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Earth as a System: Energy, Matter, and Life – Class 9 Science Chapter 13 Notes & NCERT PDF

Class 9ScienceChapter 13NCERT book: Exploration - Science Class 9

Notes and a simple summary of Chapter 13 of the NCERT Class 9 Science book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

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Chapter 13: Earth as a System: Energy, Matter, and Life

Think It Over

  • How does the warming of Arabian Sea water affect the southwest monsoon in India?
  • If a large forest is cleared, how can that affect the flow of a river?
  • What might happen to coastal cities if glaciers and polar ice keep melting faster?
  • How would increasing carbon dioxide levels affect ocean plankton?

    • Introduction

    Life on Earth is powered by a constant flow of energy and matter. The Sun is the main source of energy; the Earth's hot interior and chemical reactions in air, water and rocks also drive flows. This chapter brings earlier ideas together into one Earth system made of interacting spheres:

  • Geosphere — rocks, soil, landforms (Deccan plateau, Thar desert) and the interior
  • Hydrosphere — liquid water: oceans, rivers (Ganga–Brahmaputra), lakes, groundwater
  • Cryosphere — ice and snow (Himalayan glaciers, snow in Ladakh, polar ice caps)
  • Atmosphere — the air around the Earth
  • Biosphere — all living organisms and their habitats (mangroves, forests, farms, plankton, coral reefs)
    • A disturbance in one sphere leads to changes in others — e.g., less winter snowfall → less water in the lake in summer → less grass for sheep.

    Key Concepts

    13.1 Uneven Heating of the Earth

  • Solar radiation reaches Earth as electromagnetic (EM) waves, which travel through vacuum at the speed of light (3 × 10⁸ m s⁻¹) — unlike sound, which needs a medium.
  • The EM spectrum runs from gamma rays and X-rays (high frequency, short wavelength, harmful) to infrared, microwaves and radio waves.
  • About 99% of the Sun's energy reaching Earth is in the UV, visible and infrared ranges.

    • - UV (100–400 nm) is mostly absorbed by the ozone layer; it can damage eyes and skin.

    - Visible light powers photosynthesis and partly warms land and water.

    - Infrared warms the surface, which re-radiates heat; greenhouse gases (CO₂, CH₄, water vapour) trap part of it.

  • Insolation: the Sun's radiation reaching the Earth's surface. Solar constant: about 1.4 kW m⁻² at the top of the atmosphere; maximum insolation at the surface is about 1 kW m⁻² under a clear sky.
  • Anna Mani mapped solar insolation over India; with S. Rangarajan she published Solar Radiation Over India (1982).

    • 13.1.1 Albedo

    Albedo is the fraction of solar radiation reflected by a surface.
  • High albedo (snow 0.80–0.90, ice 0.50–0.70) → stays cool. Low albedo (black soil, ocean water) → absorbs more, warmer. Crushed rock: 0.25–0.30.
  • Urban heat island: concrete, steel, brick and asphalt absorb and re-radiate heat, making cities warmer than rural areas, especially at night.

    • 13.1.2 Latitude

  • Because the Earth is spherical, the Sun's rays are concentrated over a smaller area at the equator and spread over a larger area at the poles — so the equator is warm and the poles cold. This uneven heating drives global winds and ocean currents.

    • 13.1.3 Role of the Atmosphere

  • Mainly nitrogen (78%) and oxygen (21%), with argon, CO₂ and water vapour.
  • Troposphere (0–12 km): nearly all weather; temperature falls with height (~6.5 °C/km); tallest over the equator.
  • Stratosphere (12–50 km): ozone layer absorbs UV; temperature rises with height, preventing vertical mixing.
  • Above: mesosphere, thermosphere, exosphere; outer space begins at about 100 km.
  • The atmosphere absorbs part of the incoming radiation and traps outgoing heat; excess CO₂ enhances the greenhouse effect → global warming. Venus is hotter than Mercury because of its runaway greenhouse effect.
  • CFCs caused the ozone hole over Antarctica; the Montreal Protocol is helping the ozone layer recover. K. R. Ramanathan measured ozone in the Himalayas in 1934.

    • 13.2 Winds and Ocean Currents

  • Wind moves from high to low pressure; pressure differences come from uneven heating.
  • Local winds: by day, sunlit slopes warm faster → valley breeze (up the slope); after sunset, slopes cool faster → mountain breeze (down into the valley) — common in Shimla and Dehradun.
  • Planetary winds: warm air rises at the equatorial low, sinks near 30° N/S (sub-tropical highs), rises again near 60° N/S (sub-polar lows); cold air sinks at the polar highs (~90°). Earth's rotation deflects winds to the right in the Northern and left in the Southern Hemisphere.
  • Ocean currents: driven by winds, temperature, salinity, rotation and land masses. Warm equatorial water flows poleward at the surface; cold, dense water returns at depth. Circular gyres turn clockwise in the Northern and anticlockwise in the Southern Hemisphere. The North Atlantic Drift (extension of the Gulf Stream) keeps north-western European ports ice-free.
  • IITM Pune runs coupled models of atmosphere, ocean, land and ice to forecast the monsoon.

    • 13.3 Biogeochemical Cycles

    The cyclic movement of matter and energy between the abiotic and biotic components is the biogeochemical cycle.
  • Water cycle: evaporation, transpiration, condensation, precipitation, run-off, infiltration. A warmer atmosphere holds more moisture → heavier rains in some places and droughts elsewhere; melting glaciers raise sea levels, threatening cities like Mumbai and Chennai.
  • Carbon cycle: fast cycle (days to years) — photosynthesis, respiration, decomposition; slow cycle (millions of years) — fossil fuel formation. Oceans absorb CO₂ as carbonate and bicarbonate ions. Atmospheric CO₂ rose from about 315 ppm (1960) to about 420 ppm (the Keeling curve). Carbon is about 49% of the dry weight of organisms; 71% of global carbon is in oceans and only about 1% in the atmosphere.
  • Nitrogen cycle: N₂ is non-reactive. Nitrogen fixation by Rhizobium (root nodules of legumes) and Azotobacter → ammonia; nitrification by Nitrosomonas (ammonia → nitrite) and Nitrobacter (nitrite → nitrate); assimilation by plants; ammonification by decomposers; denitrification by Pseudomonas → N₂. Lightning also fixes some nitrogen; the Haber–Bosch process makes most fertiliser ('Bread from Air').
  • Oxygen cycle: about 21% of the atmosphere is O₂; used in respiration and combustion, restored by photosynthesis.

    • 13.4 Human Impact

  • Excess CO₂ makes sea water more acidic, threatening plankton and coral reefs; warmer water absorbs less CO₂.
  • Over-use of fertilisers causes eutrophication (algal blooms that deplete oxygen and kill fish).
  • Deforestation reduces photosynthesis and transpiration (less local rainfall), changes albedo and increases soil erosion.
  • Vehicle emissions + sunlight → smog and harmful ground-level ozone.
  • The Montreal Protocol worked; the Kyoto Protocol and Paris Agreement have been less successful. Mission LiFE (2021) promotes eco-friendly lifestyles.

    • Worked Numerical

  • Ex 13.1: Energy received by 1 m² in one hour at 1 kW m⁻²: E = 1000 J s⁻¹ m⁻² × 1 m² × 3600 s = 3.6 × 10⁶ J — about one unit of household electricity.

    • Activities in the Chapter

  • 13.1 Identify the five spheres in a picture and trace how less snowfall affects the lake and grass.
  • 13.2 Complete an albedo table for snow, ice, crushed rock, soils and ocean water.
  • PhET simulation on greenhouse gas concentration and surface temperature.

    • Key Terms

  • Geosphere (भूमंडल), Hydrosphere (जलमंडल), Cryosphere (हिममंडल), Atmosphere (वायुमंडल), Biosphere (जैवमंडल)
  • Insolation (सूर्यातप), Albedo (एल्बिडो / परावर्तकता), Greenhouse effect (हरितगृह प्रभाव)
  • Troposphere (क्षोभमंडल), Stratosphere (समतापमंडल), Ozone layer (ओज़ोन परत)
  • Ocean current (महासागरीय धारा), Nitrogen fixation (नाइट्रोजन स्थिरीकरण), Eutrophication (सुपोषण)

    • Real-life Connections

  • A fraction of the Thar desert covered with solar panels could supply India's electricity needs.
  • Warmer Arabian Sea water causes monsoon fluctuations — floods in some regions and drought in others.
  • Planting trees, saving water and energy, and sustainable farming help restore the balance.

    • Common Misconceptions

  • "The greenhouse effect is always harmful." — Some greenhouse gases keep Earth warm enough for life; the problem is the excess.
  • "Ozone is always good." — Stratospheric ozone protects us; ground-level ozone in smog is harmful.
  • "The atmosphere holds most of Earth's carbon." — Only about 1%; about 71% is in the oceans.
  • "The Sun heats the air directly." — Mostly the surface absorbs radiation and re-radiates heat that greenhouse gases trap.

    • Exercise Hints

  • Warming of the Earth: the surface absorbs solar radiation, which is re-radiated and trapped by greenhouse gases (Q2).
  • Biogeochemical cycles recycle essential nutrients between biotic and abiotic components (Q1).
  • For breeze questions, compare how fast slopes and valley floors heat and cool.

    • 💡 Key Learning Points

    • ✓The Earth works as one system of interacting spheres — geosphere, hydrosphere, cryosphere, atmosphere and biosphere — so a change in one affects the others.
    • ✓Solar EM radiation (mainly UV, visible and IR) is unevenly absorbed because of albedo, latitude and the Earth's shape; the atmosphere filters UV and traps heat.
    • ✓Uneven heating creates pressure differences that drive local breezes, planetary winds and ocean currents, which redistribute heat.
    • ✓Water, carbon, nitrogen and oxygen cycle between living and non-living components through biogeochemical cycles.
    • ✓Human activities such as burning fossil fuels, deforestation and fertiliser overuse disturb these cycles, but local action and global cooperation can restore balance.

    👨‍🏫 Teaching Tips

    • →Open with Activity 13.1: give groups the landscape picture and ask them to circle each sphere and trace a chain of effects.
    • →Place black and white papers (or dark and light soil) in sunlight with thermometers to demonstrate albedo.
    • →Use the Keeling curve to let students read CO₂ values in 1960 and today and discuss the seasonal sawtooth.
    • →Draw the three-cell pressure belt model step by step: equatorial low, 30° highs, 60° lows and polar highs.
    • →Work Example 13.1 and connect 3.6 × 10⁶ J to one unit on a household electricity bill.
    • →Run the PhET greenhouse effect simulation and let students change greenhouse gas levels.
    • →End with Mission LiFE: each student lists three actions to save energy, water or food at home.

    📋 Assessment Questions

    1. Can the student name the five spheres with Indian examples and explain how a change in one affects another?
    2. Can the student explain albedo, insolation and the solar constant, and why the equator is warmer than the poles?
    3. Can the student describe the troposphere and stratosphere and the two protective roles of the atmosphere?
    4. Can the student explain how valley and mountain breezes, planetary winds and ocean currents form?
    5. Can the student describe the carbon and nitrogen cycles and give examples of human impacts on them?

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    Keep learning

    All chapters in Exploration - Science Class 9

    1. 1. Exploration: Entering the World of Secondary Science
    2. 2. Cell: The Building Block of Life
    3. 3. Tissues in Action
    4. 4. Describing Motion Around Us
    5. 5. Exploring Mixtures and their Separation
    6. 6. How Forces Affect Motion
    7. 7. Work, Energy, and Simple Machines
    8. 8. Journey Inside the Atom
    9. 9. Atomic Foundations of Matter
    10. 10. Sound Waves: Characteristics and Applications
    11. 11. Reproduction: How Life Continues
    12. 12. Patterns in Life: Diversity and Classification
    13. 13. Earth as a System: Energy, Matter, and Life