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Entropy

Entropy

The first law of thermodynamics is quantitatively formulated via an equation relating the internal energy of a system, the heat exchanged by it, and the work done on it. A quantitative formulation of the second law of thermodynamics leads to defining a state function, the entropy.
When an ideal gas expands isothermally, the disorder in the gas increases. From the molecular perspective, the gas molecules have more volume to move around in.
Consider an infinitesimal step in the expansion, which...

Entropy

Entropy

Salt particles that have dissolved in water never spontaneously come back together in solution to reform solid particles. Moreover, a gas that has expanded in a vacuum remains dispersed and never spontaneously reassembles. The unidirectional nature of these phenomena is the result of a thermodynamic state function called entropy (S). Entropy is the measure of the extent to which the energy is dispersed throughout a system, or in other words, it is proportional to the degree of disorder of a...

Entropy and the Second Law of Thermodynamics

Entropy and the Second Law of Thermodynamics

The second law of thermodynamics can be stated quantitatively using the concept of entropy. Entropy is the measure of disorder of the system.
The relation between entropy and disorder can be illustrated with the example of the phase change of ice to water. In ice, the molecules are located at specific sites giving a solid state, whereas, in a liquid form, these molecules are much freer to move. The molecular arrangement has therefore become more randomized. Although the change in average...

The Second Law of Thermodynamics

The Second Law of Thermodynamics

In the quest to identify a property that may reliably predict the spontaneity of a process, a promising candidate has been identified: entropy. Scientists refer to the measure of randomness or disorder within a system as entropy. High entropy means high disorder and low energy. To better understand entropy, think of a student’s bedroom. If no energy or work were put into it, the room would quickly become messy. It would exist in a very disordered state, one of high entropy. Energy must be...

Entropy Change in Reversible Processes

Entropy Change in Reversible Processes

In the Carnot engine, which achieves the maximum efficiency between two reservoirs of fixed temperatures, the total change in entropy is zero. The observation can be generalized by considering any reversible cyclic process consisting of many Carnot cycles. Thus, it can be stated that the total entropy change of any ideal reversible cycle is zero.
The statement can be further generalized to prove that entropy is a state function. Take a cyclic process between any two points on a p-V diagram.

Second Law of Thermodynamics

Second Law of Thermodynamics

In the quest to identify a property that may reliably predict the spontaneity of a process, a promising candidate has been identified: entropy. Processes that involve an increase in entropy of the system (ΔS > 0) are very often spontaneous; however, examples to the contrary are plentiful. By expanding consideration of entropy changes to include the surroundings, a significant conclusion regarding the relation between this property and spontaneity may be reached. In thermodynamic models, the...

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Author Spotlight: Advancing the Study of Brain-Heart Interplay with a Comprehensive EEGLAB Plugin for Multimodal Signal Analysis

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Review of Recent (2015-2024) Popular Entropy Definitions Applied to Physiological Signals.

Dimitrios Platakis¹, George Manis¹

¹Department of Computer Science and Engineering, University of Ioannina, 45110 Ioannina, Greece.

Entropy (Basel, Switzerland)

|September 27, 2025

Summary

This summary is machine-generated.

This review compares recent entropy estimation methods for physiological signals. It highlights their citation counts and applications in biomedical engineering for analyzing signal complexity.

Keywords:

Bubble Entropy Dispersion Entropy Distribution Entropy Phase Entropy Slope Entropy biomedical engineering entropy physiological signals review

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Area of Science:

Biomedical Engineering
Time Series Analysis
Signal Processing

Background:

Entropy estimation quantifies physiological signal complexity.
Approximate Entropy and Sample Entropy are widely used.
Numerous new entropy definitions have emerged in the last decade.

Purpose of the Study:

To systematically review and compare prominent entropy definitions from 2015-2024.
To analyze citation counts and application domains of these methods.
To document the specific entropy definitions and physiological signals used in recent research.

Main Methods:

Systematic literature review adhering to PRISMA methodology.
Search conducted on December 20, 2024.
Analysis of selected articles for entropy definitions, physiological signals, citations, and applications.

Main Results:

Identified and compared key entropy estimation techniques published in the last decade.
Presented statistical data on method citations and their use across various physiological signals.
Detailed which entropy methods are applied to which physiological signals.

Conclusions:

The field of entropy estimation in biomedical engineering is rapidly evolving.
Understanding the landscape of new entropy methods is crucial for accurate physiological signal analysis.
This review provides a comprehensive overview to guide future research and application choices.