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Heat and Energy
Published in Alan Cottrell, An Introduction to Metallurgy, 2019
The internal energy is the sum of all the individual kinetic energies of motion and energies of interaction (potential energies) of the particles in the system. When a system is left to itself its internal energy remains constant but when it is brought into interaction with its surroundings it can alter its internal energy by receiving or giving energy. This energy may be transferred either as work, through the displacement of an applied force, or as heat, through the fine-scale individual interactions of the particles (including photons) in thermal contact. Suppose that a system changes its internal energy from E (before) to E + dE (after) by absorbing a small amount of heat dQ from its surroundings and by doing a small amount of work dW on its surroundings (e.g. pushing the atmosphere back by expanding against atmospheric pressure). Then the first law of thermodynamics, i.e. the principle of conservation of energy and heat, requires that dE=dQ−dW
The Laws of Thermodynamics
Published in Robert E. Masterson, Nuclear Reactor Thermal Hydraulics, 2019
Work can be transferred into or out of a system by any mechanical device. Energy changes that are NOT caused by temperature differences between a system and its environment are caused by the application of this work. Work can be transferred to a system by a rotating shaft, a piston, or an electrical wire that crosses the system boundaries. Work transfer into a system increases its internal energy, and work transfer out of a system decreases its internal energy. Steam turbines produce mechanical work while reactor coolant pumps and compressors consume it. In nuclear science and engineering, work is given the symbol W. In the SI unit system, work has the units of joules (J). The rate at which work is performed per second is called the power, and power is measured in J/s or W. In commercial power reactors, the electrical power that is produced is normally measured in MW (or MWE). Large power reactors can produce up to 3,500 MW of thermal power each second.
Phase and State Transitions and Transformations in Food Systems
Published in Dennis R. Heldman, Daryl B. Lund, Cristina M. Sabliov, Handbook of Food Engineering, 2018
The internal energy, U, is the sum of all forms of energy within a system, that is, all kinetic and potential energy of all molecules within the system. A change in internal energy, ΔU, may occur as a result of energy transfer, and the amount of internal energy changes from the initial state of the system, Ui, to a final state, Uf . The internal energy is a state function. This means that the internal energy is dependent on the state of the system, but independent of how that state may have been achieved. State functions are properties which are dependent on state variables, for example, pressure. The internal energy of a system may change as a result of transfer of heat, q, or work, w, with the surroundings of the system. This is quantified by Equation 4.1, which is also known as the First Law of Thermodynamics. ΔU=q+w
Pyrolysis kinetics of regional agro-industrial wastes using isoconversional methods
Published in Biofuels, 2019
Alejandra Saffe, Anabel Fernandez, Marcelo Echegaray, Germán Mazza, Rosa Rodriguez
The thermodynamic parameters, ∆H, ∆G and ∆S, were calculated at the temperature that the maximum mass loss rate is produced [16]. Enthalpy is a measurement of the energy in a thermodynamic system. Enthalpy is defined as a state function, and it depends only on the prevailing equilibrium state identified by the internal energy, pressure, and volume. It is an extensive quantity. If ΔH is positive, the reaction is endothermic. The exothermic processes exhibit a negative value of this variation. ΔH is equal to the change in the internal energy of the system, plus the pressure–volume work that the system has done on its surroundings. ΔH, under such conditions, is the heat absorbed (or released) by the material through a chemical reaction or by external heat transfer. The activation ΔH also shows the energy differences between the activated complex and the reagents. If this difference is small, the formation of an activated complex is favored, because the potential energy barrier is low [60].