Thermodynamics

two requirements for a work interaction between a system and its surroundings to exist:

(1) there must be a force acting on the boundary, and (2) the boundary must move

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.

Second law of thermodynamics

(increase of entropy principle) is expressed as the entropy of an isolated system during a process always increases or, in the limiting case of a reversible process, remains constant. In other words, the entropy of an isolated system never decreases. It also asserts that energy has quality as well as quantity, and actual processes occur in the direction of decreasing quality of energy.

State postulate

(specifies the number of properties required to fix the state of a system:) The state of a simple compressible system is completely specified by two independent, intensive properties.

work unit

1 Joules (J)- or 1 Newton-meter

Watt

A measure of power equal to one joule of work per second.

Isochoric process

A process in which specific volume remains constant and in which no net pressure-volume work is done

Isobaric process

A process that occurs at a constant pressure

Cycle

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.

Dimensional Homogeneity

For an equation to be valid, it must be dimensionally homogeneous, that is the dimensions on the left-hand side must equal the dimensions on the right hand-side.

density

Mass per unit volume (mass/volume)

Gage Pressure Equation

Pgage = Pabs - Patm

Vacuum Pressure

Pressures below atmospheric pressure

vacuum pressure equation

Pvac = Patm - Pabs

Weight Equation

W=mg

Quasi-static, or quasi-equilibrium, process

When a process proceeds in such a manner that the system remains infinitesimally close to an equilibrium state at all times. A quasi-equilibrium process can be viewed as a sufficiently slow process that allows the system to adjust itself internally so that properties in one part of the system do not change any faster than those at other parts.

Intensive properties

a physical property that remains the same no matter how much of a substance is present^^are those that are independent of the size of a system, such as temperature, pressure, and density. Intensive properties of a nonreacting ideal- or real-gas mixture are obtained by dividing the extensive properties by the mass or the mole number of the mixture in the gas mixture. The internal energy, enthalpy, and entropy of a gas mixture per unit mass or per unit mole of the mixture can be determined by summing the products of the mass fractions and the specific property or summing the products of the mole fractions and the molar specific property. That is, the intensive properties of a gas mixture are determined by either a mass weighted or a mole weighted average of the properties.

Isothermal process

a process that occurs at constant temperature

system

a quantity of matter or a region in space chosen for study

Work, like heat, is an

an energy interaction between a system and its surroundings.

Dimensions

are any physical characterizations of a quantity.

Specific properties

are extensive properties per unit mass. Some examples of specific properties are specific volume (v = V/m) and specific total energy (e = E/m).

Path functions

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

Unity conversion ratios

are ratios of units that are based on the definitions of the units in question that are identically equal to 1, are unitless, and can be inserted into any calculation to properly convert units.

Units

are the arbitrary magnitudes assigned to the dimensions.

atmospheric pressure is measured by a

barometer

Closed system or control mass

consists of a fixed amount of mass (control mass), and no mass can cross its boundary. But energy, in the form of heat or work, can cross the boundary.

extensive properties

depend on the amount of matter that is present-^^are those whose values depend on the size—or extent—of the system. Mass m, volume V, and total energy E are some examples of extensive properties. Extensive properties of a nonreacting ideal- or real-gas mixture are obtained by just adding the contributions of each component of the mixture.

Heat and work are _____________ quantities

directional

IndepeIndependent propertiesndent properties

exist when one property can be varied while another property is held constant.

energy can cross the boundary of a closed system in the form of

heat or work

The pressure in a stationary fluid remains constant in the

horizontal direction

Equilibrium

implies a state of balance. In an equilibrium state there are no unbalanced potentials (or driving forces) within the system. A system in equilibrium experiences no changes when it is isolated from its surroundings.

The state postulate requires that the two properties specified be

independent to fix the state

To describe a process completely, one should specify the

initial and final states of the process, as well as the path it follows, and the interactions with the surroundings.

Isolated system

is a closed system in which energy is not allowed to cross the boundary.

Steady-flow process

is a process during which a fluid flows through a control volume steadily. That is, the fluid properties can change from point to point within the control volume, but at any point, they remain constant during the entire process. During a steady-flow process, no intensive or extensive properties within the control volume change with time.

Simple compressible system

is a system in which there is the absence of electrical, magnetic, gravitational, motion, and surface tension effects. These effects are due to external force fields and are negligible for most engineering problems.

Continuum

is a view of mass as continuous, homogeneous matter with no holes. Matter is made up of atoms that are widely spaced in the gas phase. Yet it is very convenient to disregard the atomic nature of a substance. The continuum idealization allows us to treat properties as point functions, and to assume the properties to vary continually in space with no jump discontinuities. This idealization is valid as long as the size of the system we deal with is large relative to the space between the molecules. This is the case in practically all problems, except some specialized ones.

Open system or control volume

is any arbitrary region in space through which mass and energy can pass across the boundary.

Process

is any change that a system undergoes from one equilibrium state to another. To describe a process completely, one should specify the initial and final states of the process, as well as the path it follows, and the interactions with the surroundings.

Property

is any characteristic of a system. Some familiar properties are pressure P, temperature T, volume V, and mass m. The list can be extended to include less familiar ones such as viscosity, thermal conductivity, modulus of elasticity, thermal expansion coefficient, electric resistivity, and even velocity and elevation.

Specific gravity, or relative density

is defined as the ratio of the density of a substance to the density of some standard substance at a specified temperature (usually water at 4°C, for which the density is 1000 kg/m3). **dimensionless quantity

Chemical equilibrium

is established in a system when its chemical composition does not change with time.

Mechanical equilibrium

is related to pressure, and a system is in mechanical equilibrium if there is no change in pressure at any point of the system with time.

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.

Phase equilibrium

is the condition in which the two phases of a pure substance are in equilibrium when each phase has the same value of specific Gibbs function. Also, at the triple point (the state at which all three phases coexist in equilibrium), the specific Gibbs function of each one of the three phases is equal.

Weight

is the gravitational force applied to a body, and its magnitude is determined from Newton's second law. (force not mass)

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.

Boundary

is the real or imaginary surface that separates the system from its surroundings. The boundary of a system can be fixed or movable.

Specific weight

is the weight of a unit volume of a substance and is determined from the product of the local acceleration of gravity and the substance density.

Power

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

if the energy crossing the boundary of a closed system is not heat

it must be work

the complete description of a heat or work interaction requires the specification of both the

magnitude and direction

Small to moderate pressure differences are measured by a

manometer

Thermal equilibrium

means that the temperature is the same throughout the entire system.

The work done on a system by an external force acting in the direction of motion is

negative

The density of liquids and solids depends more strongly on

on temperature than it does on pressure

work done by a system against an external force acting in the opposite direction to motion is

positive

The density of most gases is

proportional to pressure and inversely proportional to temperature

Specific volume

reciprocal of density; volume per unit mass volume/mass

Pascal's principle

states that the pressure applied to a confined fluid increases the pressure throughout by the same amount.

Primary or fundamental dimensions

such as mass m, length L, time t, and temperature T, are the basis for the derivation of secondary dimensions.

Secondary or derived dimensions

such as velocity V, energy E, and volume V, are expressed in terms of the primary dimensions.

The density of a substance, in general, depends on

temperature and pressure.

State

the condition of a system not undergoing any change gives a set of properties that completely describes the condition of that system. At this point, all the properties can be measured or calculated throughout the entire system.

Gage Pressure

the difference between the absolute pressure and the local atmospheric pressure

work

the energy transfer associated with a force acting through a distance

sorroundings

the mass or region outside the system/are everything outside the system boundaries.

path

the series of states through which a system passes during a process