Reversible and are key concepts in thermodynamics. They help us understand how energy changes and flows in real-world systems. are ideal, while irreversible ones reflect reality.
The second law of thermodynamics ties these ideas to entropy, which always increases in isolated systems. This law explains why some processes happen spontaneously and others don't, shaping our understanding of energy efficiency and natural phenomena.
Reversible and Irreversible Processes
Reversible vs irreversible processes
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Reversible processes occur infinitely slowly, allowing the system to remain in with its surroundings at all times
Can be reversed without any net change in the system or surroundings
Require infinitesimal changes in thermodynamic variables (pressure, volume, temperature)
Idealized processes that do not occur in reality but serve as a theoretical limit (frictionless motion, perfect insulation)
Irreversible processes occur at a finite rate, causing the system to deviate from equilibrium with its surroundings
Cannot be reversed without net changes in the system or surroundings
Involve finite changes in thermodynamic variables
Real-world processes that always result in an increase in entropy (heat transfer, gas expansion, mixing)
An adiabatic process, where no heat is exchanged with the surroundings, is typically irreversible in practice
Second law and irreversibility
The second law of thermodynamics states that the total entropy of an always increases over time
Entropy measures the disorder or randomness of a system (gas molecules, energy distribution)
Irreversible processes always lead to an increase in the total entropy of the universe
Heat transfer from a hot object to a cold object, gas expansion into a vacuum, and mixing of two gases increase entropy
Heat naturally flows from a higher temperature to a lower temperature
This process is irreversible, as heat cannot spontaneously flow from a colder object to a hotter object without external work being done
The second law places constraints on the direction of heat flow and the efficiency of heat engines
No heat engine can be 100% efficient, as some heat must always be rejected to a low-temperature reservoir (exhaust, cooling towers)
The Carnot cycle represents the most efficient theoretical heat engine operating between two temperature reservoirs
Entropy and process spontaneity
The change in entropy (ΔS) determines the spontaneity of a process
For a , ΔSuniverse>0 (ice melting, gas expanding)
For a , ΔSuniverse<0 (water freezing, gas compressing)
For a process at equilibrium, ΔSuniverse=0 (sealed container, isolated system)
The entropy change of the universe is the sum of the entropy changes of the system and its surroundings
ΔSuniverse=ΔSsystem+ΔSsurroundings
The entropy change of a system can be calculated using the following equations:
For a reversible process: ΔS=∫TdQrev, where dQrev is the heat exchanged reversibly and T is the absolute temperature
For an irreversible process: ΔS>∫TdQ, where dQ is the actual heat exchanged
The states that for any cyclic process, ∮TdQ≤0, with equality holding only for reversible processes
The entropy change of the surroundings can be calculated using: ΔSsurroundings=−TsurroundingsQ, where Q is the heat exchanged with the surroundings and Tsurroundings is the absolute temperature of the surroundings
Thermodynamic potentials and efficiency
is a thermodynamic potential that combines the internal energy, temperature, and entropy of a system
It helps determine the spontaneity and maximum work output of processes at constant temperature and pressure
measures the ratio of useful work output to total energy input in a thermodynamic process or cycle
It is always less than 100% due to irreversibilities and heat losses