IB Environmental systems and societies 2.2C Second Law of Thermodynamics Notes

The Second Law: every energy transfer wastes usable energy as heat

What the Second Law says

1. The Second Law of Thermodynamics states that in every energy transformation some usable energy is degraded and dispersed as low-quality heat.
2. It is about the quality of energy falling, not its quantity.
   1. The First Law (in 2.2A) says the total amount of energy is conserved.
   2. The Second Law says the amount that stays useful falls after each transfer.
3. As energy is transformed it becomes more spread out, so entropy (disorder) increases.

Energy quality falls at every step, even though the total quantity of energy never changes.

Why energy is lost at every trophic transfer

1. Energy enters an ecosystem as concentrated, high-quality sunlight and is fixed into chemical energy in biomass by photosynthesis.
2. At each transfer up a food chain, a large share of that chemical energy is degraded to heat and lost from the ecosystem.
   1. Most energy is lost as heat from cellular respiration as organisms release energy for movement, growth, and life processes.
   2. Not all of a trophic level is eaten, because some organisms die or are not caught.
   3. Not all that is eaten is digested and absorbed, so undigested food leaves the body as faeces (egestion).
3. This lost heat cannot be recaptured, so energy flow through an ecosystem is one-directional and cannot be recycled.

Why food chains are short and only about 10% passes on

1. Because so much energy is degraded at each step, only a small fraction reaches the next trophic level.
2. On average about 10% of the energy in one trophic level reaches the next, though real values vary widely.
   1. This 10% figure is a rough average, not a law, and the worked calculation is covered in 2.2D.
3. So little energy remains after a few transfers that there is not enough to support many levels, which is why food chains are short.

Short food chains are a direct consequence of the Second Law, because usable energy runs out quickly.

How ecosystems stay ordered despite rising entropy

1. Living things are highly ordered, which is a state of low entropy.
2. Building and maintaining that order constantly degrades energy to heat, so it cannot continue on its own.
3. Ecosystems keep their low-entropy order only because of a constant input of solar energy.
4. Matter can be recycled within an ecosystem, but energy cannot, because it leaves as heat.

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