Thermodynamics 2nd Law Physics Lesson 21 by Owen Borville 12.18.2025
A reversible process is one in which both the system and its environment can return to the states they were in exactly by following the reverse path. An irreversible process is one in which the system and its environment cannot return together to exactly the states that they were in. The irreversibility of any natural process results from the second law of thermodynamics.
The work done by a heat engine is the difference between the heat absorbed from the hot reservoir and the heat discharged to the cold reservoir: W = Qhot-Qcold = Qh-Qc. This is the result of energy conservation. The ratio of the work done by the engine and the heat absorbed from the hot reservoir provides the efficiency of the engine: e (EFF) = W/Qh = 1-Qc/Qh
The second law of thermodynamics allows the ability to heat an interior space using a heat pump. Heat pumps compress cold air and heat it to room temperature without violation of conservation principles. A refrigerator or a heat pump is a heat engine run in reverse. The focus of a refrigerator is on removing the heat from the cold reservoir with a coefficient of performance KR = Qc/W = Qc/Qh-Qc = Tc/Th-Tc. The focus of a heat pump is on dumping heat to the hot reservoir with a coefficient of performance KP = Qh/W = Qh/Qh-Qc = Th/Th-Tc.
2nd Law of Thermodynamics: (1) Heat transfer occurs spontaneously from higher to lower temperature bodies but never spontaneously in the reverse direction. (2) It is impossible in any system for heat transfer from a reservoir to completely convert to work in a cyclical process in which the system returns to its initial state.
Irreversible processes depend on path and do not return to their original state. Cyclical processes are processes that return to their original state at the end of every cycle. In a cyclical process, such as a heat engine, the net work done by the system equals the net heat transfer into the system W = Qh-Qc, where Qh is the heat transfer from the hot object or reservoir and Qc is the heat transfer into the cold object or reservoir.
The four-stroke gasoline engine is often explained in terms of the Otto cycle, which is a repeating sequence of processes that convert heat into work.
The Kelvin statement of the second law of thermodynamics: It is impossible to convert the heat from a single source into work without any other effect. The Kelvin statement and Clausius statement of the second law of thermodynamics are equivalent.
The Carnot Cycle is the most efficient engine for a reversible cycle designed between two reservoirs. The Carnot Principle is another way of stating the second law of thermodynamics. The resulting efficiency of a Carnot Cycle is e = 1-Tc/Th The Carnot cycle is a theoretical cycle that is the most efficient cyclical process possible. An engine using the Carnot cycle, which uses only reversible processes (adiabatic and isothermal), is known as a Carnot engine. Any engine that uses the Carnot cycle enjoys the maximum theoretical efficiency.
While Carnot engines are ideal engines, in reality, no engine achieves Carnot's theoretical maximum efficiency, since dissipative processes, such as friction, affect performance. Carnot cycles without heat loss may be possible at absolute zero, but this has never been seen in nature.
Entropy is the loss of energy available to do work. Another form of the second law of thermodynamics states that the total entropy of a system either increases or remains constant and never decreases. Entropy is zero in a reversible process and it increases in an irreversible process. The ultimate fate of the universe is likely to be thermodynamic equilibrium, where the universal temperature is constant and no energy is available to do work. Entropy is also associated with the tendency toward disorder in a closed system.
The change in entropy for a reversible process at constant temperature is equal to the heat divided by the temperature. The entropy of a system undergoing a reversible process at a constant temperature is ΔS = Q/T. The entropy change of a system under a reversible process is = ΔS = SB-SA = ∫ (B-A) dQ/T. The entropy of a system undergoing any complete reversible cyclic process is ∫dS = dQ/T = 0.
A system's change in entropy between two states is independent of the reversible thermodynamic path taken by the system when it makes a transition between the states.
Entropy can be related to how disordered a system is. The more a system is disordered, the higher its entropy. In any irreversible process, the universe becomes more disordered. Disorder is far more likely than order, according to statistical principles. The entropy of a system in a given state or macrostate is = S = klnW, where k = 1.38x10^-23 J/K is Boltzmann's constant, and lnW is the natural logarithm of the number of microstates W corresponding to the given macrostate.
The change in entropy of a closed system under an irreversible process is ΔS >= 0.
The change in entropy of the system along an isotherm is zero if the system's state does not change, such as at equilibrium or absolute zero. The total entropy of a system and its surroundings is zero for reversible isothermal processes.
The Third Law of Thermodynamics states that absolute zero temperature is unreachable.
A reversible process is one in which both the system and its environment can return to the states they were in exactly by following the reverse path. An irreversible process is one in which the system and its environment cannot return together to exactly the states that they were in. The irreversibility of any natural process results from the second law of thermodynamics.
The work done by a heat engine is the difference between the heat absorbed from the hot reservoir and the heat discharged to the cold reservoir: W = Qhot-Qcold = Qh-Qc. This is the result of energy conservation. The ratio of the work done by the engine and the heat absorbed from the hot reservoir provides the efficiency of the engine: e (EFF) = W/Qh = 1-Qc/Qh
The second law of thermodynamics allows the ability to heat an interior space using a heat pump. Heat pumps compress cold air and heat it to room temperature without violation of conservation principles. A refrigerator or a heat pump is a heat engine run in reverse. The focus of a refrigerator is on removing the heat from the cold reservoir with a coefficient of performance KR = Qc/W = Qc/Qh-Qc = Tc/Th-Tc. The focus of a heat pump is on dumping heat to the hot reservoir with a coefficient of performance KP = Qh/W = Qh/Qh-Qc = Th/Th-Tc.
2nd Law of Thermodynamics: (1) Heat transfer occurs spontaneously from higher to lower temperature bodies but never spontaneously in the reverse direction. (2) It is impossible in any system for heat transfer from a reservoir to completely convert to work in a cyclical process in which the system returns to its initial state.
Irreversible processes depend on path and do not return to their original state. Cyclical processes are processes that return to their original state at the end of every cycle. In a cyclical process, such as a heat engine, the net work done by the system equals the net heat transfer into the system W = Qh-Qc, where Qh is the heat transfer from the hot object or reservoir and Qc is the heat transfer into the cold object or reservoir.
The four-stroke gasoline engine is often explained in terms of the Otto cycle, which is a repeating sequence of processes that convert heat into work.
The Kelvin statement of the second law of thermodynamics: It is impossible to convert the heat from a single source into work without any other effect. The Kelvin statement and Clausius statement of the second law of thermodynamics are equivalent.
The Carnot Cycle is the most efficient engine for a reversible cycle designed between two reservoirs. The Carnot Principle is another way of stating the second law of thermodynamics. The resulting efficiency of a Carnot Cycle is e = 1-Tc/Th The Carnot cycle is a theoretical cycle that is the most efficient cyclical process possible. An engine using the Carnot cycle, which uses only reversible processes (adiabatic and isothermal), is known as a Carnot engine. Any engine that uses the Carnot cycle enjoys the maximum theoretical efficiency.
While Carnot engines are ideal engines, in reality, no engine achieves Carnot's theoretical maximum efficiency, since dissipative processes, such as friction, affect performance. Carnot cycles without heat loss may be possible at absolute zero, but this has never been seen in nature.
Entropy is the loss of energy available to do work. Another form of the second law of thermodynamics states that the total entropy of a system either increases or remains constant and never decreases. Entropy is zero in a reversible process and it increases in an irreversible process. The ultimate fate of the universe is likely to be thermodynamic equilibrium, where the universal temperature is constant and no energy is available to do work. Entropy is also associated with the tendency toward disorder in a closed system.
The change in entropy for a reversible process at constant temperature is equal to the heat divided by the temperature. The entropy of a system undergoing a reversible process at a constant temperature is ΔS = Q/T. The entropy change of a system under a reversible process is = ΔS = SB-SA = ∫ (B-A) dQ/T. The entropy of a system undergoing any complete reversible cyclic process is ∫dS = dQ/T = 0.
A system's change in entropy between two states is independent of the reversible thermodynamic path taken by the system when it makes a transition between the states.
Entropy can be related to how disordered a system is. The more a system is disordered, the higher its entropy. In any irreversible process, the universe becomes more disordered. Disorder is far more likely than order, according to statistical principles. The entropy of a system in a given state or macrostate is = S = klnW, where k = 1.38x10^-23 J/K is Boltzmann's constant, and lnW is the natural logarithm of the number of microstates W corresponding to the given macrostate.
The change in entropy of a closed system under an irreversible process is ΔS >= 0.
The change in entropy of the system along an isotherm is zero if the system's state does not change, such as at equilibrium or absolute zero. The total entropy of a system and its surroundings is zero for reversible isothermal processes.
The Third Law of Thermodynamics states that absolute zero temperature is unreachable.