NCERT Solutions for Class 12 Physics Atoms

Activity 13.1 - Exploring how the Earth's spheres are interconnected

Part 1 - Identifying examples of each sphere from Fig. 13.1:

• Geosphere: Rocky mountains, soil, or landforms visible in the landscape.
• Hydrosphere: The lake or river water in the valley.
• Cryosphere: Snow and ice on the mountain peaks.
• Atmosphere: The air and clouds above the mountains.
• Biosphere: The grass on the slopes and the sheep grazing on it.

Part 2 - How snow (cryosphere) becomes part of the lake (hydrosphere):

Snow on mountain peaks melts gradually during warmer months due to solar heating. The meltwater flows down the slopes through streams and collects in the valley lake. In this way, solid water from the cryosphere enters the liquid water of the hydrosphere.

Part 3 - Effect of reduced snowfall on the lake and grass:

Less snowfall means less snow accumulates on the peaks. During spring and summer, less meltwater flows into the lake, so the lake level drops. Drier soil means less grass grows, reducing food available for the sheep. A change in the cryosphere directly affects the hydrosphere and biosphere.

Part 4 - How all spheres are interconnected:

• Atmosphere-Geosphere: Solar radiation heats the land surface and drives weather.
• Cryosphere-Hydrosphere: Glacier melt adds water to rivers and lakes.
• Biosphere-Atmosphere: Plants absorb CO₂ and release O₂ through photosynthesis; animals and decomposers return CO₂ through respiration.
• Geosphere-Biosphere: Soil provides nutrients for plants; plant roots prevent soil erosion.

A disturbance in one sphere spreads to others. For example, rising atmospheric temperatures (atmosphere) melt glaciers (cryosphere), raise sea levels (hydrosphere), erode coasts (geosphere), and destroy habitats (biosphere).

Climate change - caused mainly by rising global temperatures due to excess greenhouse gases - disrupts and intensifies several stages of the water cycle:

1. Increased evaporation: A warmer atmosphere causes more water to evaporate from oceans, lakes, rivers, and soil. A warmer atmosphere can also hold more water vapour (roughly 7% more for every 1 °C rise in temperature), loading the atmosphere with extra moisture.

2. Heavier and more uneven precipitation: Extra atmospheric moisture leads to more intense rainfall in some regions. For example, India experiences more intense monsoon events due to the warmer Arabian Sea supplying more moisture. However, other regions see less rainfall and more droughts - making the water cycle more extreme and uneven.

3. Melting glaciers: Rising temperatures melt glaciers and polar ice caps. Himalayan glaciers that supply water to rivers like the Ganga and Brahmaputra will initially cause flooding but will eventually shrink, reducing long-term river flow and threatening millions who depend on these rivers. Globally, melting polar ice raises sea levels, threatening coastal cities like Mumbai and Chennai.

4. Reduced groundwater recharge: Heavy bursts of rainfall cause rapid surface run-off rather than slow infiltration into the ground. This reduces groundwater recharge, making it harder to maintain water supply during dry months.

5. Soil erosion: Intense rainfall on deforested land washes away topsoil, reducing the land's ability to absorb water and further disrupting the local water cycle.

In summary, climate change makes the water cycle more extreme - wetter in some places, drier in others - and disrupts the balance between the cryosphere, hydrosphere, atmosphere, and biosphere.

Estimation: How much of India's land needs to be covered with solar panels to meet its electricity needs?

Step 1 - India's electricity consumption: India's annual electricity consumption is approximately 1,600-1,800 billion kWh per year. This works out to roughly 4.5 billion kWh per day.

Step 2 - Solar energy available at Earth's surface: The solar constant is about 1.4 kWm^-2 at the top of the atmosphere. After atmospheric losses, about 1 kWm^-2 reaches the surface under clear skies. In the Thar desert (Rajasthan), the daily solar energy received is about 5.5 kWh per square metre per day - one of the highest in the world.

Step 3 - Solar panel efficiency: Assume commercially available panels operate at about 15-20% efficiency. Use 15% (0.15) for this estimate.

Step 4 - Calculate area needed:

• Energy produced per m² per day = 5.5 kWh × 0.15 = 0.825 kWh/m²/day
• Total area needed = 4.5 × 10⁹ kWh ÷ 0.825 kWh/m² ≈ 5.5 × 10⁹ m² = about 5,500 km²

Conclusion: The Thar desert covers about 200,000 km². Even covering just 2-3% of it with solar panels could, in theory, supply all of India's electricity. This shows the enormous solar energy potential of India's tropical location and explains why India is rapidly expanding its solar power capacity.

Albedo is the fraction of solar radiation reflected by a surface (the word comes from Latin, meaning whiteness). It directly controls how much solar energy a surface absorbs versus reflects, and therefore affects the temperature of that surface and the climate of the region.

High albedo surfaces (reflect more, absorb less, stay cooler):

• Snow: Albedo 0.80-0.90 - reflects 80-90% of sunlight, keeping polar regions very cold.
• Ice: Albedo 0.50-0.70 - glaciers and sea ice stay cold because they reflect most incoming radiation.
• Light coloured soil: Albedo 0.25-0.45 - cooler than dark soil under the same sunshine.

Low albedo surfaces (absorb more, reflect less, warm up faster):

• Ocean water: Albedo 0.06-0.10 - absorbs most solar energy, warming tropical oceans significantly.
• Black soil: Albedo 0.05-0.15 - heats up quickly, as seen in Deccan plateau soils.

Impact on climate:

• Global temperature differences: Albedo differences between the equator (low albedo, warm oceans) and poles (high albedo, snow and ice) create temperature contrasts that drive planetary winds and ocean currents.
• Ice-albedo feedback: When ice melts due to global warming, darker ocean or land is exposed. Darker surfaces have lower albedo and absorb more radiation, causing more warming and more melting - a self-reinforcing feedback loop that accelerates climate change.
• Urban heat island effect: Cities have concrete roads, buildings, and asphalt surfaces with low albedo. They absorb more radiation and retain more heat than surrounding forests and farmland, making cities warmer - especially at night.

PhET Greenhouse Effect Simulation - Guided Activity

Website: https://phet.colorado.edu/en/simulations/greenhouse-effect

Steps:

1. Open the simulation in a browser and select the "Greenhouse Effect" tab.
2. Start with no greenhouse gases - observe the surface temperature shown. It will be very low (around -18 °C), too cold for life.
3. Gradually increase greenhouse gas concentration (e.g., CO₂) using the slider. Note how the surface temperature rises.
4. Observe the radiation arrows - more greenhouse gas means more outgoing infrared (heat) radiation is trapped and sent back to the surface.
5. Try different gas levels and record surface temperature for each setting.

Expected observations:

• No greenhouse gases: Surface temperature is very low - the Earth would be frozen and unable to support life.
• Moderate greenhouse gases (natural level): Surface temperature is around 15 °C - warm enough for life to thrive.
• Very high greenhouse gases: Surface temperature rises sharply - this models the effect of burning fossil fuels and deforestation. It demonstrates the risk of global warming.

Conclusion: A natural amount of greenhouse gases is essential to keep the Earth warm. However, human activities are increasing their concentration beyond the natural level, causing global warming and climate change.