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THERMODYNAMICS
Content:
- Definition
- Thermodynamics System
- Properties of a System
- Gas Laws
- Processes
- Heat and Entropy
- Enthalpy and Internal Energy
- Laws of Thermodynamics
- Cycles Thermodynamics is the branch of physics that deals with the conversion from one to another of various forms of energy and how these affect temperature, pressure, volume, mechanical action and work. Thermodynamics is an axiomatic science which deals with the relations among heat, work and properties of system which are in equilibrium. It describes state and changes in state of physical systems Thermodynamics is the science that deals with the interaction between energy and material systems. I. Thermodynamics System System, Boundary and Surroundings System. A system is a finite quantity of matter or a prescribed region of space Boundary. The actual or hypothetical envelope enclosing the system is the boundary of the system. The boundary may be fixed or it may move, as and when a system containing a gas is compressed or expanded. The boundary may be real or imaginary. Kinds of Thermodynamics system CLOSED SYSTEM: also known as Control Mass, is a system consisting of a fixed amount of mass, and no mass can cross its boundary. That is, no mass can enter or leave a closed system. However, energy in the form of heat or work can cross the boundary. OPEN SYSTEM: also known as Control volume, is a system in which mass is allowed to cross the boundary. ISOLATED SYSTEM: is a system in which neither mass nor energy is allowed to cross the boundary. II. Properties of a System A property is any quantity, which serve to describe a system. It can be divided into two general types. Intensive property is one, which does not depend on the mass of the system. Extensive property is one, which depends on the mass of the system. TEMPERATURE Temperature is a measure of the intensity of heat of a substance Celsius Scale (ºC) Coined by Anders Celsius Freezing points: 0 ºC Boiling point: 100 ºC Fahrenheit Scale (ºF) Coined by Gabriel Daniel Fahrenheit
Freezing point: 32 ºF Boiling point: 212 ºF Kelvin (K) Coined by Lord Kelvin / Sir William Thompson Absolute temperature scale Rankine Coined by William Macquorn Rankine Thermodynamics Temperature Conversion Formulas: Temperature Change:
∆ T C =
∆ T F
∆ T F =
∆ TC
DENSITY
Mass Density of a material is defined as the mass per unit volume of the material
ρm =
m
V
Weight Density or specific weight of a material is defined as the weight per unit volume of the material.
ρw =
W
V
SPECIFIC VOLUME
Specific volume is the volume per unit mass
v =
V
m
SPECIFIC GRAVITY
Specific gravity of relative density of a
substance is the ratio of the density of
the substance to the density of some
standard substance. The standard is
usually water at 4ºC for liquids and
solid, while for gases, it is usually
air.
sp gr =
ρstandard
PRESSURE
Pressure is defined as the force per unit
area.
P =
F
A
, N / m
2
P = ρgh
Where:
P = Pressure ρ = Density (kg/ m^3 )
F = Force g = gravity (m/ s^2 ¿
A = Area h = height (m) Absolute pressure is the total pressure exerted on the system. Gauge pressure is the amount by which the absolute pressure exceeds atmospheric pressure
P |¿|= Patm + Pgauge ¿
AVOGADRO’S LAW
At equal volume, under the same pressure and temperature conditions, gases contain the same number of molecules
m 1
m 2
M 1
M 2
R 1
R 2
Where m = mass M = molecular weight R = gas constant IV. Processes A process is any change that a system undergoes from one equilibrium state to another. A path refers to the series of states through which a system passes during a process. A reversible process is one that is performed in such a way that at the end of the process, both the system and the local surroundings can be restored to their initial states. A process that does not meet these requirements is said to be irreversible. Types of processes:
1. Isobaric Process: a process by which the sate variable of a system is changed while the pressure is held constant. 2. Isovolumic process: also known as isometric or isochoric process, is a process carried out at constant volume 3. Adiabatic process: is one which no heat or other energy is transferred to or from the system. a. Isentropic process: is an adiabatic process in which there is no change in the system entropy. b. Throttling process: is an adiabatic process in which there is no change in the system enthalpy but for which there is significant pressure drop 4. Isothermal process: is a process carried out at constant temperature. Polytropic Process: A polytropic process is one that obeys the polytropic equation of state
PV
n
= C
Where: P = pressure V = volume n = polytropic exponent n = 0; Isobaric process n = 1; Isothermal process n = k; Isentropic process
n = ∞ ; Isometric process
V. Heat and entropy Heat, (Q) Heat is a form of transferred energy that arises from the random motion of molecules. Transmission of heat
- Conduction – mode of transfer in which heat transfer takes place from molecule to molecule through a body or through bodies in contact.
- Convection – transmission in which the transfer is due to the motion of molecules of the medium.
- Radiation – mode of transfer in high the heat transfer takes place without any intervening medium. Latent heat: Latent heat is the amount of heat necessary to change the phase of the system without changing its temperature.
QL = ± m ( H ) fusion ∨ vaporization
Use (+) if heat is absorbed by the substance (substance melts) Use (-) if heat released by the substance (substance freeze) Where: Q – Heat needed m – Mass H – Latent heat (fusion or vaporization) Latent heat of fusion is the heat necessary to change a unit mass of a substance from solid to liquid state or from liquid to solid state at its melting point.
Hf = 80
cal
gm
= 144 BTU/lb = 334 kJ/kg Sensible Heat Sensible heat is the amount of heat necessary to change the temperature of the system without changing its phase.
Qs = mc ∆ T
Where: Q = heat needed c = specific heat of the substance
∆ T = change in temperature
Specific heat is the amount of heat required to raise the temperature of 1gm of the substance by 1ºC. For water and ice:
cw = 1
cal
g ∙ ℃
ci =0.
cal
g ∙℃
Total heat The total heat entering a substance is the sum of the heat that changes the phase the substance (latent heat) and the heat that changes the temperature of the substance (sensible heat)
QT = QL + QS
ENTROPY
Absolute Entropy is a measure of the energy that is no longer available to perform useful work within the current environment. Other definition is that it is measure of “randomness” or “disorder” of the system.
S =
Q
T K
Where: S – Entropy (J/K) Q – Heat (J) T – Temperature (K) Change in entropy ISOBARIC Process
∆ S = mC p ∈(
T 2
T 1
Change in Entropy ISOMETRIC process
First Law equation for ISOBARIC Process ΔQ = ΔU + pΔV First law equation for ISOVOLUMIC PROCESS ΔQ = ΔU ΔW,since ΔV= Note: for an isovolumic (also called Isochoric or isometric) process, any heat flow into the system appears as increased internal energy of the system First law equation for ISOTHERMAL PROCESS
∆ Q = ∆ W = p 1 V 1 ∈
V 2
V 2
First law equation for adiabatic PROCESS 0 = ΔU + ΔW
3. Second Law of Thermodynamics Heat energy flows spontaneously from hotter to a colder object, but not vice versa. No heat engine that cycle continuously can change all its input energy to useful work. If a system undergoes spontaneous change, it will change in such a way that its entropy will increase or at best remain constant. 4. Third law of thermodynamics The absolute entropy of a pure substance approaches zero as the absolute thermodynamic temperature approaches zero.
lim
T → 0 K
s = 0
VIII. Cycles A cycle is a series of processes that eventually brings the system back to its original condition Thermal Efficiency The thermal efficiency of a power cycle is defined as the ratio of the useful work output to the supplied input energy.
ηthermal =
network output
energy input
W net
Qinput
Q ¿− Qout
Q ¿
Carnot cycle The carnot cycle is the most efficient power cycle. The efficiency of carnot cycle is the maximum possible for any power cycle. Processes: Process 1-2: Isothermal Head Addition Process Process 2-3: Isentropic Expansion Process Process 3-4: Isothermal Heat Rejction Process Process 4-1: Isentropic Compression Process Efficiency Equation:
W net
Q A
T H − T L
T H
Where:
η – Efficiency
W net – Net work
QA – Heat added
T H – High temperature
T L – Low temperature
Notes: Efficiency is increased by rising the temperature
T H at which heat is added or by lowering the
temperature T L at which heat is rejected.
Other thermodynamics cycles: Rankine Cycle Otto Cycle Diesel Cycle Brayton Cycle Reversed Carnot Cycle REVIEW QUESTIONS:
- The term “thermodynamics” comes from Greek words “therme” and “dynamis” which means _______. a. Heat power b. Heat transfer c. Heat energy d. Heat motion
- Any characteristic of a thermodynamics system is called a _____. a. Property b. Process c. Phase d. Cycle
- A system is in ______ equilibrium of its chemical composition does not change with time, i.e., no chemical reaction occurs. a. Chemical b. Thermal c. Mechanical d. Phase
- “The state of a simple compressible system is completely specified by two independent, intensive properties”. This is known as ______. a. Equilibrium postulate b. State postulate c. Environment postulate d. Compressible system postulate
- What is a process in which the system remains infinitesimally closed to an equilibrium state at all times? a. Path equilibrium process b. Cycle equilibrium process c. Phase equilibrium process d. Quasi-state or quasi- equilibrium process
- What is defined as a process during which a fluid flows through a control volume steadily? a. Transient-flow process b. Steady and uniform process c. Uniform-flow process d. Steady-flow process
- Who coined the word “energy” in 1807? a. William Rankine b. Rudolph Clausius c. Lord Kelvin d. Thomas Young
- The electrons in an atom which rotate about the nucleus possess what kind of energy? a. Translational energy b. Spin energy c. Rotational kinetic energy d. Sensible energy
- Who formulated the zeroth law of thermodynamics in 1931? a. A. Celsuis
b. Sonny Carnot c. Sadi Carnot d. Suri Carnot
- “The total volume of a mixture of non- reacting gases is equal to the sum of the partial volumes.” This statement is known as ______. a. Law of Dulong and Petit b. Maxwell-Boltzmann law c. Amagat’s law d. Avogadro’s law
- A simple steam engine receives steam from the boiler at 180˚C and exhausts directly into the air at 100˚C. What is the upper limit of its efficiency? a. 11.28 % b. 36.77 % c. 20.36 % d. 17.66 %
- A gas is enclosed in a cylinder with a weighted piston as the top boundary. The gas is heated and expands from a volume of 0. m3 to 0.10 m3 at a constant pressure of 200 kPa. Find the work done on the system. a. 5 kJ b. 15 kJ c. 10 kJ d. 12 kJ
- The flow energy of 150 L of a fluid passing a boundary to a system is 110 kJ. Determine the pressure at this point a. 733.33 kPa b. 833.33 kPa c. 933.33 kPa d. 633.33 kPa 24. If a system absorbs 500 cal of heat at the same time does 400J of work, find the change in internal energy of the system. a. 1400 J b. 1700 J c. 1900 J d. 1500 J 25. Twenty grams of ice at 0˚C melts to water at 0˚C. How much does the entropy of the 20g change in this process? a. 30.5 J/K b. 24.6 J/K c. 21.3 J/K d. 15.7 J/K
TAKE HOME EXAM! Answer to this exercise will be provided next meeting
- It states that if an external stress is applied to a system at equilibrium, the system adjusts in such a way to decrease the effect of the stress. a. Hess’s law b. Raoult’s law c. Le Chatelier’s Principle d. Carnot cycle
- In the process of freeze drying, ice goes directly into water vapor. The temperature at which this can take place is ____. a. Below the triple point of water b. At the triple point of water c. Above the triple point of water d. Any of the above, depending on the pressure 3. The amount of energy needed to change a given mass of ice to water at constant temperature is called the heat of ___. a. Fusion b. Formation c. Condensation d. Crystallization 4. A 1-kg steam water mixture at 1.0 MPa is contained in an inflexible tank. Heat is added until the pressure rises to 3.5MPa and the temperature to 400º. Determine the heat added. a. 1378.7 kJ c. 1348.5kJ b. 1278.7kJ d. 1246.5kJ 5. If the ΔH of a reaction is a negative quantity, the reaction is definitely ____. a. Endothermic c. exothermic b. Unstable d. reversible 6. Which of the following equation of state has two constants? a. Vander waals equation b. Beattie-Bridgemen Equation c. Benedict-Webb-Rubin Equation d. Strobridge Equation 7. How many constant are there in strobridge equation of state? a. 8 b. 14 c. 13 d. 16 8. How many constant are there in Benedict-Webb- Rubin equation of state? a. 8 b. 14 c. 13 d. 16 9. Ten lb of ice at 0ºF is added to 100 lb of water at 50ºF. the temperature of the resulting mixture is ___. a. 19ºF b. 31ºF c. 32ºF d. 34ºF 10. A piston cylinder device initially contains
0.8 m^3 of air at 110kPa and 80ºC. The air is
1.5 ft^3 , at a temperature of 320K. How much work
is done? a. -8.12kJ c.-9.61kJ b. -10.44kJ d.-11.59kJ
24. How many moles of O 2 are present in 44.8L of
O 2 at STP?
a. 1.2moles b.1.4moles b. 2.0moles d.2.8moles
- It is impossible to convert heat completely into work. a. 1 st^ law of thermodynamics b. 2 nd^ law of thermodynamics c. 3 rd^ law of thermodynamics d. Zeroth law of thermodynamics
- A carnot engines turns heat into work ___. a. With 100% efficiency b. With 0% efficiency c. Without itself undergoing a permanent change d. With the help of expanding steam
- Which of the following engines is the most efficient? a. Gasoline engine b. Diesel engine c. Gas turbine d. Carnot cycle
- Which of the following engines is the least efficient? a. Gasoline engine b. Diesel engine c. Gas turbine d. Carnot cycle
- What pressure must be applied to 225ml of gas at 1atm to reduce its volume to 100ml? a. 0.44atm c. 22500atm b. 2.25atm d. 1710atm
- A frictionless heat engine can be 100% efficient only if its exhaust temperature is a. Equal to its input temperature b. Less than its input temperature c. 0ºC d. 0K
- A process not involved in the operating cycle of a Carnot engine is __ a. An isothermal expansion b. An isobaric expansion c. An adiabatic expansion d. An adiabatic expansion
- The value for the ΔU of a system is -120J. If the system is known to have absorbed 420J of hat, how much work was done? a. -540J b.-640j c.-740J d.-840J
- The fuel in a diesel engine is ignited by___ a. A spark plug b. Being compressed until it is hot enough c. The hot compressed air into which it is injected d. Exhaust gases remaining from the previous cycle
- The volume of a gas sample is proportional to its ___ a. Fahrenheit temperature b. Celsius temperature c. Absolute temperature d. Pressure
- It states that the vapour pressure in a solution is the sum of the vapour pressure of the pure component multiplied by the mole fraction of that component.
a. Raoult’s law b. Henry’s law c. Hess’ law d. Zeroth law of thermodynamios
36. If the absolute pressure on 10 ft^3 of air is
increased from 30psi to 120psi, the new volume of the air will be __.
a. 2.5 ft^3 b.5 ft^3 c.40 ft^3 d.900 ft^3
- An absolute temperature of 100K is the same as a Celsius temperature of __. a. -173ºC b.32ºC c.212ºC d.373ºC
- The boiling point of water on the rankine scale is a. -248ºR b.-61ºR c.485ºR d.672ºR
- When a vapor condense into a liquid, a. It absorbs heat b. It evolves heat c. Its temperature rises d. Its temperature drops
- The heat of vaporization of a substance is ___. a. Less than its heat of fusion b. Equal to its heat of fusion c. Greater than its heat of fusion d. Any of the above, depending on the nature of the substance
- A refrigerator exhausts . a. Less heat thgan it absorbs from its contents b. The same amount of heat absorbs from its content c. More heat that it absorbs from its contents d. Any of the above, depending on the circumstances 42. The principle that the amount of gas dissolved under equilibrium in a volume of liquid is in direct proportion to the pressure of the gas that contact the liquid surface. a. Raoult’s law b. Henry’s law c. Hess’law d. Zeroth law of thermodynamics 43. The heat of vaporization of a substance is a. Less than its heat of fusion b. Equal to its heat of fusion c. Greater than its heat of fusion d. Any of the above, depending on the nature of the substance 44. A process in which the system is always infinitesimally close to equilibrium. Such a process can never be observed, it is only of theoretical interest. a. Spontaneous process b. Reversible process c. Irreversible process d. Exothermic process 45. Absolute zero may be regarded as that temperature at which. a. Water freezes b. All gases become liquids c. All substances are solid d. Molecular motion in a gas would be the minimum possible
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