Block 1 - Introductory Thermo Concepts

Kinetic energy

1/2mv^2 Disregards how velocity was attained Kinetic energy is a property of the body Extensive property because it is associated with the body as a whole

Equilibrium

A condition of balance maintained by an equality of opposing forces Several types of equilibrium must exist individually to fulfill the condition of complete equilibrium, such as mechanical, thermal, phase, and chemical equilibrium

Thermometric property

A measurable property that changes as its temperature changes

Closed system

A particular quantity of matter is under study Always contains the same matter No transfer of mass across its boundary Special type that does not interact in any way is an isolated system

Isothermal process

A process occurring at constant temperature

Phase

A quantity of matter that is homogeneous throughout in both chemical composition and physical structure In physical structure it means that matter is all solid, or all liquid, or all vapor (all gas)

Control volume

A region within a prescribed boundary is studied Mass may cross the boundary

Molar basis

Amount of a substance in terms of kilomole (kmol) or the pound mole (lbmol) n = m/M Number of kilomoles, n, obtained by dividing the mass, m, in kilograms by the molecular weight, M

Internal energy

An extensive property of the system, as is the total energy Represented by the symbol U The specific internal energy is symbolized by u E = KE + PE + U

Energy transfer of heat

Closed system also can interact with their surroundings in a way that cannot be categorized as work An example is a gas in a container undergoing a process while in contact with a flame at a temperature greater than that of the gas

Heat Transfer

Conduction - heat transfer through a medium across which a temperature difference exists Convection - heat transfer between a surface and a moving or still fluid having a different temperature Radiation - Represents the net exchange of energy between surfaces at different temperatures by electromagnetic waves independent of any intervening medium Multiple Modes - involving multiple ways of heat transfer

Physical laws

Conservation of mass Conservation of energy Conservation of momentum Second law of thermodynamics

Boundary

Distinguishes the system from its surroundings and may be at rest or in motion

Energy balance

E2-E1=Q-W KE+PE+U=Q-W which shows that an energy transfer across the system boundary results in a change in one or more of the macroscopic energy forms: kinetic energy, gravitational potential energy, and internal energy

Converted

Energy can be converted from energy stored in the chemical bonds of fuels to electrical or mechanical power in fuel cells and internal combustion engines

Stored

Energy can be stored in forms such as kinetic energy and gravitational potential energy or within the matter making up the system

Transferred

Energy can be transferred by work, heat transfer, and the flow of hot or cold streams of matter

Convection

Energy transfer between a surface and a moving or still fluid having a different temperature

Conduction

Energy transfer by heat through a medium across which a temperature difference exists

Surroundings

Everything external to the system

Fluid Mechanics

Fluid statics Conservation of momentum Mechanical energy equation Similitude and modeling

MLtT/FLtT

Four primary dimensions of thermodynamics, fluid mechanics, and heat transfer Mass (M), length (L), time (t), and Temperature (T) Force (F) can be used in place of mass (M)

Modes

Heat transfer processes Conduction, convection, radiation

Macroscopic contributions

In engineering thermodynamics the change in the total energy of a system is considered to be made up of three macroscopic contributions One is the change in kinetic energy, associated with the motion of the system as a whole relative to an external coordinate frame Another is the change in gravitational potential energy, associated with the position of the system as a whole in the earth's gravitational field All other changes are lumped together in the internal energy

Intensive properties

Independent of the size or extent of a system and may vary from place to place within the system at any moment May be functions of both position and time, whereas extensive properties vary at most with time Specific volume, pressure, temperature, etc.

Thermal systems

Involve the storage, transfer, and conversion of energy

Extensive properties

Its value for an overall system is the sum of its values for the parts into which the system is divided Mass, volume, energy, etc. Depend on the size or extent of a system Can change with time

Property

Macroscopic characteristic of a system such as mass, volume, energy, pressure, and temperature to which a numerical value can be assigned at a given time without knowledge of the previous behavior of the system

Adiabatic process

No energy transfer by heat

Steady state

None of the properties of a system changes with time

Quasiequilibrium (or quasistatic) process

One in which the departure from thermodynamic equilibrium is at most infinitesimal All states through which the system passes may be considered equilibrium states The values of the intensive properties are uniform throughout the system, or every phase present in the system, at each state visited

Pure substance

One that is uniform and invariable in chemical composition Can exist in more than one phase but its chemical composition must be the same in each phase

Microscopic interpretation of internal energy

Part of the internal energy of the gas is the translational kinetic energy of the molecules Other contributions to the internal energy include the kinetic energy due to rotation of the molecules relative to their centers of mass and the kinetic energy associated with vibrational motions within the molecules Energy is stored in the chemical bonds between the atoms that make up the molecules

Energy units

SI, newton-meter, N m, called the joule, J Foot-pound force, ft lbf, and the British thermal unit Btu

Thermodynamic cycle

Sequence of processes that begins and ends at the same state At the conclusion all properties have the same values they had at the beginning Over the cycle there is no net change of state

Power units

Since power is a time rate of doing work, it can be expressed in terms of any units for energy and time SI, the unit for power is J/s, called the watt Others includes ft . lbf/s, Btu/h, and horsepower

State

The condition of a system as described by its properties

Macroscopic

The gross or overall behavior of matter

Thermal radiation

The net exchange of energy between surfaces at different temperatures by electromagnetic waves independent of any intervening medium

Specific volume, v

The reciprocal of the density v = 1/p Volume per unit mass Like density, specific volume is an intensive property and may vary from point to point

Polytropic process

The relationship between pressure and volume during an expansion or compression process A quasi equilibrium process described by pV^n = constant, where the value of n is a constant for the particular process

Microscopic

The structure of matter Characterize by statistical means the average behavior of the particles making up a system of interest and relate this information to the observed macroscopic behavior of the system

Sign Convention

The symbol Q denotes an amount of energy transferred across the boundary of a system in a heat interaction with the system's surroundings Q>0, heat transfer to the system Q<0, heat transfer from the system The sign convention is the reverse of the one adopted for work

Equilibrium state

The system was in equilibrium at the moment it was isolated if there are no changes to it

Work is not a property of the system or the surroundings

The value of W depends on the details of the interactions taking place between the system and surroundings during a process and not just the initial and final states of the system

Heat is not a property

The value of a heat transfer depends on the details of a process and not just the end states

Power, W

Time rate at which energy transfer occurs The rate of energy transfer by work When a work interaction involves an observable force, the rate of energy transfer by work is equal to the product of the force and the velocity at the point of application of the force W = F . V

Sign convention

W>0, work done by the system W<0, work done on the system The direction of energy transfer is shown by an arrow on a sketch of the system, and work is regarded as positive in the direction of the arrow

System

Whatever we want to study

Work

When a body moving along a path is acted on by a resultant force that may vary in magnitude from position to position along the path, the work of the force is written as the dot product of the force vector F and the displacement vector of the body along the path When the resultant force causes elevation to be increased and/or the body to be accelerated, the work done by the force can be considered a transfer of energy to the body, where it is stored as gravitational energy and/or kinetic energy

Thermal equilibrium

When all changes in observable properties cease, the interaction is at an end

Phase boundaries

When more than one phase is present, the phases are separated by phase boundaries

Process

When the properties of a system change and cause the state and the system to change Transformation from one state to another

Thermodynamic definition of work

Work is done by a system on its surroundings if the sole effect on everything external to the system could have been the raising of a weight

First law of thermodynamics

change in the amount of energy contained within the system during some time interval = net amount of energy transferred in across the system boundary by heat transfer during the time interval - net amount of energy transferred out across the system boundary by work during the time interval

Thermodynamics

conservation of mass conservation of energy second law of thermodynamics properties

Instantaneous time rate form of the energy balance

dE/dt = Q-W time rate of change of the energy contained within the system at time t = net rate at which energy is being transferred in by heat transfer at time t - net rate at which energy is being transferred out by work at time t

SI base units

mass - kilogram (kg) length - meter (m) time - second (s) force - newton (N = 1 kg * m/s^2)

Gravitational potential energy

mgz An attribute of a system consisting of the body and the earth together Regarded as an extensive property of the body