Thermodynamics Laws: Friend Or Foe Of Machines?

The laws of thermodynamics are a set of scientific laws that define a group of physical quantities, such as temperature, energy, and entropy, that characterise thermodynamic systems. These laws are the foundation of heat transfer and energy work and are applied by engineers when designing or implementing a system. The laws of thermodynamics are important fundamental laws of physics and are applicable in other natural sciences.

The laws of thermodynamics apply to machines and govern the work done by them. For example, the first law of thermodynamics, also known as the law of conservation of energy, states that energy can neither be created nor destroyed. This means that energy is conserved and can be converted from one form to another. This law applies to machines such as refrigerators, which take heat from a low-temperature reservoir and pump it into a high-temperature reservoir.

The second law of thermodynamics states that the entropy of the universe tends to a maximum and that heat does not flow spontaneously from a colder region to a hotter region. This law helps to explain why some machines, such as refrigerators, require a source of work to function.

The third law of thermodynamics states that the entropy of a system approaches a constant value as the temperature of the system approaches absolute zero. This law reveals that refrigeration techniques must fail as absolute zero is approached.

Therefore, the laws of thermodynamics apply to machines and govern the work they can do.

The First Law of Thermodynamics

> ΔU = Q - W

Where:

• ΔU is the change in the internal energy of the system
• Q is the heat added to the system from its surroundings
• W is the work done by the system on its surroundings

The Second Law of Thermodynamics

A simple statement of the law is that heat always flows spontaneously from hotter to colder regions of matter. In other words, heat does not flow spontaneously from a colder region to a hotter region. This is equivalent to saying that "all closed systems tend toward an equilibrium state in which entropy is at a maximum and no energy is available to do useful work'.

The second law can be expressed in many ways, including:

• "It is impossible for a self-acting machine, unaided by any external agency, to convey heat from one body to another at a higher temperature" (Lord Kelvin)
• "It is impossible to construct an engine which will work in a complete cycle, and produce no effect except the production of work and cooling of a heat reservoir" (Planck)
• "In every neighbourhood of any state S of an adiabatically enclosed system there are states inaccessible from S" (Constantin Carathéodory)

The Third Law of Thermodynamics

At absolute zero, the system must be in a state with the minimum possible energy. Entropy is related to the number of accessible microstates, and there is typically one unique state, known as the ground state, with minimum energy. In such cases, the entropy at absolute zero is exactly zero.

However, if the system does not have a well-defined order, it may retain some finite entropy at very low temperatures. This can occur if the system becomes locked into a configuration with non-minimal energy or if the minimum energy state is non-unique.

The third law was formulated by Walther Nernst between 1906 and 1912 and is sometimes referred to as the Nernst heat theorem or the Nernst-Simon heat theorem, acknowledging the contribution of Nernst's doctoral student, Francis Simon.

The third law has several alternative formulations, including:

• The Nernst statement: It is impossible for any process to reduce the entropy of a system to its absolute-zero value in a finite number of operations.
• The statement in adiabatic accessibility: It is impossible to start from a state of positive temperature and adiabatically reach a state with zero temperature.
• The Einstein statement: The entropy of any substance approaches a finite value as the temperature approaches absolute zero.
• The Nernst-Simon statement: For a condensed system undergoing a reversible isothermal process, the associated entropy change approaches zero as the temperature approaches zero.

The third law has important implications. It implies that it is impossible to reach absolute zero temperature, as the further a system gets to this temperature, the more difficult it becomes to extract energy from it. It also provides an absolute reference point for determining the entropy of a system at any other temperature.

Perpetual Motion Machines

Perpetual motion is the motion of bodies that continue forever in an unperturbed system. A perpetual motion machine is a hypothetical machine that can perform work indefinitely without an external energy source. This kind of machine is impossible as its existence would violate the first and/or second laws of thermodynamics.

The first law of thermodynamics, also known as the law of conservation of energy, states that energy within a closed system remains constant. Energy can be transformed from one form to another but can be neither created nor destroyed. A perpetual motion machine would have to produce more energy than it takes to operate, which is impossible.

The second law of thermodynamics, as defined by Scottish physicist William Thomson, states that "a cyclic transformation whose only final result is to transfer heat extracted from a source that is at the same temperature throughout into work is impossible." In other words, natural processes in a closed system always cause some energy loss in the form of heat.

The history of perpetual motion machines dates back to the Middle Ages. For millennia, it was unclear whether perpetual motion devices were possible until the development of modern theories of thermodynamics showed they were impossible. Many attempts have been made to create such machines, continuing into modern times.

There are three types of perpetual motion machines, each corresponding to a particular law of thermodynamics that they purport to violate:

• A perpetual motion machine of the first kind produces work without the input of energy, violating the first law of thermodynamics.
• A perpetual motion machine of the second kind spontaneously converts thermal energy into mechanical work, violating the second law of thermodynamics.
• A perpetual motion machine of the third kind is defined as one that eliminates friction and other dissipative forces, allowing motion to be maintained forever due to its mass inertia. This machine is also impossible, as dissipation cannot be eliminated in a mechanical system.

Heat Transfer and Energy Work

The laws of thermodynamics are a set of scientific laws that define a group of physical quantities, such as temperature, energy, and entropy, which characterise thermodynamic systems in thermodynamic equilibrium. The laws of thermodynamics are the foundation of heat transfer and energy work.

The first law of thermodynamics, also known as the law of conservation of energy, states that energy can neither be created nor destroyed. In other words, energy is conserved. This means that the total energy of an isolated system is constant, and energy can only be converted from one form to another. For example, internal energy can be converted to potential energy (stored in the system) or kinetic energy (for state changes).

The second law of thermodynamics establishes the concept of entropy as a physical property of a thermodynamic system. Entropy is defined as the tendency for natural systems to break down and for energy in a system to be dissipated. It is a measure of disorder and randomness in a system. The second law states that the entropy of the universe tends to a maximum, and that heat does not flow spontaneously from a colder region to a hotter region.

The third law of thermodynamics states that the entropy of a system approaches a constant value as the temperature of the system approaches absolute zero. This implies that it is impossible to attain absolute zero, as it becomes increasingly difficult to extract energy from the system as its temperature decreases.

The zeroth law of thermodynamics was developed after the first three laws and is a basic concept that states that if two thermodynamic systems are equal to a third thermodynamic system, then the first two systems are equal to each other. This law is necessary to define thermodynamic equilibrium and temperature.

These laws of thermodynamics are applicable to all physical and biological systems and provide a complete description of all changes in the energy state of any system and its ability to perform useful work on its surroundings.

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