Applying Thermodynamics Laws: A Practical Guide To Energy And Entropy

The laws of thermodynamics are a set of scientific laws that define a group of physical quantities, such as temperature, energy and entropy, and how they behave under various circumstances. They also establish relationships between thermodynamic processes, such as thermodynamic work and heat. These laws are the result of progress made in the field over the nineteenth and early twentieth centuries. Traditionally, there are three fundamental laws of thermodynamics, but a fourth zeroth law was added later to allow for a self-consistent definition of temperature. The laws of thermodynamics are applicable to all physical and biological systems.

The First Law of Thermodynamics: Energy can be converted from one form to another but cannot be created or destroyed

The First Law of Thermodynamics states that energy cannot be created or destroyed; it can only be converted from one form to another. This law is derived from the law of conservation of energy, which states that the total energy of a system remains constant.

The First Law distinguishes between two principal forms of energy transfer: heat and thermodynamic work. These two forms of energy transfer modify a thermodynamic system containing a constant amount of matter. The law also defines the internal energy of a system, which is an extensive property that accounts for the balance of heat and work in the system.

The First Law can be expressed mathematically as:

ΔU = Q - W

Where:

• ΔU is the change in internal energy of the system
• Q is the net heat transferred into the system (the sum of all heat transfers into and out of the system)
• W is the net work done by the system (the sum of all work done on or by the system)

This equation shows that the change in internal energy of a system is equal to the net heat transfer into the system minus the net work done by the system.

The First Law of Thermodynamics has been experimentally validated numerous times and is considered a fundamental law of physics. It allows for the conversion of energy from one form to another but never permits the creation or destruction of energy during this process.

A practical application of the First Law is the heat engine, which converts thermal energy into mechanical energy and vice versa. Most heat engines fall into the category of open systems, where there is a free exchange of energy and matter with the surroundings. Examples of working fluids in heat engines include steam in a steam engine and hydrofluorocarbons in refrigeration systems.

In summary, the First Law of Thermodynamics states that energy can be converted from one form to another but cannot be created or destroyed. This law is based on the principle of conservation of energy and provides a framework for understanding energy transfers in thermodynamic systems.

The Second Law of Thermodynamics: The entropy of the universe increases over time

The second law of thermodynamics is one of the most fundamental laws of nature, and it states that the total entropy of a system either increases or remains constant in any spontaneous process; it never decreases. This law implies that the universe will eventually reach a state of maximum entropy, often called the "heat death" of the universe, where everything is at the same temperature, and no work can be done.

Entropy is a measure of the disorder of a system and is related to the unavailability of energy to do work. In other words, it measures how much energy in a system is not available to do work. The more disordered a system is, the higher its entropy, and the less of its energy is available to do work. For example, when a hot object is placed in a room, it quickly spreads heat energy in all directions, increasing the entropy of the room.

The second law of thermodynamics has several important implications. Firstly, it indicates that heat flows spontaneously from a hotter region to a cooler region but not the other way around. This is because entropy increases when heat is transferred from hot to cold. Secondly, it shows that many processes are irreversible, as they lead to an increase in disorder. For example, a pendulum will gradually lose energy and come to a stop, but it won't start moving again spontaneously.

The second law also has implications for the evolution of life. While complex organisms have evolved from simpler ancestors, resulting in a decrease in entropy, this does not violate the second law. This is because the total entropy of the universe, or the sum of the entropies of all interacting systems, always increases, even if the entropy of a local system decreases. For instance, energy from the Sun has decreased the entropy of local systems on Earth, but the overall entropy of the universe has increased by a greater amount due to the disorder created in the process.

The Third Law of Thermodynamics: A perfect crystal at absolute zero has zero entropy

The Third Law of Thermodynamics states that a perfect crystal at absolute zero (0 Kelvin) has zero entropy. This law was formulated by German chemist Walther Nernst between 1906 and 1912.

A perfect crystal is one with no impurities, that has achieved thermodynamic equilibrium, and is in a crystalline state where all atoms, ions, and molecules are in well-defined positions in a highly ordered lattice structure. This excludes amorphous solids like glass, which lack an ordered, crystalline structure and have not achieved thermodynamic equilibrium.

At absolute zero, a system is in a state with the minimum thermal energy, known as the ground state. In this state, the system's entropy is at a constant value, which is typically close to zero. This is because the system has only one accessible microstate, its ground state.

Mathematically, the Third Law can be expressed as:

> S – S0 = kB lnΩ

Where:

• S is the entropy of the system
• S0 is the initial entropy
• KB is the Boltzmann constant
• Ω refers to the total number of microstates consistent with the system's macroscopic configuration

For a perfect crystal with a unique ground state, Ω = 1. Therefore, the equation can be rewritten as:

> S – S0 = kB ln(1) = 0

When the initial entropy of the system is set to zero, the following value of S is obtained:

> S – 0 = 0 ⇒ S = 0

Thus, the Third Law of Thermodynamics states that the entropy of a perfect crystal at absolute zero is zero.

The Third Law provides a reference point for determining the absolute entropy of a substance at any temperature, which is useful for calculating the entropy change in a reaction. It also implies that it is impossible to reach absolute zero in a finite number of steps, as an infinite number of steps would be required to remove all heat from a system.

The Zeroth Law of Thermodynamics: If two systems are in thermal equilibrium with a third, they are in equilibrium with each other

The Zeroth Law of Thermodynamics is one of the four principal laws of thermodynamics. It was formulated by Ralph H. Fowler in the 1930s, long after the first, second, and third laws were established.

The Zeroth Law states that if two systems are in thermal equilibrium with a third system, the two original systems are also in thermal equilibrium with each other. For example, if system A is in thermal equilibrium with system C, and system B is also in thermal equilibrium with system C, then systems A and B are in thermal equilibrium with each other. This law is based on temperature measurement and provides an independent definition of temperature without referring to entropy, which is defined in the second law.

The Zeroth Law is important for the mathematical formulation of thermodynamics and is needed for the definition of temperature scales. It establishes thermal equilibrium as an equivalence relationship, allowing any member of a subset of systems to be uniquely "tagged" with a label identifying the subset to which it belongs. This property justifies the use of empirical temperature as a tagging system and the use of practical thermometers.

The Zeroth Law also establishes that temperature is a fundamental and measurable property of matter. It is a transitive property, meaning that if A=C and B=C, then A=B. This law also implies that heat will only flow from a region of high temperature to a region of low temperature.

The Zeroth Law is a prerequisite for the other laws of thermodynamics and allows for a self-consistent definition of temperature.

Applications: The laws of thermodynamics are applied in power plants and vehicle engines

The laws of thermodynamics are applied in power plants and vehicle engines. The First Law of Thermodynamics, based on the law of conservation of energy, states that energy cannot be created or destroyed, only altered in form. This law is used to categorise the performance of cyclic conversion systems, such as fossil-fired steam power cycles or geothermal cycles. For example, in a geothermal power plant, the First Law states that the electricity generated must be balanced by the energy extracted from the geothermal resource, minus any other energy uses and losses to the environment.

The Second Law of Thermodynamics is considered the most fundamental law of science. It explains how natural processes tend to lead towards spatial homogeneity of matter and energy, especially temperature. This law is applied to internal combustion engines in cars, motorcycles, ships, and airplanes. In these engines, heat is generated by the combustion of fuel, which takes place due to a spark (in gasoline engines) or fuel compression (in diesel engines). Some of the heat generated is used to move the piston inside the engine cylinder, and the rest is released into the atmosphere as exhaust gases. According to the Second Law, the higher the temperature of the source of heat and the lower the temperature of where the heat is released, the higher the efficiency of the engine.

The Third Law of Thermodynamics states that the entropy of a closed system at thermodynamic equilibrium approaches a constant value when its temperature approaches absolute zero. This law provides an absolute reference point for determining the entropy of any system at any temperature.

Frequently asked questions