CLASS 9 SCIENCE · CHAPTER 13 · STUDY NOTES
Earth as a System: Energy, Matter and Life
Understand earth as a system: energy, matter and life through short explanations, labelled models and original practice. Read one topic, explain it in your own words, then check.
EXPLORE
One change can reach several parts of Earth
Follow a link from snow to water to life.Snowmelt connects the landscape
- Geosphere
- Earth’s rocks, soil, landforms and interior.
- Hydrosphere and cryosphere
- Hydrosphere covers Earth’s water; cryosphere specifically describes its snow and ice.
- Atmosphere
- The gases surrounding Earth.
- Biosphere
- Living organisms and their environments.
The spheres are connected descriptions, not isolated sealed compartments. Melting mountain snow can enter a lake, wet soil and support grass used by animals.
Less snowfall can change seasonal water supply, but rainfall, temperature, storage and water use also matter. Predict an effect by tracing the mechanism rather than claiming one factor decides every outcome.
Take a moment. What do you think?
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Question
Name three spheres linked when glacier melt enters a river used by plants.
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EXPLORE
Matter returns; energy needs a continuing supply
These two ideas should not use the same circular arrow.Use different arrows for different ideas
- Matter cycle
- Movement and transformation of material between stores.
- Energy flow
- Transfer of energy through processes, with much eventually emitted to space as radiation.
Carbon, water and nitrogen atoms move between air, water, living organisms and rocks. Matter changes form and can be reused in cycles.
Solar energy enters the Earth system, is transformed and transferred, and ultimately leaves mainly as thermal radiation. Earth’s internal energy also drives processes such as tectonics. Energy is conserved, but ecosystems cannot recycle heat back into food indefinitely.
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Question
Why does a food web still need sunlight even when nutrients are recycled?
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EXPLORE
Sunlight crosses space as electromagnetic radiation
It does not need air to reach Earth.Read the spectrum in order
- Electromagnetic spectrum
- The range of electromagnetic wavelengths and frequencies.
- Insolation
- Incoming solar radiation, described as energy or power per area with the stated measurement.
The Sun supplies mainly visible, infrared and ultraviolet radiation at Earth. Electromagnetic waves can cross vacuum at about 3×10⁸ m/s; sound cannot.
For electromagnetic waves in vacuum, greater frequency means shorter wavelength. Visible light supports photosynthesis; absorbed radiation warms surfaces. The atmosphere filters much harmful radiation, but not every ultraviolet ray.
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Question
Why can sunlight reach Earth across space while an ordinary acoustic wave cannot?
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EXPLORE
Multiply intensity, area and time
A watt measures rate; a joule measures energy.Energy grows while the rate stays fixed
Incident energy (MJ)
Time (min)
- 1600 W incident power · 800 W/m² × 2 m².
Assumptions: Original fixed-intensity example; incident radiation is not all converted into electricity.
Read the plotted values
1600 W incident power
| Time (min) | Incident energy (MJ) |
|---|---|
| 0 | 0 |
| 10 | 0.96 |
| 20 | 1.92 |
| 30 | 2.88 |
Original fixed-intensity example; incident radiation is not all converted into electricity.
- Solar constant
- Approximate solar power per unit area normal to the rays near Earth’s orbit, at the top of the atmosphere.
- Kilowatt-hour
- An energy unit equal to 3.6×10⁶ joules.
The textbook rounds the solar constant to 1.4 kW/m². Surface sunlight varies with time, location, clouds and orientation; 1 kW/m² is a useful clear-sky classroom value, not a universal maximum.
Original example: intensity 800 W/m² on a 2 m² panel for 30 min gives incident energy 800×2×1800=2.88×10⁶ J. At an assumed 20% conversion efficiency, electrical output is 5.76×10⁵ J=0.16 kWh. Real output changes with conditions.
Keep time in seconds
Constant intensity and orientation are assumed.
E = IAtW/m² × m² × s = joules. Efficiency multiplies incident energy to estimate output.
- 1
Convert 30 min to 1800 s.
- 2
Compute power: 800×2=1600 W.
- 3
Multiply by time for 2.88 MJ.
- 4
Apply 0.20 efficiency for 0.576 MJ electrical energy.
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Question
What is received by 1 m² in 1 h at 1000 W/m², before conversion losses?
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EXPLORE
Reflected energy is not absorbed by that surface
Light colour is one clue, not a complete temperature rule.Follow a 100-unit energy budget
- Albedo
- The fraction of incoming radiation reflected by a surface; between 0 and 1.
Clean snow generally reflects more sunlight than dark soil. For an opaque surface receiving 100 illustrative energy units, an albedo of 0.8 reflects 80 units and leaves 20 to be absorbed.
Temperature also depends on water, heat storage, wind, shade and heat transfer. Oceans may have low albedo yet warm more slowly than land. All warm materials radiate; mud walls do not remain cool because they emit no heat.
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Question
Two opaque model surfaces receive 100 units. Their albedos are 0.2 and 0.7. Which absorbs more?
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EXPLORE
The same beam can spread over a larger area
Angle and day length change the heating pattern.Compare footprints of equal beams
- Latitude
- Angular position north or south of the equator.
- Axial tilt
- The tilt of Earth’s rotation axis relative to its orbital plane’s perpendicular.
More slanting rays spread a given incoming beam over a larger surface area. Earth’s shape contributes to warmer low latitudes and colder high latitudes, with other controls also important.
Seasons mainly follow axial tilt as Earth orbits the Sun, changing solar angle and day length. They are not caused simply by Earth being nearer the Sun in summer everywhere. The hemispheres have opposite seasons.
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Question
Why is a slanting beam less concentrated on a horizontal surface?
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EXPLORE
Most weather happens in the lowest layer
Approximate boundaries vary.Layers are not a stack of equal thickness
- Troposphere
- The lowest atmospheric layer, where most weather occurs.
- Stratosphere
- The layer above the troposphere, containing most atmospheric ozone.
Dry air is mainly nitrogen, about 78%, and oxygen, about 21%; water vapour varies. The troposphere’s height varies with latitude and season, so “12 km” is an approximate reference. Its temperature generally falls with height, with local inversions possible.
In the stratosphere, ozone absorbs ultraviolet radiation and temperature generally rises with height. Above lie the mesosphere, thermosphere and exosphere. Around 100 km is a commonly used space boundary; it is not where air abruptly ends.
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Question
Why should 12 km not be treated as the exact troposphere height everywhere?
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EXPLORE
Two protective roles use different wavelengths
The ozone hole and greenhouse warming are different problems.Two radiation routes
- Greenhouse effect
- Warming associated with gases absorbing and re-emitting some outgoing thermal infrared radiation.
- Ozone layer
- A stratospheric ozone-rich region that absorbs much harmful solar ultraviolet radiation.
Earth absorbs solar energy and emits infrared radiation. Greenhouse gases, including water vapour, carbon dioxide and methane, absorb some infrared and emit in different directions. Energy still escapes to space; it is not trapped forever.
Extra greenhouse gases from human activities change the energy balance and warm the climate. Stratospheric ozone helps shield life from UV; near-ground ozone is a pollutant. Restrictions on ozone-depleting substances support ozone-layer recovery.
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Question
Does the ozone layer mainly stop outgoing infrared from Earth?
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EXPLORE
Land use can change local temperature
Trees help through shade and water exchange.A hypothetical evening temperature profile
- Urban heat island
- A built-up area being warmer than surrounding less-built areas under many conditions.
Buildings and roads change absorption, heat storage and airflow. Waste heat also contributes. More vegetation can cool surroundings through shade and evapotranspiration, if water is available.
The effect varies by time and place, often being strong at night. Compare sites with the same measurement method; one hot afternoon alone does not establish a long-term climate trend.
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Question
Why can a tree cool a pavement even when both receive sunlight?
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EXPLORE
Daily heating reverses a local circulation
Follow the arrow along the slope.Reverse the local direction
- Valley breeze
- A daytime upslope airflow produced by differential heating.
- Mountain breeze
- A nighttime downslope airflow of relatively cool, dense air.
During suitable daytime conditions, sun-warmed slopes heat nearby air, producing upslope flow. At night, cooling can produce denser air that flows downslope.
These are simplified local patterns. Large-scale weather, terrain, clouds and season can weaken or override them. They are not guaranteed at the same hour every day.
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Question
Which local slope-flow direction is expected in the basic clear-night model?
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