Chapter 2 Flashcards
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 50 constituents. The total energy of a system on a unit mass basis is denoted by e and is defined as E/m.
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.
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.
Internal energy
(U) of a system is the sum of all the microscopic forms of energy.
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.
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.
Stationary systems
are closed systems whose velocity and elevation of the center of gravity remain constant during a process.
Mass flow rate
is the amount of mass flowing through a cross section per unit time.
Volume flow rate
is the volume of the fluid flowing through a cross section per unit of time.
Sensible energy
is the portion of the internal energy of a system associated with the kinetic energies of the molecules.
Latent energy
is the internal energy associated with the phase of a system.
Chemical energy
is the internal energy associated with the atomic bonds in a molecule.
Nuclear energy
is the tremendous amount of energy associated with the strong bonds within the nucleus of the atom itself.
Heat transfer
(heat) 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. It is the area under the process curve on a T-S diagram during an internally reversible process; however, this area has no meaning for irreversible processes.
Work
is the energy transfer associated with a force acting through a distance.
Thermal energy
is the sensible and latent forms of internal energy.
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.
Heat transfer
(heat) 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. It is the area under the process curve on a T-S diagram during an internally reversible process; however, this area has no meaning for irreversible processes.
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.
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.
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.
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.
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.
Radiation
is the transfer of energy due to the emission of electromagnetic waves (or photons).
Power
is the work done per unit time and has the unit kJ/s, or kW.
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.
Path functions
are functions whose magnitudes depend on the path followed during a process as well as the end states.
Inexact differentials
are the differential amount of change for path functions and are designated by the symbol δ. Therefore, since heat and work are path functions, a differential amount of heat or work is represented by δQ or δW, respectively, instead of dQ or dW.
Point functions
depend on the state only. They do not depend on the path followed to reach that state. Properties are point functions.
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.
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.
Surface tension
is the force per unit length used to overcome the microscopic forces between molecules at the liquid–air interfaces.
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.
Magnetic work
is the product of the generalized force as the magnetic field strength and the generalized displacement as the total magnetic dipole moment.
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).
First law of thermodynamics
is simply a statement of the conservation of energy principle, and it asserts that total energy is a thermodynamic property. Joule’s experiments indicate the following: For all adiabatic processes between two specified states of a closed system, the net work done is the same regardless of the nature of the closed system and the details of the process. It 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 Images
Energy balance
is the net change (increase or decrease) in the total energy of the system during a process; it is equal to the difference between the total energy entering and the total energy leaving the system during that process.
Stationary systems
are closed systems whose velocity and elevation of the center of gravity remain constant during a process.
Rate form
is the form of a quantity expressed per unit time.
Per unit mass
in thermodynamics, it is common and usually more convenient to express the quantities on a per unit mass basis. For example, energy per unit mass is expressed as kJ/kg or Btu/lbm.
Efficiency of a water heater
is the ratio of the energy delivered to the house by hot water to the energy supplied to the water heater.
Heating value of a fuel
is the amount of heat released when a fuel is burned completely in a steady-flow process and the products are returned to the state of the reactants. In other words, the heating value of a fuel is equal to the absolute value of the enthalpy of combustion of the fuel.
Combustion equipment efficiency
is the ratio of the useful amount of heat delivered by the combustion equipment to the heating value of the fuel burned.
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.
Generator efficiency
is the ratio of the electrical power output to the mechanical power input to a generator.
Overall efficiency
(combined efficiency) for a power plant is 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.
Lighting efficacy
is the ratio of the amount of light output by lighting devices in lumens of light output to the electrical energy input in W.
Efficiency of a cooking appliance
is the ratio of the useful energy transferred to the food to the energy consumed by the appliance.
Environment
refers to the region beyond the immediate surroundings whose properties are not affected by the process at any point.
Mechanical efficiency
of a device or process is the ratio of the mechanical energy output to the mechanical energy input.
Pump efficiency
is the ratio of the mechanical energy increase of the fluid as it flows through the pump to the mechanical energy input to the pump.
Turbine efficiency
is the ratio of the mechanical energy output of the turbine to the mechanical energy decrease of the fluid flow through the turbine.
Useful pumping power
is the rate of increase in the mechanical energy of a fluid as it flows through a pump.
Motor efficiency
is the ratio of the mechanical energy output of a motor to the electrical energy input.
Generator efficiency
is the ratio of the electrical power output to the mechanical power input to a generator.
Acid rain
is rain or snow that washes acid-laden droplets from the air onto the soil.
Greenhouse effect
is the heating effect that causes the temperature of the earth’s atmosphere to increase as solar radiation enters the atmosphere during the day but heat radiated by the earth at night is blocked by carbon dioxide and trace amounts of other gases such as methane and nitrogen oxides.
Global warming
(global climate change) is the undesirable consequence of the greenhouse effect.
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.
Thermal conductivity
is a measure of the ability of a material to conduct heat.
Fourier’s law of heat conduction
states that rate of heat conduction in a direction is proportional to the temperature gradient in that direction.
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
Forced convection
is convection heat transfer that occurs 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.
Free convection
(natural convection) is convection heat transfer that occurs when the fluid motion is caused by buoyancy forces induced by density differences due to the variation of temperature in the fluid.
Newton’s law of cooling
defines 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.
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.
Radiation
is the transfer of energy due to the emission of electromagnetic waves (or photons).
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.
Blackbody
is an idealized surface that emits radiation at the maximum rate given by the Stefan-Boltzmann law.
Blackbody radiation
is the amount of radiation emitted by a blackbody
Emissivity
is a surface property that is a measure of how closely a surface approximates a blackbody for which the emissivity equal to one.
Absorptivity
is the fraction of the radiation energy incident on a surface that is absorbed by the surface.
Kirchhoff’s law
is defined for radiation that the emissivity and the absorptivity of a surface are equal at the same temperature and wavelength.
