in mechanical equilibrium when no external forces are acting on the system.
3. Chemical equilibrium: A system is said to be in chemical equilibrium when no chemical reaction is taking place in the system.

There are four essential laws in thermodynamics:

Zeroth law of thermodynamics: It explains the concept of thermal equilibrium and temperature. When 2 objects are both in contact, they attain the same temperature.

The first law of thermodynamics: corresponds to the law of energy, stating that an energy form can neither be created nor be destroyed but only can transfer from one form to another.

It explains heat and mechanical work.

The second law of thermodynamics states that the disorderliness(entropy) in a system or universe will never decrease and reach a negative point.

The Third law of thermodynamics: states that the entropy of a system reaches a constant value when the temperature of the system is absolute zero.

To understand the concept of thermodynamics, there are a few terminologies to be understood:

State variables: these variables can be measured directly. They explain the captivity of the system. The state variables of a gas system are temperature (T), pressure, volume, entropy, internal energy, and concentration.

There are 2 types of variables : 

1. Extensive – dependent on the size of the thermodynamic system
2. Intensive – Independent on the size of the thermodynamic system.

To define the state of any system, we need at least two state variables.

State functions: state functions can be defined as dependent on state variables.

The second law of thermodynamics: 

The second law of thermodynamics explains heat energy’s direction, quality, and quantity. 

We need to convert heat to mechanical energy to operate vehicles and convert heat to mechanical energy as any thermal system would lose heat in the surroundings.

The second law gives a gist of how naturally a system evolves with time in one direction and not the reverse. i.e., the flow of heat can be in one direction only that is higher to lower.

It also provides significant points on how energy transfer and conversion processes occur.

Heat engines:

It can be defined as the device that converts heat energy to mechanical energy or works in a cyclic process.

Examples: steam engine, automobile engine.

 In a cyclic process, the substance in the system must return to its initial stage for the process to occur once again.

The internal energy for a cyclic process is zero as it returns to its initial stage.

Mechanism : 

A heat engine absorbs the heat from a high-temperature heat reservoir. Then the absorbed heat is converted to work (W). then the remaining heat goes to the low-temperature reservoir.

The second law of thermodynamics clearly states that not all absorbed energy is converted into work. This principle was distinctively stated in Kevin – Planck’s statement.

It is cited that “ it is impossible to construct a particular device that runs in a cyclic process to completely convert all of the heat absorbed from the reservoir to mechanical energy completely.”

Efficiency :

The efficiency of a heat engine is the total work output by the total amount of heat input.

The work done is calculated by the principles of the 1st law of thermodynamics principles.

Efficiency = 1 – ( heat released / heat absorbed).

Heat pump : 

It is a type of heat engine that does not require work input to transfer the heat from a cold reservoir to a hot reservoir.

Heat pumps are widely used in buildings in cold countries. Heat pumps are significantly used in refrigerators and air conditioners.

The second law gives us the information that this transfer of heat from a reservoir without work input is not possible.

The Clausius statement of the second law of thermodynamics states that “ No cyclic transformations can transfer heat from a low-temperature reservoir to high-temperature reservoir without mechanical work”.

As we know, the flow of heat is in one direction for the reverse to take place, an outside force or energy is required in the form of mechanical work.

The working process of the heat pump is explained by an effectiveness called the “coefficient of performance “.

COP = heat transferred to high-temperature reservoir/work input

The heat pumps are more effective at a higher temperature surroundings than at a low temperature, as the amount of mechanical input required increases as the temperature goes down.

The Carnot’s Engine :

The Carnot engine involves the Carnot cycle that is a reversible cyclic process closed by two isothermal processes and two adiabatic processes.

The engine absorbs heat from the high-temperature reservoir, it should be made sure that the substance that we are working upon also must be of the same temperature that is there should be no loss of heat. In the same way, when the unused heat is transferred to the low-temperature reservoir, the substance that we are working upon should be maintained at the same temperature.

Therefore, every process taking place should be isothermal, that is, the temperature must be kept constant. When the temperature of the substance is in between the temperature of the two reservoirs, then an adiabatic process takes place, where no heat transfer takes place.All 4 processes of carnot cycle are reversible in nature. 

 The Carnot’s cycle: 

It has 4 consecutive steps :

1. First process

• Isothermal expansion takes place
• No change in temperature.
• Volume changes from V1 to V2

2. Second process

• Adiabatic expansion takes place 
• Decrease in temperature.
• Volume changes from V2 to V3

3. Third process

• Isothermal Compression takes place.
• Decrease in temperature 
• Volume changes from V3 to V4

4. Fourth process

• Adiabatic compression takes place.
• The temperature rises low to high 
• Volume changes from V4 to V1.

Efficiency: 

η = performed work / absorbed heat 

η = W /Q1 = (Q1 + Q2) /Q1

η  = (Q1 − |Q2|) /Q1

η  = 1 –  Q2/Q1

Here Q1  = absorbed heat and Q2  = released heat

 η = 100% if there is no amount of heat wasted, and this condition is not possible for any working device.

Carnot’s theorem :

It is a principle obtained from the result of the 2nd law of thermodynamics:

1. Any heat engine working between two hot reservoirs is less efficient than the Carnot’s engine working at the same two temperatures.
2. The same efficiency is observed for any reversible heat engine working between the same two reservoirs.

Conclusion 

Thermodynamics being the study of heat, work, and energy, play an important role in the field of science. As it is also part of our biological system where the exchange of heat takes place between our system and surrounding. The energy created in our body for all other purposes also involves the principle of thermodynamics.

Therefore the application of thermodynamics is vast. This article highlights the laws of thermodynamics and mainly the second law of thermodynamics, heat engines, heat pumps, and Carnot’s engine.