Syllabus Edition
First teaching 2015
Last exams 2025
Laws of Thermodynamics & Environmental Systems (DP IB Environmental Systems & Societies (ESS))
Revision Note
Written by: Alistair Marjot
Reviewed by: Bridgette Barrett
The First Law of Thermodynamics
Energy exists in many different forms, including light energy, heat energy, chemical energy, electrical energy, and kinetic energy
The way in which energy behaves within systems can be explained by the laws of thermodynamics
There are two laws of thermodynamics
The first law of thermodynamics is as follows:
Energy can neither be created nor destroyed, it can only be transformed from one form to another
This is also known as the principle of conservation of energy
It means that the energy entering a system equals the energy leaving it
The transfer of energy in food chains within ecosystems demonstrates the principle of conservation of energy
Energy enters the system (the food chain or food web) in the form of sunlight
Producers convert this light energy into biomass (stored chemical energy) via photosynthesis
This chemical energy is passed along the food chain, via consumers, as biomass
All energy ultimately leaves the food chain, food web or ecosystem as heat energy
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 is 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
The second law of thermodynamics explains the decrease in available energy within ecosystems
In a food chain, for example, energy is transformed from a more concentrated (ordered) form (e.g. light energy the Sun), into a more dispersed (disordered) form (heat energy)
Initially, light energy from the Sun is absorbed by producers
However, even at this initial stage, energy absorption and transfer by producers is inefficient due to reflection, transmission (light passing through leaves) and inefficient energy transfer during photosynthesis
The energy that is converted to plant biomass is then inefficiently transferred along the food chain through respiration and production of waste heat energy
As a result of these inefficient energy transfers, food chains are often short (they rarely contain more than five trophic levels)
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)
Worked Example
Explain the implications of the first and second laws of thermodynamics for a named ecological system.
Answer
The implications of the first and second laws of thermodynamics on the Arctic tundra ecosystem can be explained as follows:
First Law of Thermodynamics:
The first law states that energy cannot be created or destroyed, only transformed from one form to another. In the Arctic tundra ecosystem, solar energy is the primary source of energy, which is captured by the plants through photosynthesis and transformed into chemical energy.
The herbivores then consume the plants and obtain the chemical energy stored in the plants, while the carnivores consume the herbivores and obtain the chemical energy stored in their bodies. Therefore, in the Arctic tundra ecosystem, the energy flow follows the principle of the first law of thermodynamics.
Second Law of Thermodynamics:
The second law states that in every energy transfer or transformation, some energy is lost as unusable energy, such as heat or waste. In the Arctic tundra ecosystem, energy loss occurs at every trophic level due to inefficient energy conversion and energy loss as heat during respiration. As a result, the amount of energy available to the next trophic level decreases, leading to a decrease in the number of individuals in higher trophic levels.
This also means that the energy available to the top predators in the Arctic tundra ecosystem is much lower than that available to the producers, and the number of predators is limited due to the scarcity of energy. Therefore, the second law of thermodynamics limits the complexity and carrying capacity of the Arctic tundra ecosystem
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