Chapter 2

Question 1

Total energy

Answer 1

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.

Question 2

Macroscopic forms of energy

Answer 2

are those a system possesses as a whole with respect to some outside reference frame, such as kinetic and potential energies.

Question 3

Microscopic forms of energy

Answer 3

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.

Question 4

Internal energy

Answer 4

(U) of a system is the sum of all the microscopic forms of energy.

Question 5

Kinetic energy

Answer 5

(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.

Question 6

Potential energy

Answer 6

(PE) is the energy that a system possesses as a result of its elevation in a gravitational field and is expressed as PE = mgz.

Question 7

Stationary systems

Answer 7

are closed systems whose velocity and elevation of the center of gravity remain constant during a process.

Question 8

Mass flow rate

Answer 8

is the amount of mass flowing through a cross section per unit time.

Question 9

Volume flow rate

Answer 9

is the volume of the fluid flowing through a cross section per unit of time.

Question 10

Sensible energy

Answer 10

is the portion of the internal energy of a system associated with the kinetic energies of the molecules.

Question 11

Latent energy

Answer 11

is the internal energy associated with the phase of a system.

Question 12

Chemical energy

Answer 12

is the internal energy associated with the atomic bonds in a molecule.

Question 13

Nuclear energy

Answer 13

is the tremendous amount of energy associated with the strong bonds within the nucleus of the atom itself.

Question 14

Heat transfer

Answer 14

(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.

Question 15

Work

Answer 15

is the energy transfer associated with a force acting through a distance.

Question 16

Thermal energy

Answer 16

is the sensible and latent forms of internal energy.

Question 17

Mechanical energy

Answer 17

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.

Question 18

Heat transfer

Answer 18

(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.

Question 19

Adiabatic process

Answer 19

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.

Question 20

Kinetic theory

Answer 20

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.

Question 21

Caloric

Answer 21

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.

Question 22

Conduction

Answer 22

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.

Question 23

Convection

Answer 23

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.

Question 24

Radiation

Answer 24

is the transfer of energy due to the emission of electromagnetic waves (or photons).

Question 25

Power

Answer 25

is the work done per unit time and has the unit kJ/s, or kW.

Question 26

Formal sign convention

Answer 26

(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.

Question 27

Path functions

Answer 27

are functions whose magnitudes depend on the path followed during a process as well as the end states.

Question 28

Inexact differentials

Answer 28

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.

Question 29

Point functions

Answer 29

depend on the state only. They do not depend on the path followed to reach that state. Properties are point functions.

Question 30

Exact differentials

Answer 30

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.

Question 31

Electrical power

Answer 31

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.

Question 32

Surface tension

Answer 32

is the force per unit length used to overcome the microscopic forces between molecules at the liquid–air interfaces.

Question 33

Electrical work

Answer 33

is work done on a system as electrons in a wire move under the effect of electromotive forces while crossing the system boundary.

Question 34

Magnetic work

Answer 34

is the product of the generalized force as the magnetic field strength and the generalized displacement as the total magnetic dipole moment.

Question 35

Electrical polarization work

Answer 35

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).

Question 36

First law of thermodynamics

Answer 36

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

Question 37

Energy balance

Answer 37

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.

Question 38

Stationary systems

Answer 38

are closed systems whose velocity and elevation of the center of gravity remain constant during a process.

Question 39

Rate form

Answer 39

is the form of a quantity expressed per unit time.

Question 40

Per unit mass

Answer 40

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.

Question 41

Efficiency of a water heater

Answer 41

is the ratio of the energy delivered to the house by hot water to the energy supplied to the water heater.

Question 42

Heating value of a fuel

Answer 42

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.

Question 43

Combustion equipment efficiency

Answer 43

is the ratio of the useful amount of heat delivered by the combustion equipment to the heating value of the fuel burned.

Question 44

Annual fuel utilization efficiency

Answer 44

(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.

Question 45

Generator efficiency

Answer 45

is the ratio of the electrical power output to the mechanical power input to a generator.

Question 46

Overall efficiency

Answer 46

(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.

Question 47

Lighting efficacy

Answer 47

is the ratio of the amount of light output by lighting devices in lumens of light output to the electrical energy input in W.

Question 48

Efficiency of a cooking appliance

Answer 48

is the ratio of the useful energy transferred to the food to the energy consumed by the appliance.

Question 49

Environment

Answer 49

refers to the region beyond the immediate surroundings whose properties are not affected by the process at any point.

Question 50

Mechanical efficiency

Answer 50

of a device or process is the ratio of the mechanical energy output to the mechanical energy input.

Question 51

Pump efficiency

Answer 51

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.

Question 52

Turbine efficiency

Answer 52

is the ratio of the mechanical energy output of the turbine to the mechanical energy decrease of the fluid flow through the turbine.

Question 53

Useful pumping power

Answer 53

is the rate of increase in the mechanical energy of a fluid as it flows through a pump.

Question 54

Motor efficiency

Answer 54

is the ratio of the mechanical energy output of a motor to the electrical energy input.

Question 55

Generator efficiency

Answer 55

is the ratio of the electrical power output to the mechanical power input to a generator.

Question 56

Acid rain

Answer 56

is rain or snow that washes acid-laden droplets from the air onto the soil.

Question 57

Greenhouse effect

Answer 57

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.

Question 58

Global warming

Answer 58

(global climate change) is the undesirable consequence of the greenhouse effect.

Question 59

Conduction

Answer 59

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.

Question 60

Thermal conductivity

Answer 60

is a measure of the ability of a material to conduct heat.

Question 61

Fourier’s law of heat conduction

Answer 61

states that rate of heat conduction in a direction is proportional to the temperature gradient in that direction.

Question 62

Convection

Answer 62

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

Question 63

Forced convection

Answer 63

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.

Question 64

Free convection

Answer 64

(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.

Question 65

Newton’s law of cooling

Answer 65

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.

Question 66

Convection heat transfer coefficient

Answer 66

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.

Question 67

Radiation

Answer 67

is the transfer of energy due to the emission of electromagnetic waves (or photons).

Question 68

Stefan-Boltzmann law

Answer 68

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.

Question 69

Blackbody

Answer 69

is an idealized surface that emits radiation at the maximum rate given by the Stefan-Boltzmann law.

Question 70

Blackbody radiation

Answer 70

is the amount of radiation emitted by a blackbody

Question 71

Emissivity

Answer 71

is a surface property that is a measure of how closely a surface approximates a blackbody for which the emissivity equal to one.

Question 72

Absorptivity

Answer 72

is the fraction of the radiation energy incident on a surface that is absorbed by the surface.

Question 73

Kirchhoff’s law

Answer 73

is defined for radiation that the emissivity and the absorptivity of a surface are equal at the same temperature and wavelength.