| Acid rain | is defined as rain or snow that washes acid-laden droplets from the air on to the soil.
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| Absorptivity | is the fraction of the radiation energy incident on a surface that is absorbed by the surface.
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| Adiabatic process | is a process during which there is no heat transfer. The word adiabatic comes from the Greek word adiabatos, which means not to be passed.
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| Annual fuel utilization efficiency AFUE | is the efficiency of space heating systems of residential and commercial buildings which accounts for the combustion efficiency as well as other losses such as heat losses to unheated areas and start-up and cool-down losses.
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| Blackbody | is an idealized surface that emits radiation at the maximum rate given by the Stefan-Boltzmann law.
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| Blackbody radiation | is amount of radiation emitted by a blackbody.
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| Caloric | is heat treated as a fluidlike substance, according to the caloric theory, that is a massless, colorless, odorless, and tasteless substance that can be poured from one body into another.
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| Chemical energy | is the internal energy associated with the atomic bonds in a molecule.
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| Combined efficiency | (see overall efficiency)
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| Combustion efficiency | is defined as the ratio of the amount of heat released during combustion to the heating value of the fuel burned.
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| Conduction | is the transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interaction between particles.
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| Conservation of energy principle | (see first law of thermodynamics).
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| Conservation of mass principle | is expressed as net mass transfer to or from a system during a process equal to the net change (increase or decrease) in the total mass of the system during that process.
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| Continuity equation | is the conservation of mass equation as it is often referred to in fluid mechanics.
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| Control volume | is any arbitrary region in space through which mass and energy can pass across the boundary. Most control volumes have fixed boundaries and thus do not involve any moving boundaries. A control volume may also involve heat and work interactions just as a closed system, in addition to mass interaction.
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| Convection | is the mode of energy transfer between a solid surface and the adjacent fluid that is in motion, and it involves the combined effects of conduction and fluid motion.
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| Convection heat transfer coefficient | is the experimentally determined parameter that is the ratio of the rate of convection heat transfer and the product of the heat transfer area and surface to bulk fluid temperature.
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| Cycle | is a process, or series of processes, that allows a system to undergo state changes and returns the system to the initial state at the end of the process. That is, for a cycle the initial and final states are identical.
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| Efficiency | is defined as the ratio of desired result for an event to the input required to accomplish the event.
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| Efficiency of a cooking appliance | is defined as the ratio of the useful energy transferred to the food to the energy consumed by the appliance.
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| Efficiency of a water heater | is defined as the ratio of the energy delivered to the house by hot water to the energy supplied to the water heater.
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| Efficiency of resistance heaters | is 100 percent as they convert all the electrical energy they consume into heat.
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| Electrical polarization work | is the product of the generalized force taken as the electric field strength and the generalized displacement taken as the polarization of the medium (the sum of the electric dipole rotation moments of the molecules).
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| Electrical power | is the rate of electrical work done as electrons in a wire move under the effect of electromotive forces, doing work. It is the product of the potential difference measured in volts and the current flow measured in amperes.
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| Electrical work | is work done on a system as electrons in a wire move under the effect of electromotive forces while crossing the system boundary.
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| Emissivity | is a surface property that is a measure of how closely a surface approximates a blackbody for which the emissivity equal to one.
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| Energy Balance | is the net change (increase or decrease) in the total energy of the system during a process is equal to the difference between the total energy entering and the total energy leaving the system during that process.
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| Energy transport by mass | is the product of the mass of the flowing fluid and its total energy. The rate of energy transport by mass is the product of the mass flow rate and the total energy of the flow.
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| Exact differentials | are the differential changes for point functions (i.e., they depend on the state only, and not on how a system reaches that state), and they are designated by the symbol d. Properties are an example of point functions that have exact differentials.
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| First law | (see first law of thermodynamics)
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| First law of thermodynamics | may be expressed as follows: Energy can be neither created nor destroyed; it can only change forms. The net change (increase or decrease) in the total energy of the system during a process is equal to the difference between the total energy entering and the total energy leaving the system during that process. The energy balance can be written explicitly as
Ein – Eout =(Qin – Qout ) + (Win – Wout ) + (Emass, in – Emass, out ) = ΔEsystem
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| Flow work, or flow energy | is work required to push mass into or out of control volumes. On a unit mass basis this energy is equivalent to the product of the pressure and specific volume of the mass Pv.
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| Forced convection | is convection heat transfer when the fluid is forced to flow in a tube or over a surface by external means such as a fan, pump, or the wind.
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| Formal sign convention | (classical thermodynamics sign convention) for heat and work interactions is as follows: heat transfer to a system and work done by a system are positive; heat transfer from a system and work done on a system are negative.
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| Fourier's law | of heat conduction states that rate of heat conduction in a direction is proportional to the temperature gradient in that direction.
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| Free convection | (natural convection) is convection heat transfer when the fluid motion is caused by buoyancy forces induced by density differences due to the variation of temperature in the fluid.
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| Generator efficiency | is defined as the ratio of the electrical power output to the mechanical power input to a generator.
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| Greenhouse effect | is the heating effect causing the increase in temperature of the earth's atmosphere as the result of solar radiation entering the earth's atmosphere during the day, but heat radiated by the earth at night is blocked by gases such as carbon dioxide and trace amounts of methane, nitrogen oxides and other gases.
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| Global climate change | (see global warming).
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| Global warming | is the undesirable consequence of the greenhouse effect.
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| Heat | (see heat transfer).
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| Heat transfer | is defined as the form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature difference.
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| Higher heating value HHV of fuel | is the amount of heat released when a specified amount of fuel (usually a unit of mass) at room temperature is completely burned and the combustion products are cooled to the room temperature when the water formed during the combustion process is completely condensed and leaves as a liquid.
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| Inexact differentials | are the differential amount of change for path functions and are designated by the symbol d. Therefore, since heat and work are path functions, a differential amount of heat or work is represented by dQ or dW, respectively, instead of dQ or dW.
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| Internal energy | U of a system is the sum of all the microscopic forms of energy.
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| Kinetic energy | KE is energy that a system possesses as a result of its motion relative to some reference frame. When all parts of a system move with the same velocity, the kinetic energy is expressed as KE = m V2/2.
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| Kinetic theory | treats molecules as tiny balls that are in motion and thus possess kinetic energy. Heat is then defined as the energy associated with the random motion of atoms and molecules.
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| Kinetic theory | treats molecules as tiny balls that are in motion and thus possess kinetic energy that allows heat to be defined as the energy associated with the random motion of atoms and molecules.
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| Kirchhoff's law | is defined for radiation that the emissivity and the absorptivity of a surface are equal at the same temperature and wavelength.
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| Latent energy | is the internal energy associated with the phase of a system.
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| Lighting efficacy | is defined as the ratio of the amount of light output by lighting devices in lumens of light output to the electrical energy input in W.
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| Lower heating value of fuel | is the amount of heat released when a specified amount of fuel (usually a unit of mass) at room temperature is completely burned, and the combustion products are cooled to the room temperature when the water formed during the combustion process leaves as a vapor.
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| Macroscopic forms of energy | are those a system possesses as a whole with respect to some outside reference frame, such as kinetic and potential energies.
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| Magnetic work is the product of | the generalized force as the magnetic field strength and the generalized displacement as the total magnetic dipole moment.
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| Mass flow rate | is the amount of mass flowing through a cross section per unit time.
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| Mechanical efficiency | of a device or process is the ratio of the mechanical energy output to the mechanical energy input.
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| Mechanical energy | is the form of energy that can be converted to mechanical work completely and directly by an ideal mechanical device such as an ideal turbine.
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| Mechanical work | is work associated a force acting in the direction of motion that causes the movement of the boundary of a system or the movement of the entire system as a whole
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| Microscopic | forms of energy are those related to the molecular structure of a system and the degree of the molecular activity, and they are independent of outside reference frames.
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| Motor efficiency | is defined as the ratio of the mechanical energy output of a motor to the electrical energy input.
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| Natural convection | (see free convection)
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| Newton's law of cooling | defines the heat transfer by convection as the product of the convection heat transfer coefficient, heat transfer area, and the difference between the heat transfer surface temperature and the fluid bulk temperature away from the surface.
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| Nuclear energy | is the tremendous amount of energy associated with the strong bonds within the nucleus of the atom itself.
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| Overall efficiency | (combined efficiency) for a power plant is defined as the ratio of the net electrical power output to the rate of fuel energy input and is expressed as the product of the combustion efficiency, thermal efficiency and generator efficiency.
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| Path functions | are functions whose magnitudes depend on the path followed during a process as well as the end states.
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| Potential energy | PE is the energy that a system possesses as a result of its elevation in a gravitational field and is expressed as PE = mgz.
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| Power | is the work done per unit time is called and has the unit kJ/s, or kW.
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| Pump efficiency | is defined as the ratio of the mechanical energy increase of the fluid as it flows through the pump to the mechanical energy input to the pump.
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| Radiation | is the transfer of energy due to the emission of electromagnetic waves (or photons).
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| Rate form | is the form of a quantity expressed per unit time.
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| Rate of heat transfer | is the amount of heat transferred per unit time.
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| Sensible energy | is the portion of the internal energy of a system associated with the kinetic energies of the molecules.
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| Shaft work | is energy transmitted by a rotating shaft and is the related to the torque T applied to the shaft and the number of revolutions of the shaft per unit time.
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| Spring work | is the work done to change the length of a spring.
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| Stationary systems | are closed systems whose velocity and elevation of the center of gravity remain constant during a process.
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| Stefan-Boltzmann law | gives the maximum rate of radiation that can be emitted from a surface as product of the Stefan-Boltzmann constant, surface area, and the fourth power of the surface absolute temperature.
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| Surface tension | is the force per unit length used to overcome the microscopic forces between molecules at the liquid-air interfaces.
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| Therm | is defined as an amount of energy produced by the combustion of natural gas and is equal to 29.3 kWh.
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| Thermal conductivity | is defined as a measure of the ability of a material to conduct heat.
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| Thermal efficiency | of a power cycle is defined as the ratio of the net work done by the cycle to the heat supplied to the cycle.
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| Thermal energy | is the sensible and latent forms of internal energy.
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| Total energy | E of a system is the sum of the numerous forms of energy such as thermal, mechanical, kinetic, potential, electric, magnetic, chemical, and nuclear, and their constituents. The total energy of a system on a unit mass basis is denoted by e and is defined as E/m.
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| Total energy of a flowing fluid | is the sum of the enthalpy, kinetic, and potential energies of the flowing fluid.
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| Triple point | of water is the state at which all three phases of water coexist in equilibrium.
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| Turbine efficiency | is defined as the ratio of the mechanical energy output of the turbine to the mechanical energy decrease of the fluid flow through the turbine.
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| Useful pumping power | is the rate of increase in the mechanical energy of a fluid as it flows through a pump.
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| Uniform | implies no change with location over a specified region.
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| Volume flow rate | is the volume of the fluid flowing through a cross section per unit time.
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| Water heater efficiency | is defined as the ratio of the energy delivered to a house by hot water to the energy supplied to the water heater.
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| Work | is the energy transfer associated with a force acting through a distance.
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| Work transfer | is the energy in the form of work that is transferred across a system boundary.
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