Chapter 2 Flashcards
As a result of the conversion of electric energy consumed by the device to heat, the room temperature will ___
Rise
Total Energy E
The sum of thermal, mechanical, kinetic, potential, electric, magnetic, chemical and nuclear energy
Thermodynamics deals only with…
The change of the total energy
Those a system possesses as a whole with respect to some outside reference frame
Macroscopic forms of energy
Macroscopic forms of energy
Kinetic and potential energies
Those related to the molecular structure of a system and the degree of the molecular activity
Microscopic forms of energy - internal energy
The energy that a system possesses as a result of its motion relative to some reference from
Kinetic energy (KE)
The energy that a system possesses as a result of its elevation in a gravitational field
Potential Energy (PE)
The portion of the internal energy if a system associated with the kinetic energies of the molecules
Sensible energy
The internal energy associated with the phase of a system
Latent energy
The internal energy associated with the atomic bonds in a molecule
Chemical energy
The tremendous amount of energy associated with the strong bonds within the nucleus of the atom itself
Nuclear energy
Thermal
Sensible + latent
Internal
Sensible + latent + chemical + nuclear
The total energy of a system, can be contained or stored in a system, and thus can be viewed as the
Static forms of energy
The forms of energy not stored in a system can be viewed as the
Dynamic forms of energy or as energy interactions
Dynamic forms of energy
Recognized at the system boundary as they cross it, and they represent the energy gained or lost by a system during a process
The only two forms of energy interactions associated with a closed system are
Heat transfer and work
The difference between heat transfer and work
An energy interaction is heat transfer if its driving force is a temperature difference. Otherwise it is work.
The form of energy that can be converted to mechanical work completely and directly by an ideal mechanical device such as an ideal turbine
Mechanical energy
The familiar forms of mechanical energy
Kinetic and potential energies
The form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature difference
Heat
Temperature difference is the driving force for ____
Heat transfer
Energy is recognized as heat transfer only as it…
Crosses the system boundary
Treats molecules as tiny balls that are in motion and thus possess kinetic energy
Kinetic theory
The energy associated with the random motion of atoms and molecules
Heat
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
Conduction
The transfer of energy between a solid surface and the adjacent fluid that is in motion, and it involves the combined effects of conduction and fluid motion
Convection
The transfer of energy due to the emission of electromagnetic waves (or photons)
Radiation
Heat was thought to be an invisible fluid called the _____ that flowed from warmer bodies to the cooler ones
Caloric
The energy transfer associated with a force acting through a distance
Work
Work interactions
A rising piston, a rotating shaft, and an electric wire crossing the system boundaries
Formal sign convention
Heat transfer to a system (Qin) and work done by a system (Wout) are positive
Heat transfer to a system (Qout) and work done on a system (Win) are negative
Both heat and work are _____
Boundary phenomena - recognized at the boundaries of a system as they cross the boundaries
Systems posses ___
Energy but not heat or work
Heat vs. work
Both are associated with a process not a state
Has no meaning at a state
Both are path functions
Path functions
Their magnitudes depend on the path followed during a process as well as the end states
Properties are ___
Point functions
There are two requirements for a work interaction between a system and its surrounds to exist
There must be a force acting on the boundary
The boundary must move
Work done to raise or to accelerate a body
- The work transfer needed to raise a body is equal to the change in the potential energy of the body
- The work transfer needed to accelerate a body is equal to the change in the kinetic energy of the body
Nonmechanical forms of work
Electrical work
Magnetic work
Electrical polarization work
Electrical work
Generalized force - voltage (electrical potential)
Generalized displacement - electrical charge
Magnetic work
Generalized force - magnetic field strength
Generalized displacement - magnetic dipole moment
Electrical polarization work
Generalized force - electric field strength
Generalized displacement - polarization of the medium
Provides a sound basis for studying the relationships among the various forms of energy and energy interactions
The first law of thermodynamics (the conversation of energy principle)
The first law of thermodynamics
Energy can be neither created nor destroyed during a process; it can only change forms
Energy balance
(Total energy entering the system) - (total energy leaving the system) = (change in the total energy of the system)
Energy change of a system
Energy change = energy at final state - energy at initial state
Net energy transfer by heat, work, and mass
Ein- Eout
Change in internal, kinetic, potential, etc. energies
Change in Esystem
Mechanisms of energy transfer
Heat transfer
Work transfer
Mass flow
A closed mass involves only
Heat transfer and work
Is one of the most frequently used terms in thermodynamics, and it indicates how well an energy conversion or transfer process is accomplished
Efficiency
The amount of heat released when a unit amount of fuel at room temperature is completely burned and the combustion products are cooled to the room temperature
Heating value of the fuel
Lower heating value (LHV)
When the water leaves as a vapor
Higher heating value (HHV)
When the water in the combustion gases is completely condensed and thus the heat of vaporization is also recovered
A device that converts mechanical energy to electrical energy
Generator
The ratio of the electrical power output to the mechanical power input
Generator efficiency
The ratio of the net electrical power output to the rate of fuel energy input
Thermal efficiency of a power plant
The amount of light output in lumens per W of electricity consumed
Lighting efficacy
Using energy-efficient appliances ___
Conserve energy
The combustion of fuel produces
Carbon dioxide (global warming)
Nitrogen oxides and hydrocarbons (smog)
Carbon monoxide
Sulfur dioxide (acid rain)
The effectiveness of the conversion process between the mechanical work supplied or extracted and the mechanical energy of the fluid is expressed by the
Pump efficiency and turbine efficiency
There are 3 modes of heat transfer:
Conduction
Convection
Radiation
All modes require temperature difference between
Source and sink
Heat transfers how?
From high temperature to low temperature
Transfer of more energetic particles to less energetic particles
Conduction
Occurs in solids, liquids, and gasses
Conduction
Conduction: liquids and gasses
Collision of molecules during random motion (Brownian motion)
Conduction: solids
Combination of crystal lattice vibrations and energy transport by free electrons
Heat of conduction is proportional to…
Temperature difference and area normal to heat transfer
Proportionality constant measures material’s ability to conduct heat
Thermal conductivity
High k
Conductors (most metals)
Low k
Insulators (wood, styrofoam)
Fourier’s law of heat conduction
Negative sign is used to due to negative temperature
Transfer of energy to/from a solid surface from/to an adjacent moving fluid (liquid or gas)
Convection
Combination of conduction and fluid motion
Convection
____ fluid motion results in more heat transfer
Faster
If fluid is motionless
Pure conduction occurs
Fluid is forced over surface by external means (fan, pump, wind, etc.)
Forced convection
Fluid motion is caused by buoyancy force due to changes in density (warmer air rises because it is less dense)
Free (or natural) convection
Heat transfer in processes that involve phase changes in fluids are considered to be
Convection - evaporation, condensation
Movement of vapor bubbles through a liquid
Evaporation
Falling of liquid droplets
Condensation
Convective heat transfer coefficient
Not a property of the fluid
Determined experimentally; affected by surface geometry, fluid properties, fluid velocity
Ts
Surface temperature
Temp of fluid at surface
Tf
Bulk fluid
Radiation
Energy is emitted by matter in the form of electromagnetic waves (or photons)
Results from the change of electronic configurations of atoms or molecules
Radiation
Does not require presence of medium between source and sink; can work in a vacuum
Radiation
All bodies at a temperature above ____ emit thermal radiation
Absolute zero
Maximum radiation from a surface
Stefan-Boltzman’s Law
Surface that emits maximum radiation is called a
Black body
___ surfaces emit less than the maximum
Real
Emissivity
Material property between 0 and 1
1 for blackbodies
Pollutants emitted during the combustion of fossil fuels are responsible for
Smog, acid rain, global warming
The environmental pollution has reached such high levels that it became a serious threat to
Vegetation, wild life, human health
