Thermodynamics Flashcards
what is thermodynamics
the study of heat, work, and temperature and their relation to energy with matter
define heat transfer
how energy is transferred when there is a temperature difference
3 types of heat transfer
conduction, convection, and radiation
equation for heat transfer
Ein − Eout =∆𝑈=mcv∆T
define conduction
transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interactions between the particles
what does k represent in the equation for conductivity
thermal conductivity f the material, which is a measure of the ability of a material to conduct heat
rate of heat conduction through a plane layer equation
Q.con = -kA (∆T/∆x)
define convection
mode of energy transfer between a solid surface and the adjacent liquid of gas that is in motion, and it involves the combined effects of conduction and fluid motion
faster the fluid motion, the ______ the convection heat transfer
greater
when there is no bulk fluid in motion, heat transfer between solid surface and adjacent fluid is by pure ________
conduction
what is the name for this: fluid is forced to flow over the surface by external means such as fan, pump, or the wind
forced convection
what is the name for this: fluid motion is caused by buoyancy forces that are induced by density differences due to the variation of temperature in the fluid
natural convection
define radiation
energy emitted by matter in the form of electromagnetic waves (or photons) as a result of changes in the electronic configuration of the atoms or molecules
does radiation require the presence of an intervening medium
no
heat transfer at the ______ ____ ______ and suffers no attenuation in a vacuum.
speed of light
in an isolated system, can mass and/or energy cross the system boundary
no, the amount of matter and energy in the system is fixed
in an closed system, can mass and/or energy cross the system boundary
mass cannot pass the system boundary, but the system may exchange heat, receive/exert forces. matter is fixed - control mass
in an open system, can mass and/or energy cross the system boundary
mass, heat, and work can cross the system boundary. - control volume
define state
the condition of the system as described by the systems thermodynamic properties
define reference state
In thermodynamics, we are only
concerned with the changes of energy in the system, not
the energy values at the particular states. Therefore, we
can choose any state as the and assign zero value to the
internal energy or enthalpy of a substance at that state.
define steady state
the system properties are not changing
as function of time
define process
a path between two states. Adiabatic,
isothermal, isobaric process are of particular interest
define equilibrium
when the system is unchanging in
terms of thermal, mechanical, phase, and chemical
characteristics
define extensive properties
depends on the extent/amount of material in the system; these are properties that are additive
define intensive properties
do not depend on the extent/amount of material in the system; these are properties that are not additive
is viscosity an intensive or extensive property
intensive
is enthalpy an intensive or extensive property
extensive
is color an intensive or extensive property
intensive
is temperature intensive or extensive property
intensive
is internal energy intensive or extensive property
extensive
define fuel
any material that can be burned to release thermal energy, most familiar fuels are composed of mainly hydrogen and carbon - hydrocarbon
define combustion
a chemical reaction during which a fuel is oxidized and large quantity of energy is released
what two things do you need to start combustion
1: fuel into contact with oxygen
2: above the fuels ignition temperature
define complete combustion
all the carbon in the fuel burns to CO2, all the hydrogen burns to H2O etc
basic definition of energy
the ability to do work
Work is done when a ______ acts over some distance or
displacement. this can also be expressed as a _______
acting over some change in volume
force
pressure
is work transfer a system property
no
work transfer depends on the ________ _______
process path
what is the first law of thermodynmics
conservation of energy, energy can not be destroyed or created, it can only be converted
heat is defined as the form of energy, which can only be transferred when two systems have a _________ difference
temperature
Q>0: heat transfer _____ the system
into
Q<0: heat transfer _____ of the system
out
W>0:
system is doing the work
W<0:
work is done on the system
what is the second law of thermodynamics
Entropy, in an isolated system, can never be reduced, it
always increases, or, at best, remains unchanged
is entropy a thermodynamic property or a state function
both! entropy being a thermodynamic property makes it state function
is entropy an extensive or intensive property
extensive
entropy GENERATION can be used to quantity the _____________ of a process
irreversibility
No process is
possible whose sole result is the transfer of _____ from a body of
lower temperature to a body of higher temperature.
heat
does the first law describe the balance of energy or the direction of a process
balance of energy
does the second law describe the balance of energy or the direction of a process
direction of process
does water have higher entropy when it exists as a liquid or vapor
vapor
how to calculate the net work out of the system
Wnet,out = Qin - Qout (kj)
thermal efficiency equation
nth = network output/ total heat input = Wnet,out/ Qin
what is the most efficient heat engine, theoretically
Carnot engine
equation for Carnot engine
nth = 1 - TL / TH
what are the 4 components of a steam power plant
pump, boiler, turbine, condenser
what component is considered as Qin of the steam power plant
the boiler
what component is considered as Qout of the steam power plant
condenser
what component is considered as Wout of the steam power plant
turbine
heat engines transform a ______form of energy, into a ______ form
degraded
noble
what are the two noble forms of energy
electrical or mechanical
why is efficiency always less than 1
we have to reject part of the input heat
why is the carnot engine the most efficient heat engine
the entropy at the output is equal to that ar the input
what two diagrams do we use for a heat engine
p-v and T-s
what do you call it when entropy is constant
isentropic process
for a carnot engine, what should the temperature be in
kelvin
define voltaic cells
devices that convert chemical energy directly into electricity
examples of voltaic cells
flashlight battery, automobile battery, fuel cells
describe how a voltaic cell works
provides a flow of electrons through redox reaction in an external circuit, where useful work can be done. To harness this energy, the reaction is split into two separate half reactions: the oxidation and reduction reactions, and they are put into two different containers with a wire to drive the electrons from one side to the other
describe these two types of voltaic cells: expendable and reusable
expendable: not worth preserving after its first discharge
reusable: can be rechargeable or refuellable
describe rechargeable
activity is restored by electric charging current, such as car batteries
describe refuellable
deliver a sustained output because their consumables, fuel, and oxidant are replenished - fuel cell
consider fuel cell as a battery that has a continuous supply of _____ and _____
fuel and oxidant
do fuel cells run down or need recharge
no, that’s how they differ from batteries
describe a fuel cell
require a continuous source of fuel, such as hydrogen,
natural gas, propane, and biogas, and oxygen (usually from air)
to sustain the chemical reaction. They continuously produce
electricity and heat as long as fuel is supplied.
describe a battery
the chemical energy usually comes from
substances that are already present in the battery.
Rechargeable batteries consist of reversible cell reactions
that allow them to regain their cell potential, through the
work done by passing currents of electricity.
a negative electrode, catalyst causes the fuel to undergo oxidation reactions that generate ions and electrons
anode
a positive electrode, another catalyst causes ions, electrons, and oxygen to react, forming water and possibly other products
cathode
in addition t electricity, fuel cells produce ______ _______, ______, and depending on fuel source, very small amounts of _____ _____ and other emissions
water vapor, heat, nitrogen
allow ions, often positively charged hydrogen ions (protons) to move between the two sides of the fuel cell
electrolyte
what is the general energy efficiency of a fuel cell
40-60%
how can a fuel cell efficiency reach 85%
if waste heat is captured in a cogeneration scheme
Fuel cells can operate at __________ efficiencies than combustion engines
higher
fuel cells can convert the __________ energy in the fuel
directly to _________ energy without requiring an intermediate degradation into _____
chemical
electrical
heat
Fuel cells have ______ or _____ emissions compared to combustion engines. Hydrogen fuel cells emit only water,
addressing critical climate challenges as there are no __________ _________ emissions.
lower or zero
carbon dioxide
Fuel cell: There also are no air pollutants that create ______ and
cause health problems at the point of operation.
smog
Fuel cells are ________ during operation as they have
few moving parts
quieter
Fuel cells: Expansion is easy because of __________ construction
modular
Fuel cells: It can use __________ heat for ambient heating, or high
temperature exhaust gases for co-generation
rejected
_____ ______ has fast response to __________ changes and good
overload characteristics
fuel cell
demand
why fuel cells
-higher efficiencies, lower or zero emissions, no air pollutants, quieter, modular, fast response
The _________ the temperature, the larger
the _______ the cell can deliver, but tend to
reduce _________ of components.
higher
current
lifetime
Most early fuel cells used _______
electrolytes which can leak, require liquid _____
and ___________ management, and may be
corrosive. Modern cells tend to use _______
electrolytes.
liquid
level and concentration
solid
Simplest fuel is hydrogen, which is difficult to
_______. Higher _________ fuel cells can use CO
as fuel. Methanol is used in portable _________
equipment.
store
temperature
electronic
Electrolytes can be _________, ______, molten
carbonates, or ceramic.
alkaline, acid
Electrolytes can be alkaline, acid, ______
_________, or ________.
molten, carbonates, ceramic
Hydrogen fuel cells - uses hydrogen to produce ________ and _______ with only _____ and _______ as byproducts
electricity and water
water and heat
What does the catalysts at the anode do for a hydrogen fuel cell
separates hydrogen mcl into protons and electrons, which take different paths to the cathode
what happens at the cathode of the hydrogen fuel cell
The protons migrate through the electrolyte to the cathode, where they unite with oxygen and the electrons to produce water and heat
The electrons go through an ______ _______, creating a flow of electrons
external circuit
what are the two applications of fuel cells
stationary power plants and fuel cell electric vehicles(FCEVs)
what is the central capacity of stationary power plants. What is the dispersed capacity of stationary power plants.
what is the on-site electrical generators wattage
central utility-operated power plants of large capacity (up to 1 GW)
* dispersed utility-operated power plants (tens of MW)
* on-site electrical generators (some 10–100 kW).
What are the pros of FCEVs
fast refueling time in just a few minutes
more efficient than combustion engines, and zero carbon emissions
what are the cons of FCEVs
more expensive than battery EV, require expensive hydrogen fuel stations, fuel is expensive and may come from carbon-intensive sources, safety concerns
what are the 5 main components of a hydrogen fuel cell system vehicle
hydrogen tanks, air intake, fuel cell system, battery, electric motor
