Chapter 10: Thermodynamics. 10-1 Relationship Between Heat and Work In a closed system there’s a...
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Transcript of Chapter 10: Thermodynamics. 10-1 Relationship Between Heat and Work In a closed system there’s a...
Chapter 10: Thermodynamics
10-1 Relationship Between Heat and Work
• In a closed system there’s a direct relationship between heat and work. Heat and work both transfer energy to or from a system.
• Key idea: A system never has “heat” or “work”, it has internal energy which is affected by heat in/out or work done on/by the system
Transfer of heat and work• System: a set of particles or interacting
particles considered to be a distinct physical entity
• Environment: the combination of conditions and influences outside a system that affects the behavior of the system
• Ex of closed systems: a gas confined in a cylinder by a piston, a calorimeter, a thermos…
Work Done on or By a Gas
• Is represented in the equation:
W = PΔV
W - in Joules (J) P = pressure in Pascal (Pa) 1 Pa = 1 N/m2
ΔV= volume change in (m3)
Sample Problem
• Gas in a container is at a pressure of 1.6x105 Pa and a volume of 4.0 m3. What is the work by the gas if it expands at a constant pressure to twice its initial volume?
Solution
P= 1.6x105 Pa =1.6x105 N/m2
∆V=Vf – Vi = 8.0 m3 - 4.0 m3 =4.0 m3
W= P ∆ V
W = (1.6x105 N/m2)(4.0 m3)
W= 6.4x105 J
Thermodynamic Processes• Isovolumetric process: a
thermodynamic process that takes place at a constant volume so that no work is done on or by the system, ex: a car with closed windows parked in a hot garage.
• Isothermal process: a thermodynamic process that takes place at a constant temperature, ex usually a slow process like a balloon expanding slowly during the day.
• Adiabatic process: a thermodynamic process during which heat energy is transferred to or from the system. ex: usually a fast process like filling a tank
• Isobaric process: a process that takes place at a constant pressure. ex: heating an open pot of water
10.2 The First Law of Thermodynamics• The first law is a statement of conservation of
energy that takes into account a system’s internal energy (U) as well as the energy transfer to/from the system by work and heat.
• It is expressed as:
Signs of Q and W For a System
ΔQ = positive if heat is added to a systemΔQ = negative if heat is released from a systemΔW = positive if work is done by the systemΔW = negative if work is done on the system
First Law – Isovolumetric Process
Δ U = Q – WΔV = 0Since W = P ΔV, W = 0therefore, Δ U = Q
First Law – Isothermal Process
ΔU = Q – Wsince ΔT = 0 , ΔU = 0therefore Q = W
First Law – Adiabatic Process
Δ U = Q – WQ = 0, thereforeΔ U = – W
First Law – Isobaric Process
Δ U = Q – Wsince W = PΔVΔ U = Q – PΔV
First Law – Isolated System
Δ U = Q – Wsince Q = W = 0Δ U = 0
Sample Problem
• A total of 135 J of work is done on a gaseous refrigerant as it undergoes compression. If the internal energy of the gas increases by 114 J during the process, what is the total amount of energy transferred as heat?
Solution
W= -135 J (work done on the system is -)
∆ U= 114 J
∆ U = Q - W
Q = ∆ U + W
Q= 114 J + (-135 J)= -21 J
Q= -21 J
In this problem, energy is removed from the gas as heat, which is indicated by the negative sign on the Q value ( Q < 0 ).
Cyclic Processes
• A thermodynamic process in which a system returns to the same conditions under which it started (no change in system’s energy)
ΔUnet = 0 and Qnet = Wnet • Resembles an isothermal process in that all
energy is transferred as work and heat.
The Heat Engine
• Any device that exploits a temperature difference to do mechanical work
• The net work done is equal to the difference in energy taken in as heat from a high-temp. reservoir (Qh) and the energy expelled as heat to the low temp. reservoir(Qc).
• Wnet = Qnet = Qh − Qc
10.3 The Second Law of Thermodynamics
• States that no cyclic process that converts heat entirely into work is possible.
• Includes the requirement that a heat engine give up some energy at a lower temperature in order to do work.
• So a heat engine cannot transfer all energy as heat to do work.
Second Law of Thermodynamics
• No cyclic process that converts heat entirely into work is possible
chnetnet QQQW
Efficiency of a Heat Engine
• The smaller the fraction of usable energy that an engine can provide, the lower its efficiency is.
Qh
Qc
Qh
QcQh
Qh
Wneteff
1
heatasaddedenergy
enginebydoneworknetefficiency
Sample Problem
• Find the efficiency of a gasoline engine that, during one cycle, receives 204 J of energy from combustion and loses 153 J as heat into the exhaust.
Solution
Qh= 204 J
Qc= 153 J
eff= 1- Qc
Qh
eff= 1- 153 J 204 Jeff= 0.250 J
Entropy
• Entropy: a measure of the randomness or disorder of a system
• A greater disorder means there is less energy to do work
• The motion of the particles of a system is not well ordered and therefore is less useful for doing work
• Once a system has reached a state of the greatest disorder, it will tend to remain in that state and have maximum entropy.
• The second law of thermodynamics states that the entropy of the universe increases in all natural processes.