China
Africa
Explore chapters and articles related to this topic
Water Ecology
Published in Frank R. Spellman, Handbook of Water and Wastewater Treatment Plant Operations, 2020
Simply defined, energy is the ability or capacity to do work. For an ecosystem to exist, it must have energy. All activities of living organisms involve work, which is the expenditure of energy. This means the degradation of a higher state of energy to a lower state. Two laws govern the flow of energy through an ecosystem: The First and Second Laws of Thermodynamics. The first law sometimes called the conservation law, states that energy may not be created or destroyed. The second law states that no energy transformation is 100% efficient. That is, in every energy transformation, some energy is dissipated as heat. The term entropy is used as a measure of the non-availability of energy to a system. Entropy increases with an increase in dissipation. Because of entropy, the input of energy in any system is higher than the output or the work done; thus, the resultant efficiency is less than 100%.
General Principles
Published in Donald E. Struble, John D. Struble, Automotive Accident Reconstruction, 2020
Donald E. Struble, John D. Struble
The first great conservation law is that if a system can be defined so as to be isolated from external forces, then its momentum is conserved. This follows immediately from Newton’s Second Law, as long as the system is defined such that external forces are absent, or at least negligible. In the absence of such forces, its momentum is conserved. A pertinent example was alluded to above. For another example, define a system containing two (or more) vehicles engaged in a collision. If tire forces can be ignored, then this system’s momentum is conserved, even though both vehicles are moving and interacting within it. In fact, momentum conservation implies that the system center of mass moves in a constant speed and a constant direction throughout the crash. Thus, an observer who is stationary with respect to the system center of mass (i.e., moving with it) will see the colliding vehicles come to rest, even though they may have come together at very different speeds and angles.
Introduction
Published in Vladimir A. Dobrushkin, Applied Differential Equations, 2018
Many differential equations arise from problems in mechanics, electrical engineering, quantum mechanics, and other areas where conservation laws may be applied. In particular, a conservation law states that some physical quantity, which is usually energy, remains constant. In reality, a physical system is never conservative. However, mathematical models often neglect effects such as friction, electrical resistance, or temperature fluctuation if they are small enough. Therefore, we operate with idealized mathematical models that may obey conservative laws. We will see later that in many cases mathematical expressions that have no physical meaning behave conservatively. In this section we analyze mathematical models using systems of autonomous differential equations for which conservation laws can be applied. Consider a mechanical system that is governed by Newton's second law, F=my¨,y¨=d2y/dt2
Emergence of a new symmetry class for Bogoliubov–de Gennes (BdG) Hamiltonians: expanding 10-fold symmetry classes
Published in Phase Transitions, 2020
A symmetry is a transformation that leaves the physical system invariant. These transformations include translation, reflection, rotation, scaling, etc. One of the most important implications of symmetry in physics is the existence of conservation laws. For every global continuous symmetry, there exists an associated conserved quantity [1]. In quantum mechanics, symmetry transformation can be represented on the Hilbert space of physical states by an operator that is either linear and unitary or anti-linear and anti-unitary [2]. Any symmetry operator acts on these states and transforms them to new states. These symmetry operators can be classified as continuous (rotation, translation) and discrete (parity, lattice translations, time reversal). Continuous symmetry transformations give rise to the conservation of probabilities and discrete symmetry transformations give rise to the quantum numbers. Another important implication of symmetry in quantum mechanics is the symmetry on exchanging identical particles [3].
Challenge-based instruction promotes students’ development of transferable frameworks and confidence for engineering problem solving
Published in European Journal of Engineering Education, 2019
John R. Clegg, Kenneth R. Diller
Biotransport is a required course in the undergraduate BME curriculum, at the upper division level. The course began with a review of concepts of thermodynamics necessary for proper system definition and the application of conservation laws. Thereafter, students learned about the fundamentals and biomedical applications of fluid mechanics and bioheat transfer. Prior to the course, students attended an on-campus orientation and an off-campus send-off gathering. During these meetings, the instructor detailed the unique inquiry-driven structure of the course, as well as discussed pertinent travel and planning details. The course entailed 17 class sessions, each approximately three and a half hours in duration, over five weeks. The primary text resource was Biotransport: Principles and Applications by Roselli and Diller (Roselli and Diller 2011), which was written specifically to be amenable to challenge-based instruction.
Equivalent contact temperature (ECT) for personal comfort assessment – analytical description and definition of comfort limits
Published in Ergonomics, 2023
Alexander Warthmann, Itsuhei Kohri, Yoshiichi Ozeki, Hideaki Nagano, Christoph van Treeck
The thermodynamic rules as well as the conservation laws of energy and mass still hold. The energy balance is slightly modified compared to the full physics model. The energy and mass balances are described in Equations 16 and 2, respectively.