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is a measure of disorder in systems, quantifying randomness and the number of possible configurations. It's a key concept in thermodynamics, relating to energy distribution and the natural tendency of systems to become more disordered over time.

The second law of thermodynamics states that the total of an isolated system always increases. This principle explains why certain processes occur spontaneously and others don't, shaping our understanding of heat flow and energy conversion in natural and engineered systems.

Entropy and the Second Law of Thermodynamics

Measure of disorder in systems

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  • Entropy quantifies the degree of disorder or randomness in a system
    • Higher entropy indicates greater disorder (shuffled deck of cards), while lower entropy indicates a more ordered state (new deck of cards in sequence)
  • Entropy relates to the number of possible microscopic configurations or a system can have
    • A system with more possible microstates has higher entropy (a room with toys scattered vs. toys organized in bins)
  • In a closed system, entropy tends to increase over time
    • There are typically more ways for a system to be disordered than ordered (a broken glass vs. an intact glass)
  • Entropy is represented by the symbol SS and measured in units of joules per kelvin (J/K)
    • The entropy of a system can be calculated using or thermodynamic equations
  • The Boltzmann constant (k) relates the entropy of a system to the number of possible microstates

Entropy changes in thermodynamic processes

  • The change in entropy ([ΔS](https://www.fiveableKeyTerm:ΔS)[ΔS](https://www.fiveableKeyTerm:ΔS)) for a is calculated using [ΔS = Q/T](https://www.fiveableKeyTerm:ΔS_=_Q/T)
    • QQ is the heat transferred during the process in joules (J)
    • TT is the absolute temperature at which the heat transfer occurs in kelvin (K)
  • For an (constant temperature), entropy change is ΔS=Q/TΔS = Q/T
    • Adding heat to the system (Q>0Q > 0) increases entropy (ΔS>0ΔS > 0) (ice melting at constant temperature)
    • Removing heat from the system (Q<0Q < 0) decreases entropy (ΔS<0ΔS < 0) (water freezing at constant temperature)
  • For an (no heat transfer), entropy change is zero (ΔS=0ΔS = 0)
    • Since Q=0Q = 0 in the equation ΔS=Q/TΔS = Q/T (rapidly compressing a gas in an insulated container)

Second law and process reversibility

  • The second law of thermodynamics states that the total entropy of an isolated system always increases over time
    • The entropy of the universe tends to increase in any spontaneous process (a drop of ink diffusing in water)
  • The second law implies that heat flows naturally from a hotter body to a colder body, but not in reverse without external work
    • Heat transfer from hot to cold increases the entropy of the system (a hot cup of coffee cooling down to room temperature)
  • Reversible processes are idealized processes where the system and surroundings can be restored to their original states without any net change in entropy
    • In reality, all processes are irreversible to some extent due to factors like friction and heat loss (a pendulum swinging back and forth, eventually coming to a stop)
  • Irreversible processes always result in an increase in the total entropy of the system and its surroundings
    • Examples of irreversible processes:
      1. Spontaneous heat transfer (a hot object cooling down)
      2. Mixing of gases (opening a perfume bottle in a room)
      3. Chemical reactions (burning of fuel)

Thermodynamic equilibrium and energy conversion

  • is the state where a system has uniform temperature and no net flow of matter or energy
  • Heat engines convert thermal energy into mechanical work, utilizing temperature differences
  • Free energy is the amount of work a thermodynamic system can perform at constant temperature and pressure
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© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
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