Nothing is 100% efficient: The second law of thermodynamics says that whenever energy changes form, some of it is degraded, mostly into heat.
The points to remember
- The second law: every energy transformation is inefficient.
- Some energy is always degraded into a less useful form, usually heat.
- The amount of energy stays the same (the first law, 2.2.2), but its quality falls.
- Light and chemical energy are concentrated and can do work; heat is spread out and cannot.
- So no transformation, in a machine or in a living thing, is ever 100% efficient.
Remember it as: Energy is never lost, but it is always downgraded.
High-quality energy
- Sunlight, chemical energy in food or fuel.
- Concentrated: it can do work.
Low-quality energy
- Heat, spread out in the surroundings.
- Cannot be used again for work.
Real example: a petrol car turns only about a quarter of the chemical energy in its fuel into movement. The rest is degraded to heat, which is why the engine gets hot.
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Energy enters an ecosystem as sunlight and leaves as heat: it flows through and never cycles, so a constant input from the Sun is needed. Every step along a food chain is an energy transformation, so the second law applies at every step.
The second law in a food chain
- Energy is transformed at every trophic level: chemical energy in food becomes movement, growth and heat.
- The biggest loss is heat from cellular respiration (see 2.2.7); heat cannot be passed on.
- Energy is also lost because not all of an organism is eaten, digested or absorbed (see 2.2.12).
- So transfers between levels are never 100% efficient: often only about 10% passes on.
- So higher levels hold less energy: fewer, often larger, organisms (see 2.2.17).
- After four or five levels too little energy is left for another level (see 2.2.14).
Remember it as: Every step up the chain, most of the energy goes up as heat.
Why it matters for food: Because energy is lost at every step, eating lower in the food chain, such as more plant foods, feeds more people from the same land (see 5.2).
Real example: in the Serengeti, the grass feeds well over a million wildebeest and zebra, but only a few thousand lions: at each step most of the energy has already been lost as heat.
Name the law, then the heat: Start with the law itself: 'energy transformations are never 100% efficient'. Then say where the energy goes: 'lost as heat from respiration', never 'destroyed' or 'used up'.
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The second law can also be written in terms of order and disorder, using entropy. Some questions credit this wording.
Order and disorder
- Entropy measures disorder: how spread out energy is.
- Energy stored in biomass is ordered and concentrated: low entropy.
- Heat is energy spread out in the surroundings: high entropy.
- Every transformation along a chain turns ordered energy into heat, so entropy increases.
- That is the second law put another way: disorder always increases.
Real example: a hot cup of tea always cools to room temperature, and never heats itself up again: heat spreads out, but never gathers back by itself.
Two ways to say it: Either wording of the law is accepted: 'transformations are never 100% efficient' or 'entropy (disorder) increases'. Then link it to heat lost at each trophic level.
How this comes up: Paper 2, Section A: a food chain. Explain how the second law applies to it [2].
Figure 1 shows a savanna food chain.
Explain how the second law of thermodynamics applies to this food chain.
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