Entropy of Systems (HL) (HL IB ESS OLD COURSE - IGNORE)

Revision Note

Alistair Marjot

Written by: Alistair Marjot

Reviewed by: Bridgette Barrett

Entropy of Systems

The second law of thermodynamics

  • The second law of thermodynamics is as follows:

The entropy of a system increases over time

  • Entropy is a measure of the amount of disorder in a system

  • As entropy increases (through inefficiencies in energy transformations) the energy available to do work decreases

    • This is because the transformation and transfer of energy in any system is never 100% efficient

  • In other words, in any energy conversion, the amount of useable energy at the end of the process is always less than the amount of energy available at the start

Diagram showing the first and second laws of thermodynamics
The laws of thermodynamics—energy cannot be created or destroyed (it can only be transformed) and energy is always lost from systems when work is done, bringing about disorder (increasing entropy)

Entropy within ecosystems

Energy loss and biomass in ecosystems

  • Energy transfer in ecosystems is not 100% efficient

  • When energy passes from one trophic level to the next, a significant amount is lost as heat

    • This energy loss increases the entropy of the system

  • Biomass decrease: as energy moves up the food chain, there is less biomass available at each successive trophic level

  • This is a result of the energy losses and increased entropy

Entropy and cellular respiration

  • Cellular respiration is the process by which living organisms convert glucose and oxygen into energy, with carbon dioxide and water as by-products.

    • During respiration, energy is released as heat, which increases the entropy of the system

  • Organisms maintain low entropy within their cells and bodies (high organisation) by using energy from food

    • But in doing so, they release heat and increase the entropy of the environment

  • Net increase in entropy:

    • While organisms can maintain internal order (low entropy), the overall entropy of the system (including the surrounding environment) increases due to the energy lost as heat

Examiner Tips and Tricks

Remember that the second law of thermodynamics is about energy loss and increased entropy, not energy destruction (energy is always conserved, but it becomes less useful).

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Alistair Marjot

Author: Alistair Marjot

Expertise: Biology & Environmental Systems and Societies

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.

Bridgette Barrett

Author: Bridgette Barrett

Expertise: Geography Lead

After graduating with a degree in Geography, Bridgette completed a PGCE over 25 years ago. She later gained an MA Learning, Technology and Education from the University of Nottingham focussing on online learning. At a time when the study of geography has never been more important, Bridgette is passionate about creating content which supports students in achieving their potential in geography and builds their confidence.