Thermal Dynamics Law: Seawater's Unique Energy Transfer

The laws of thermodynamics govern the behaviour of heat, work, and temperature and their relation to energy, entropy, and the physical properties of matter and radiation. The first law of thermodynamics is a formulation of the law of conservation of energy in the context of thermodynamic processes. The law distinguishes two principal forms of energy transfer, heat and thermodynamic work, that modify a thermodynamic system containing a constant amount of matter. The thermodynamic properties of seawater include variables such as density, sound speed, specific heat capacity, the adiabatic lapse rate, and the freezing temperature. The first law of thermodynamics specifies that energy can be transferred between physical systems as heat, as work, and with the transfer of matter. The second law defines the existence of a quantity called entropy, which describes the direction in which a system can evolve thermodynamically and quantifies the state of order of a system. The third law of thermodynamics states that as the temperature of a system approaches absolute zero, all processes cease and the entropy of the system approaches a minimum value. The laws of thermodynamics are universally valid when applied to systems that fall within the constraints implied by each law.

Seawater's thermodynamic properties

The thermodynamic properties of seawater have been redefined as the International Thermodynamic Equation of Seawater (TEOS-10). This standardises the calculation of the thermodynamic properties of seawater, humid air, and ice. TEOS-10 is used by oceanographers and climate scientists to calculate and model properties of the oceans such as heat content.

The thermodynamic properties of seawater include variables such as density (or specific volume), sound speed, specific heat capacity, the adiabatic lapse rate, and the freezing temperature. TEOS-10 also treats the "heat content" of seawater in a more consistent and natural fashion through the introduction of a new temperature variable, Conservative Temperature, which replaces potential temperature.

TEOS-10 is based on thermodynamic potentials. Fluids like humid air and liquid water in TEOS-10 are described by the Helmholtz energy F(m,T,V)=F(m,T,m/ρ) or the specific Helmholtz-energy f(T,ρ)=F(m,T,m/ρ)/m. The Helmholtz energy has a unique value across phase boundaries. For the calculation of the thermodynamic properties of seawater and ice, TEOS-10 uses the specific Gibbs potential g(T,P)=G/m, G=F+pV, because the pressure is a more easily measurable property than density in a geophysical context.

The specific volume of seawater can be expressed as a function of salinity, temperature, and pressure. This is often called the "equation of state" of seawater. The horizontal gradients of specific volume and of Practical Salinity at constant pressure in the ocean are usually very small, and measuring them is a complex task.

The "raw" physical oceanographic data collected from ships and autonomous platforms are stored in national oceanographic databases as Practical Salinity (unitless) and in-situ temperature as functions of sea pressure, at a series of longitudes, latitudes, and observation times.

The TEOS-10 properties of seawater are based on a Gibbs function (or Gibbs potential) of seawater, which is a function of Absolute Salinity SA (rather than Practical Salinity SP), temperature, and pressure. Absolute Salinity is the mass fraction of dissolved material in seawater and is preferred over Practical Salinity because the thermodynamic properties of seawater are directly influenced by the mass of dissolved constituents.

The Gibbs function approach allows the calculation of internal energy, entropy, enthalpy, potential enthalpy, and the chemical potentials of seawater, as well as the freezing temperature and the latent heats of melting and evaporation.

The first law of thermodynamics

ΔU = Q - W

Where ΔU represents the change in internal energy, Q represents the quantity of heat supplied to the system, and W represents the thermodynamic work done by the system.

The second law of thermodynamics

The second law can be simply stated as: heat always flows spontaneously from hotter to colder regions of matter. Another statement of the law is: "Not all heat can be converted into work in a cyclic process."

The second law was formulated by Rudolf Clausius in the 1850s. His statement, known as the Clausius statement, says: "Heat generally cannot flow spontaneously from a material at a lower temperature to a material at a higher temperature."

The second law has implications for renewable energy sources, as it sets a limit on the efficiency of energy conversion processes. This means that even with renewable energy sources, some energy will be lost as heat and cannot be recovered.

The third law of thermodynamics

Entropy is a measure of disorder or randomness in a closed system and is related to the number of microstates accessible to the system. The microstate in which the system's energy is at its minimum is called the ground state. In most cases, the entropy at absolute zero will be zero as there is typically only one ground state. However, if the system lacks a well-defined order, some finite entropy may remain at very low temperatures. This constant value is known as the residual entropy of the system.

The third law was formulated by German chemist Walther Nernst between 1906 and 1912 and is often referred to as the Nernst heat theorem or the Nernst-Simon heat theorem, acknowledging the contribution of Nernst's doctoral student, Francis Simon. Nernst stated that it is impossible to reach absolute zero in a finite number of steps. This implies that cooling a system to absolute zero would require an infinite number of steps or an infinite amount of time.

The third law has several alternative formulations, including the Nernst statement, which applies to thermodynamic processes at fixed, low temperatures for condensed systems (liquids and solids). It states that the entropy change associated with any condensed system undergoing a reversible isothermal process approaches zero as the temperature approaches absolute zero.

Another statement of the third law, by American physical chemists Merle Randall and Gilbert Lewis, asserts that when the entropy of each element in a perfect crystalline state is taken as zero at absolute zero temperature, every substance has a finite positive entropy. However, the entropy at absolute zero can be zero, as in the case of a perfect crystal.

The third law has important applications, such as calculating the absolute entropy of a substance at any temperature. These calculations are based on heat capacity measurements.

The thermodynamic properties of seawater are defined by the International Thermodynamic Equation of Seawater—2010 (TEOS-10). TEOS-10 incorporates the effects of spatial differences in seawater composition through a new salinity variable, Absolute Salinity. It also introduces a new temperature variable, Conservative Temperature, which replaces potential temperature and treats the "heat content" of seawater more consistently. TEOS-10 provides a comprehensive framework for calculating the thermodynamic properties of seawater, including density, sound speed, specific heat capacity, and freezing temperature.

The four laws' measurable macroscopic physical quantities

The laws of thermodynamics define a group of physical quantities that can be measured at a macroscopic scale. These include temperature, energy, entropy, work, and heat. The first law of thermodynamics is a version of the law of conservation of energy, which states that energy can be transformed from one form to another but cannot be created or destroyed. This is measurable through the change in internal energy of a system.

The second law of thermodynamics states that the sum of the entropies of interacting systems never decreases in a natural thermodynamic process. Entropy can be viewed as a measure of the microscopic details of the motion and configuration of a system, often referred to as "disorder" on a molecular scale. This is measurable through the use of a 'difference of information entropy' between two given macroscopically specified states of a system.

The third law of thermodynamics states that a system's entropy approaches a constant value as its temperature approaches absolute zero. This is measurable through the Boltzmann principle, which relates entropy to the number of possible microstates of a system.

The zeroth law, which was defined later, establishes the transitive relation between the temperatures of bodies in thermal equilibrium. While not considered a "law of thermodynamics" by all sources, the Onsager reciprocal relations, which describe the relation between thermodynamic flows and forces in non-equilibrium thermodynamics, could be considered a fourth law.

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