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Class 9 Earth as a System: Energy, Matter, and Life: cheat sheet
CBSE Science. The Sun powers the Earth system: how sunlight is reflected and absorbed, why land and sea heat differently, the cycles of matter, and human impact.
Key ideas
- About 30% of incoming sunlight is reflected to space; the rest is absorbed by the atmosphere and the surface.
- Albedo is the fraction of light a surface reflects: high for snow, low for oceans and dark roads.
- Water has a high specific heat, so land heats and cools faster than the sea: sea breezes by day, land breezes by night.
- Matter cycles: water (evaporation, transpiration, condensation, precipitation), carbon and oxygen (photosynthesis, respiration, burning), nitrogen (fixation, nitrification, denitrification).
- Greenhouse gases trap heat; burning fossil fuels and cutting forests add carbon dioxide and warm the climate.
- In a food chain only about 10% of the energy passes to the next level.
Remember
| Heat needed | Q = m × c × ΔT (water c = 4200 J/kg °C) |
| Absorbed sunlight | incoming × (100% − albedo) |
| Rainfall | 1 mm of rain on 1 m² = 1 litre |
| 10% rule | energy at the next level = energy × 0.1 |
Watch out
- Saying the breeze blows from the hotter side: air flows towards the hotter land, where warm air rises.
- Mixing up nitrogen fixation (gas to compounds) and denitrification (compounds to gas).
- Confusing the greenhouse effect (natural) with global warming (the extra rise people cause).
Worked example
The plants in a forest capture 7,000 kJ of energy. Using the 10% rule, how much reaches the top carnivores (third consumers)?
Working: 10% passes to each next level: 7,000 → 700 → 70 → 7 kJ; the herbivores (e.g. deer) get 700 kJ
Answer: 7 kJ
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