Electric Charges and Fields Class 12 Physics Practice Questions

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.

Activity: Analysing monsoon rainfall trends using IMD data

Steps:

1. Access data: Visit https://mausam.imd.gov.in/ and go to the climate data section. Search for rainfall data for your city or district for June-September (the southwest monsoon season).
2. Select years: Choose five years from the 1980s (e.g. 1981, 1983, 1985, 1987, 1989) and five from the 2020s (e.g. 2020, 2021, 2022, 2023, 2024). If IMD data is difficult to access, use newspaper records or the district agriculture office.
3. Record data: Note total monsoon rainfall (mm) and, if available, the number of days with heavy rainfall (>50 mm in a single day) for each year.
4. Compare: Calculate the average total monsoon rainfall for the 1980s years and for the 2020s years. Note whether the total has increased, decreased, or stayed similar. Also check whether the number of heavy-rain days has changed.

How to connect findings to the chapter:

• Warmer Arabian Sea temperatures: A warmer Arabian Sea evaporates more water, adding extra moisture to the southwest monsoon. This can cause more intense individual rainfall events - heavy downpours concentrated over fewer days - even if the total seasonal rainfall is similar. If your data shows more heavy-rain days in the 2020s, this is consistent with the chapter's explanation of warmer Arabian Sea driving monsoon variability.
• Land use change (forests to farms to cities): Urbanisation and deforestation reduce soil's ability to absorb rainwater. Cities with large paved areas produce rapid run-off, leading to urban flooding during heavy rain. Loss of forest cover also reduces local transpiration and humidity, which can lower overall rainfall in the long run. If your area has been significantly urbanised between the 1980s and 2020s, this may explain any changes observed.

General trend across India: IMD data broadly shows that while total monsoon rainfall across India has not changed dramatically, rainfall has become more concentrated - more heavy-rain events and more dry spells within the same season. This is consistent with the warming Arabian Sea and land-use change effects described in the chapter.

Biogeochemical cycles continuously move essential nutrients such as carbon, nitrogen, oxygen, and water between living (biotic) and non-living (abiotic) parts of the Earth. If these cycles were disrupted and stopped, life could not continue. Here is what would happen:

Water cycle disrupted:

Without evaporation, condensation, and precipitation, rivers and lakes would dry up. Plants would die from lack of water, and animals would lose their freshwater source. All life processes - photosynthesis, digestion, respiration - depend on water.

Carbon cycle disrupted:

Plants use CO₂ for photosynthesis. If the carbon cycle stopped and CO₂ was not recycled, plants could not make food. All animals that depend on plants would starve. If decomposition stopped, dead organic matter would accumulate and nutrients would not return to the soil, halting new plant growth.

Nitrogen cycle disrupted:

Nitrogen is essential for proteins and nucleic acids. Plants absorb nitrates from the soil. If nitrogen-fixing bacteria stopped working, soil would become nitrogen-deficient, crops would fail, and all animals dependent on plants for protein would die.

Oxygen cycle disrupted:

Plants produce oxygen through photosynthesis. Without it, atmospheric oxygen would be consumed by respiration and combustion. Most animals and aerobic microorganisms would suffocate. Even decomposers need oxygen to break down dead matter.

Overall consequence: Without biogeochemical cycles, nutrients would not be recycled, energy would not flow through ecosystems, and the conditions for life would disappear. The Earth would become a lifeless planet.

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.

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.

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.

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.

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).

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.

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.

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).

Activity: Analysing monsoon rainfall trends using IMD data

Steps:

1. Access data: Visit https://mausam.imd.gov.in/ and go to the climate data section. Search for rainfall data for your city or district for June-September (the southwest monsoon season).
2. Select years: Choose five years from the 1980s (e.g. 1981, 1983, 1985, 1987, 1989) and five from the 2020s (e.g. 2020, 2021, 2022, 2023, 2024). If IMD data is difficult to access, use newspaper records or the district agriculture office.
3. Record data: Note total monsoon rainfall (mm) and, if available, the number of days with heavy rainfall (>50 mm in a single day) for each year.
4. Compare: Calculate the average total monsoon rainfall for the 1980s years and for the 2020s years. Note whether the total has increased, decreased, or stayed similar. Also check whether the number of heavy-rain days has changed.

How to connect findings to the chapter:

• Warmer Arabian Sea temperatures: A warmer Arabian Sea evaporates more water, adding extra moisture to the southwest monsoon. This can cause more intense individual rainfall events - heavy downpours concentrated over fewer days - even if the total seasonal rainfall is similar. If your data shows more heavy-rain days in the 2020s, this is consistent with the chapter's explanation of warmer Arabian Sea driving monsoon variability.
• Land use change (forests to farms to cities): Urbanisation and deforestation reduce soil's ability to absorb rainwater. Cities with large paved areas produce rapid run-off, leading to urban flooding during heavy rain. Loss of forest cover also reduces local transpiration and humidity, which can lower overall rainfall in the long run. If your area has been significantly urbanised between the 1980s and 2020s, this may explain any changes observed.

General trend across India: IMD data broadly shows that while total monsoon rainfall across India has not changed dramatically, rainfall has become more concentrated - more heavy-rain events and more dry spells within the same season. This is consistent with the warming Arabian Sea and land-use change effects described in the chapter.

Biogeochemical cycles continuously move essential nutrients such as carbon, nitrogen, oxygen, and water between living (biotic) and non-living (abiotic) parts of the Earth. If these cycles were disrupted and stopped, life could not continue. Here is what would happen:

Water cycle disrupted:

Without evaporation, condensation, and precipitation, rivers and lakes would dry up. Plants would die from lack of water, and animals would lose their freshwater source. All life processes - photosynthesis, digestion, respiration - depend on water.

Carbon cycle disrupted:

Plants use CO₂ for photosynthesis. If the carbon cycle stopped and CO₂ was not recycled, plants could not make food. All animals that depend on plants would starve. If decomposition stopped, dead organic matter would accumulate and nutrients would not return to the soil, halting new plant growth.

Nitrogen cycle disrupted:

Nitrogen is essential for proteins and nucleic acids. Plants absorb nitrates from the soil. If nitrogen-fixing bacteria stopped working, soil would become nitrogen-deficient, crops would fail, and all animals dependent on plants for protein would die.

Oxygen cycle disrupted:

Plants produce oxygen through photosynthesis. Without it, atmospheric oxygen would be consumed by respiration and combustion. Most animals and aerobic microorganisms would suffocate. Even decomposers need oxygen to break down dead matter.

Overall consequence: Without biogeochemical cycles, nutrients would not be recycled, energy would not flow through ecosystems, and the conditions for life would disappear. The Earth would become a lifeless planet.

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.

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.