Atoms Class 12 Physics Practice Questions

The correct answer is (ii) To recycle essential nutrients between biotic and abiotic components.

Explanation: Biogeochemical cycles (water, carbon, nitrogen, oxygen) continuously move essential elements between living organisms (biotic) and the non-living environment - soil, water, and air (abiotic). They ensure that elements like carbon, nitrogen, and oxygen are not permanently locked away but are constantly recycled and made available for reuse by living organisms.

Why the other options are incorrect:

• (i) Wrong - Cycles move nutrients, not food. Organisms make their own food (plants through photosynthesis) or obtain it by consuming others.
• (iii) Wrong - Biogeochemical cycles do not create new elements. The same atoms have been recycled for billions of years; they only change chemical form.
• (iv) Wrong - Removing pollutants is not the primary role of biogeochemical cycles, although some natural processes like decomposition do help break down certain compounds.

The Earth's surface absorbs solar radiation and releases this energy as infrared (heat) radiation back towards space - a process called terrestrial radiation or re-radiation.

Mechanisms of heat loss:

• Infrared radiation: The primary method - the warm surface emits long-wave infrared radiation upward into and through the atmosphere.
• Convection: Warm air in contact with the heated surface rises, carrying heat to higher levels of the atmosphere.
• Conduction: Heat passes by direct contact from the warm surface to the air immediately above it.
• Evaporation and transpiration: Water evaporating from oceans, lakes, and plant leaves carries energy into the atmosphere as latent heat, which is released when water vapour condenses into clouds and rain.

Significance:

• Earth's energy balance: For the Earth's temperature to remain stable over time, the heat it absorbs from the Sun must equal the heat it loses to space. This balance is called the Earth's energy balance. If more heat is trapped than released (due to excess greenhouse gases), the planet warms up.
• Drives weather and climate: The re-radiated heat warms the lower atmosphere (troposphere), creating temperature gradients that drive winds, storms, and the water cycle.
• Greenhouse effect: Greenhouse gases (CO₂, CH₄, water vapour) absorb some of the outgoing infrared radiation and re-emit part of it back toward the surface. This natural greenhouse effect keeps the Earth warm enough for life. Excess greenhouse gases trap too much heat, causing global warming.

Completed Table 13.1 - Albedo values for common surface materials:

S. No.	Materials	Albedo
1.	Snow	0.80 - 0.90
2.	Ice	0.50 - 0.70
3.	Crushed rock	0.25 - 0.30
4.	Light coloured soil	0.25 - 0.45
5.	Black soil	0.05 - 0.15
6.	Ocean water	0.06 - 0.10

Key observations:

• Snow and ice (high albedo): Reflect most incoming sunlight, keeping polar regions cold.
• Black soil and ocean water (low albedo): Absorb most sunlight, warming up faster - this is why tropical oceans and dark agricultural soils become warm.
• Light coloured soil (moderate albedo): Reflects a moderate amount of radiation, staying cooler than dark soils.

These differences in albedo across the Earth's surface cause uneven heating, which drives winds, ocean currents, and large-scale weather patterns.

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.

The correct answer is (ii) To recycle essential nutrients between biotic and abiotic components.

Explanation: Biogeochemical cycles (water, carbon, nitrogen, oxygen) continuously move essential elements between living organisms (biotic) and the non-living environment - soil, water, and air (abiotic). They ensure that elements like carbon, nitrogen, and oxygen are not permanently locked away but are constantly recycled and made available for reuse by living organisms.

Why the other options are incorrect:

• (i) Wrong - Cycles move nutrients, not food. Organisms make their own food (plants through photosynthesis) or obtain it by consuming others.
• (iii) Wrong - Biogeochemical cycles do not create new elements. The same atoms have been recycled for billions of years; they only change chemical form.
• (iv) Wrong - Removing pollutants is not the primary role of biogeochemical cycles, although some natural processes like decomposition do help break down certain compounds.

How human activities increase greenhouse gas concentrations:

Greenhouse gases (GHGs) such as CO₂, methane (CH₄), and nitrous oxide (N₂O) trap outgoing infrared heat from the Earth's surface. Human activities have raised their concentrations well above natural levels:

• Burning fossil fuels: Coal, oil, and natural gas burned for electricity, transport, and industry release large amounts of CO₂. Atmospheric CO₂ has risen by about 35% since 1960 (from 315 ppm to over 420 ppm).
• Deforestation: Forests absorb CO₂ through photosynthesis. When cleared and burned, stored carbon is released as CO₂ and future absorption is also lost.
• Agriculture: Flooded rice paddies and livestock digestion produce methane; overuse of fertilisers releases nitrous oxide from soil.
• Industrial processes: Cement manufacturing, chemical plants, and steel production release CO₂ and other GHGs.
• Waste: Organic waste in landfills decomposes without oxygen to produce methane.

What an individual can do:

• Transport: Use public transport, cycle, or walk; prefer electric vehicles where possible.
• Energy: Use LED lights and energy-efficient appliances; switch off devices not in use; use solar energy.
• Waste: Reduce, reuse, and recycle to minimise landfill waste.
• Food: Reduce meat consumption, especially beef, and avoid burning crop residue.
• Green cover: Plant trees to increase local carbon absorption.
• Water: Conserve water - pumping and treating water requires energy.

India's Mission LiFE (Lifestyle for Environment) encourages people to adopt such eco-friendly habits to collectively reduce GHG emissions.

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.

The correct answer is (iii) The Earth's surface absorbs solar radiation, which is then re-radiated and trapped by greenhouse gases.

Explanation: Visible light from the Sun passes through the atmosphere and warms the Earth's surface. The warm surface re-radiates this energy as infrared (heat) radiation back towards space. Greenhouse gases - mainly CO₂, CH₄, and water vapour - in the atmosphere absorb this outgoing infrared radiation and re-emit some of it back toward the Earth's surface. This process, called the greenhouse effect, is primarily responsible for keeping the Earth warm enough for life.

Why the other options are incorrect:

• (i) Wrong - CO₂ does not immediately absorb incoming solar radiation. It traps the outgoing infrared radiation re-emitted by the Earth's surface - not the incoming solar radiation itself.
• (ii) Wrong - While aerosol particles do scatter some solar radiation, this is not the primary warming mechanism. In fact, some aerosols (like volcanic ash) can temporarily cool the Earth by reflecting sunlight.
• (iv) Wrong - Radiation reflected by clouds does not warm the Earth; reflection actually reduces the amount of energy reaching the surface.

Completed Table 13.1 - Albedo values for common surface materials:

S. No.	Materials	Albedo
1.	Snow	0.80 - 0.90
2.	Ice	0.50 - 0.70
3.	Crushed rock	0.25 - 0.30
4.	Light coloured soil	0.25 - 0.45
5.	Black soil	0.05 - 0.15
6.	Ocean water	0.06 - 0.10

Key observations:

• Snow and ice (high albedo): Reflect most incoming sunlight, keeping polar regions cold.
• Black soil and ocean water (low albedo): Absorb most sunlight, warming up faster - this is why tropical oceans and dark agricultural soils become warm.
• Light coloured soil (moderate albedo): Reflects a moderate amount of radiation, staying cooler than dark soils.

These differences in albedo across the Earth's surface cause uneven heating, which drives winds, ocean currents, and large-scale weather patterns.

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.

When global temperature increases, CO₂ dissolved in the ocean is disturbed, with serious consequences for marine life.

What happens to dissolved CO₂:

• Warmer water holds less dissolved gas. As ocean temperature rises, the ocean's capacity to absorb CO₂ from the atmosphere decreases.
• The CO₂ that does dissolve reacts with seawater to form carbonic acid, making the ocean more acidic. This is called ocean acidification.

Impact on marine life:

• Corals and shell-forming organisms: Acidic water dissolves calcium carbonate, which corals, molluscs, and tiny organisms like pteropods use to build their shells and skeletons. This causes coral bleaching and the collapse of coral reef ecosystems, which support enormous biodiversity.
• Phytoplankton: These tiny photosynthetic organisms form the base of the entire ocean food chain. Changes in ocean temperature and chemistry can disrupt their growth, affecting all animals that depend on them.
• Fish and larger animals: As phytoplankton decline and coral reefs collapse, fish populations fall, threatening the food security of millions of people who depend on seafood.
• Reduced carbon sink: A warmer, more acidic ocean absorbs less CO₂, leaving more in the atmosphere and accelerating further warming - a feedback loop.

The atmospheric layer mainly responsible for weather phenomena such as winds, storms, and rainfall is the troposphere.

Key facts about the troposphere:

• Extends from the Earth's surface to an average height of about 12 km (higher above the equator, lower above the poles).
• Contains about 75% of the atmosphere's total mass and almost all of its water vapour.
• Nearly all weather - clouds, rain, thunderstorms, cyclones, winds - occurs here.

Primary reason for weather activity in the troposphere:

The troposphere is heated from below by the Earth's surface, which absorbs solar radiation and re-radiates it as infrared heat. This causes temperature to decrease with altitude (about 6.5 °C per km). Warm air near the surface is light and rises; cooler air above is denser and sinks. This vertical movement (convection) drives cloud formation, winds, and precipitation.

The uneven heating of the Earth's surface - due to differences in albedo, latitude, and land-water contrast - further intensifies these air movements, producing the varied weather we observe.

Other layers (stratosphere, mesosphere, thermosphere) play little or no role in producing weather because they are heated differently and do not have the same convective instability as the troposphere. The stratosphere actually has temperature increasing with height (due to ozone absorbing UV), which prevents vertical mixing and keeps it calm.

How human activities increase greenhouse gas concentrations:

Greenhouse gases (GHGs) such as CO₂, methane (CH₄), and nitrous oxide (N₂O) trap outgoing infrared heat from the Earth's surface. Human activities have raised their concentrations well above natural levels:

• Burning fossil fuels: Coal, oil, and natural gas burned for electricity, transport, and industry release large amounts of CO₂. Atmospheric CO₂ has risen by about 35% since 1960 (from 315 ppm to over 420 ppm).
• Deforestation: Forests absorb CO₂ through photosynthesis. When cleared and burned, stored carbon is released as CO₂ and future absorption is also lost.
• Agriculture: Flooded rice paddies and livestock digestion produce methane; overuse of fertilisers releases nitrous oxide from soil.
• Industrial processes: Cement manufacturing, chemical plants, and steel production release CO₂ and other GHGs.
• Waste: Organic waste in landfills decomposes without oxygen to produce methane.

What an individual can do:

• Transport: Use public transport, cycle, or walk; prefer electric vehicles where possible.
• Energy: Use LED lights and energy-efficient appliances; switch off devices not in use; use solar energy.
• Waste: Reduce, reuse, and recycle to minimise landfill waste.
• Food: Reduce meat consumption, especially beef, and avoid burning crop residue.
• Green cover: Plant trees to increase local carbon absorption.
• Water: Conserve water - pumping and treating water requires energy.

India's Mission LiFE (Lifestyle for Environment) encourages people to adopt such eco-friendly habits to collectively reduce GHG emissions.

The Earth's surface absorbs solar radiation and releases this energy as infrared (heat) radiation back towards space - a process called terrestrial radiation or re-radiation.

Mechanisms of heat loss:

• Infrared radiation: The primary method - the warm surface emits long-wave infrared radiation upward into and through the atmosphere.
• Convection: Warm air in contact with the heated surface rises, carrying heat to higher levels of the atmosphere.
• Conduction: Heat passes by direct contact from the warm surface to the air immediately above it.
• Evaporation and transpiration: Water evaporating from oceans, lakes, and plant leaves carries energy into the atmosphere as latent heat, which is released when water vapour condenses into clouds and rain.

Significance:

• Earth's energy balance: For the Earth's temperature to remain stable over time, the heat it absorbs from the Sun must equal the heat it loses to space. This balance is called the Earth's energy balance. If more heat is trapped than released (due to excess greenhouse gases), the planet warms up.
• Drives weather and climate: The re-radiated heat warms the lower atmosphere (troposphere), creating temperature gradients that drive winds, storms, and the water cycle.
• Greenhouse effect: Greenhouse gases (CO₂, CH₄, water vapour) absorb some of the outgoing infrared radiation and re-emit part of it back toward the surface. This natural greenhouse effect keeps the Earth warm enough for life. Excess greenhouse gases trap too much heat, causing global warming.

Formation of Valley Breeze (daytime):

During the day, the Sun heats mountain slopes more rapidly than the valley floor. The air over the warm slopes becomes lighter and rises, creating a low-pressure zone on the slopes. Cooler air from the valley floor moves upslope to replace the rising warm air. This flow is called the valley breeze.

Formation of Mountain Breeze (night-time):

After sunset, mountain slopes lose heat quickly by radiation and become cooler than the valley floor. The cooled, denser air on the slopes sinks down into the valley. This downslope flow is called the mountain breeze.

Would the two mountain breezes be different?

Yes. Consider one mountain covered with grass and another with barren rocks.

Feature	Grass-covered mountain	Barren rock mountain
Albedo	Higher (grass reflects more light)	Lower (dark rock absorbs more)
Heat absorption	Slower - grass and plant transpiration cool the surface	Faster - rock heats up quickly
Daytime valley breeze	Weaker and cooler (less heating)	Stronger and warmer (more heating)
Night-time mountain breeze	Gentler and less cold (grass retains some heat)	Stronger and colder (bare rock cools rapidly)

Conclusion: The grass-covered mountain produces milder, gentler breezes due to higher albedo and moisture from transpiration. The barren rock mountain produces stronger, more extreme breezes because bare rock heats and cools rapidly. Surface cover significantly affects local wind patterns through its effect on heat absorption and radiation.