Chapter 2 Flashcards by Alyse Jones

As a result of the conversion of electric energy consumed by the device to heat, the room temperature will ___

Rise

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Total Energy E

The sum of thermal, mechanical, kinetic, potential, electric, magnetic, chemical and nuclear energy

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Thermodynamics deals only with…

The change of the total energy

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Those a system possesses as a whole with respect to some outside reference frame

Macroscopic forms of energy

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Macroscopic forms of energy

Kinetic and potential energies

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Those related to the molecular structure of a system and the degree of the molecular activity

Microscopic forms of energy - internal energy

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The energy that a system possesses as a result of its motion relative to some reference from

Kinetic energy (KE)

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The energy that a system possesses as a result of its elevation in a gravitational field

Potential Energy (PE)

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The portion of the internal energy if a system associated with the kinetic energies of the molecules

Sensible energy

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The internal energy associated with the phase of a system

Latent energy

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The internal energy associated with the atomic bonds in a molecule

Chemical energy

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

Nuclear energy

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Thermal

Sensible + latent

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Internal

Sensible + latent + chemical + nuclear

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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

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The forms of energy not stored in a system can be viewed as the

Dynamic forms of energy or as energy interactions

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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

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The only two forms of energy interactions associated with a closed system are

Heat transfer and work

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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.

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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

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The familiar forms of mechanical energy

Kinetic and potential energies

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The form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature difference

Heat

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Temperature difference is the driving force for ____

Heat transfer

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Energy is recognized as heat transfer only as it…

Crosses the system boundary

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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

1. The work transfer needed to raise a body is equal to the change in the potential energy of the body 2. 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

Surface temperature Temp of fluid at surface

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