Thermodynamics In Space: Do The Laws Apply?

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 in thermodynamic equilibrium. There are four laws of thermodynamics: the zeroth law, the first law, the second law, and the third law. The laws of thermodynamics are important fundamental laws of physics in general and are applicable in other natural sciences. But do they apply in space?

Do the laws of thermodynamics apply in space?

The laws of thermodynamics apply everywhere, including in space.

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 in thermodynamic equilibrium. They also use various parameters for thermodynamic processes, such as thermodynamic work and heat, and establish relationships between them.

The first law of thermodynamics states that the amount of energy in the universe is constant and can neither be destroyed nor created. It can, however, be transformed from one form to another. This is also known as the law of conservation of energy.

The second law of thermodynamics establishes the concept of entropy as a physical property of a thermodynamic system. It also predicts whether processes are forbidden despite obeying the requirement of conservation of energy as expressed in the first law of thermodynamics. It is concerned with the direction of natural processes and asserts that a natural process runs only in one sense and is not reversible.

The third law of thermodynamics states that a system's entropy approaches a constant value as the temperature approaches absolute zero.

The laws of thermodynamics are important fundamental laws of physics in general and are applicable in other natural sciences.

The first law of thermodynamics

The first law distinguishes two principal forms of energy transfer, heat and thermodynamic work, that modify a thermodynamic system containing a constant amount of matter. The law also defines the internal energy of a system, an extensive property for taking account of the balance of heat and work in the system.

The first law can be captured by the following equation: ΔU = Q — W, where ΔU is the change in the internal energy, Q is the heat added to the system, and W is the work done by the system. The internal energy is a state variable, just like temperature or pressure.

The first law allows for many possible states of a system to exist, but only certain states are found to exist in nature. This eventually leads to the second law of thermodynamics and the definition of another state variable called entropy.

The second law of thermodynamics

The second law states that heat always flows spontaneously from hotter to colder regions of matter. It also asserts that a natural process runs only in one direction and is not reversible. In other words, the second law is concerned with the direction of natural processes.

The second law was historically an empirical finding that was accepted as an axiom of thermodynamic theory. It was first formulated by French scientist Sadi Carnot in 1824, who showed that the efficiency of converting heat to work in a heat engine has an upper limit. However, the first rigorous definition of the second law based on the concept of entropy came from German scientist Rudolf Clausius in the 1850s.

The second law can be expressed in the following equation:

$\Delta S = \frac{\Delta Q}{T}$

Where:

• $\Delta S$ is the change in entropy
• $\Delta Q$ is the heat transfer
• $T$ is the temperature

The second law has important implications for the fate of the universe. It implies that the universe will eventually reach a state of 'heat death', where everything is at the same temperature and no work can be done. This is the ultimate level of disorder, where all the energy will be in the form of the random motion of atoms and molecules.

The third law of thermodynamics

Entropy is related to the number of accessible microstates, and there is typically one unique state (called the ground state) with minimum energy. In such a case, the entropy at absolute zero will be exactly zero. If the system does not have a well-defined order, then there may remain some finite entropy as the system is brought to very low temperatures.

The third law was developed by German chemist Walther Nernst between 1906 and 1912 and is often referred to as the Nernst heat theorem. It is because a system at zero temperature exists in its ground state, so its entropy is determined only by the degeneracy of the ground state.

The third law provides an absolute reference point for the determination of entropy at any other temperature. The entropy of a closed system, determined relative to this zero point, is then the absolute entropy of that system.

Mathematically, the absolute entropy of any system at zero temperature is the natural log of the number of ground states times the Boltzmann constant kB = 1.38×10−23 J K−1.

The zeroth law of thermodynamics

The law provides an independent definition of temperature without reference to entropy, which is defined in the second law. The zeroth law states that if two thermodynamic systems are both in thermal equilibrium with a third system, then the two systems are in thermal equilibrium with each other.

Two systems are said to be in thermal equilibrium if they are linked by a wall permeable only to heat, and they do not change over time. This means that there is no heat flow between the systems, and they have the same temperature.

The zeroth law is important for the mathematical formulation of thermodynamics. It makes the relation of thermal equilibrium between systems an equivalence relation, which can represent equality of some quantity associated with each system. A quantity that is the same for two systems, if they can be placed in thermal equilibrium with each other, is a scale of temperature.

The zeroth law is needed for the definition of temperature scales and justifies the use of practical thermometers. It establishes thermal equilibrium as an equivalence relationship, allowing any member of a subset of systems to be uniquely "tagged" with a label identifying its temperature.

The zeroth law in its usual short statement allows recognition that two bodies in a relation of thermal equilibrium have the same temperature, especially that a test body has the same temperature as a reference thermometric body. For a body in thermal equilibrium with another, there are indefinitely many empirical temperature scales, generally depending on the properties of a particular reference thermometric body. The second law allows a distinguished temperature scale, which defines an absolute, thermodynamic temperature, independent of the properties of any particular reference thermometric body.

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