Thermodynamics Flashcards
What is a thermodynamic system?
a region of space denoted by boundires containing a quantity of matter
Definition of a closed system
contains a fixed amount of mass
work and heat can cross boundaries
boundaries are impermeable but may be movable
Definition of an open system
mass can flow in and out - boundaries are permeable
work and heat can cross boundaries
Thermodynamics Properties - convenient
temperature
pressure
volume
mass
Thermodynamic Properties - Less convenient
Specific Volume
Density
Internal Energy
Enthalpy
Entropy
Thermodynamic Properties - Less convenient
Specific Volume
Density
Internal Energy
Enthalpy
Entropy
Types of Properties
Intensive
Extensive
Specific Properties
Intensive definition
does not depend on the mass of the substance in a system
Extensive properties definition
depends of the mass of the substance in a system
Specific Properties definition
are reduced to the unit mass of a substance to acquire the meaning of intensive properties
Process vs Cycle
a process is when it changes from one state to another and a cycle is where the start and end states are identical
Isothermal process
constant temperature
Isothermal process
constant temperature
Isobaric Process
constant pressure
isochoric process
constant volume
adiabatic process
no heat crosses system boundries
reversible process
the ideal case moves between continuous succession of equilibrium states, with no degradation of quality energy
Irreversible process
all real processes, heat transfer occurring and cannot be returned, energy quality is degraded
Process diagrams allow us to…
summerise whats happening in a process
PV = constant process diagram shape
Concave shape
1st law of thermodynamics
energy can be transferred, this can primarily accomplished by work and heat transfer
Work
is done by a force when the point of force application undergoes a displacement
Conservation of energy
when the total energy of an isolated system remains constant
conservation of energy in a closed system
Wnet + Qnet = 0
what does positive Wnet mean
means that work is done on a system by the surroundings
Power input
W = W per cycle x cycle/sec
Internal energy
the energy associated with the motion of molecules
when there is no work done but heat transfer to surroundings the change of energy is…
the change in internal energy
thermally insulated system
cannot have heat transfer across the boundary but work can take place
adiabatic
changes in thermal energy balance the work
thermally isolated system
where to heat of work transfer can take place
Enthalpy
H=U+pv
Specific enthalpy
h=u+pv
Specific Heat capacity
the quantity of heat required to raise the temperature of a system by 1 degree
Heat Capacity (J/K)
C=dQ/dT
Specific heat capacity equation (J/KgK)
C= dq/dT
Specific heat capacity with constant volume
cv = dq/dT=dU/dT
dq = du as there is not change in work
cv= (U1-U2) / (T1-T2)
Specific Heat Capacity with constant pressure
cp = dq/dt = dh/dT
cp = qp/T1-T2
heat engine works by
receiving heat from a hot reservoir and converts some of the heat into work it also rejects heat to the cold reservoir and work in a cycle
what must a heat engine have ( define each )
heat source - supplies heat
heat sink - absorbs rejected heat
work output - the use of heat engines
prime mover - the moving parts that deliver work
thermal energy reservoirs
large bodies of heat that do not change in temperature
thermal efficiency
n = W/Q
thermal efficeincy ( 1- equation )
n = 1- Qout/Qin
thermal efficeincy ( temperature equation )
n = 1-T1/T2
Carnot efficiency
the max efficiency for a heat engine that operates reversibly between the heat source and heat sink.
Carnot for an irreversible heat engine
n < ncarnot
Carnot for reversible heat engines
n = ncarnot
Carnot for impossible engines
n > ncarnot
thermal resistance in heat engines
engine hot is a lower temperature than the heat source and engine cold is a higher temperature than the heat sink
why is work output sometimes reduced
some of the work can be dissipated by heat
LOOK AT PV DIAGRAM FOR THE HEAT SINK
squashed parallelogram
Ordered energy
when all atoms have the same energy, they all have the same kinetic energy so move at the same speed and have the same potential energy
Disordered energy
atoms move randomly and have different amounts of energy
High Temperature vs Low temperature how disordered
low temp is more disordered
Work vs Heat
work is ordered and heat is disordered
Energy interactions
when a hot objects interacts with a cold energy moves randomly between atoms and disperses from the hot to cold
Heat sinks output … energy
ordered
Entropy
S=k loge W
k=1,38054x10^-23
most disordered = … entropy
highest
work has … entropy
0
as heat disperses from hot to cold
the entropy increases
entropy equation
change in S = Q/T
entropy in a real process
Change in S system + Change in S surroundings > 0
entropy in an ideal process
Change in S system + Change in S surroundings = 0
entropy for any process
Change in S system + Change in S surroundings >= 0
entropy in hot and cold sources
dSh = dQ/Th
dSc = dQ/Tc
dSh<dSc
