Class 10 Physics Chapter 21: Space and Environment

1. Role & Position of the Sun: The Sun is a massive, luminous star located at the centre of our Solar System. It is the greatest source of light, heat, and other forms of electromagnetic (E.M.) waves. The strong gravitational pull of the Sun (F = Gm₁m₂/r²) keeps all planets, including the Earth, revolving in stable orbits around it.

2. Chemical Composition of the Sun: According to the PECTAA 2026 Class 10 Physics textbook, the Sun is composed primarily of two light gases:

• Hydrogen (H₂): 74% of the Sun's mass (main nuclear fuel).
• Helium (He): 24% of the Sun's mass.
• Other Heavy Elements: About 2% (oxygen, carbon, neon, iron, etc.).

3. Core Nuclear Fusion Process: At the core of the Sun, where temperature (~15 million K) and pressure are extremely high, nuclear fusion takes place continuously. In this reaction, four light hydrogen nuclei (protons) fuse together to form a heavier helium nucleus:

4. Energy Transmission Through Space: This fusion reaction causes a tremendous increase in the Sun's temperature. The energy released travels across the vacuum of space as electromagnetic radiations, including infrared (heat) radiation, visible sunlight, and ultraviolet (UV) radiations.

To study and compare planets in our Solar System, physicists focus on five fundamental physical factors that make each planet unique:

1. Orbital Distance: The average distance of a planet from the Sun. Planets closer to the Sun (like Mercury and Venus) have smaller orbital distances, whereas outer planets (like Neptune) have huge orbital distances.
2. Orbital Period: The time a planet takes to complete one full orbit around the Sun. Planets closer to the Sun have shorter orbital periods (Mercury takes 88 Earth days; Earth takes 365.25 days; Neptune takes ~165 years).
3. Density (Composition Indicator): Density (ρ = m/V) reflects a planet's internal composition. Rocky planets (Earth, Mars, Mercury, Venus) have higher densities due to their solid, metallic cores, whereas gas giants (Jupiter, Saturn) have lower densities because they consist mostly of light gases (hydrogen and helium).
4. Surface Temperature & Greenhouse Effect: Depends on distance from the Sun, atmospheric composition, and greenhouse effects. Why is Venus hotter than Mars (and even Mercury)? Venus is extremely hot (~465°C) due to its thick, carbon-dioxide-rich heat-trapping atmosphere (runaway greenhouse effect), whereas Mars is very cold (~−65°C) due to its thin atmosphere and greater distance from the Sun.
5. Gravitational Field Strength (g): At a planet's surface, g depends on its mass and radius and is nearly uniform across its surface. Smaller/less massive planets like Mars (g ≈ 3.7 N/kg) have weaker gravity, while massive planets like Jupiter (g ≈ 24.8 N/kg) have much stronger gravitational pull than Earth (9.8 N/kg). Gravitational strength decreases as distance from the planet increases.

1. What are Cyclones? Cyclones refer to heavy storms containing fast and spinning winds forming around a low air pressure area over warm tropical oceans (typically where sea surface temperatures exceed 26.5°C).

2. Role of Convection in Cyclone Formation: The phenomenon of thermal convection is responsible for developing cyclones. Warm, moist air over the heated ocean surface is lighter (less dense) and rises rapidly upward, creating a low-pressure area underneath. Heavier, cooler surrounding air rushes in and moves downward to replace it. As the warm moist air rises and cools, water vapour condenses into heavy storm clouds, releasing latent heat that further fuels the storm.

3. The Coriolis Effect & Hemispheric Spin: Because the Earth is spinning on its axis, the rushing air does not travel in a straight line toward the low-pressure center. Instead, it is deflected and starts to spin around the low-pressure area. This deflection is called the Coriolis Effect:

• Northern Hemisphere: Cyclones spin anticlockwise (counter-clockwise).
• Southern Hemisphere: Cyclones spin clockwise.

4. The Eye of a Cyclone: In the very center (middle) of the cyclone, a special circular region called the eye forms, where air moves downward, making the weather inside the eye remarkably calm and clear.

5. Why Cyclones Weaken Over Land: Cyclones start to weaken when they move over land or colder ocean water because they are cut off from warm, moist air, which acts as the fuel for the storm. Without this thermal energy supply, winds lose speed and the cyclone gradually fades away.

1. Definition & Causes of Global Warming: Global warming is the long-term increase in the average surface temperature of the Earth due to:

• Burning of fossil fuels (coal, oil, natural gas).
• Release of greenhouse gases such as methane (CH₄), carbon dioxide (CO₂), and nitrous oxide (N₂O).
• Cutting down forests (deforestation) and certain industrial and farming methods.

2. Primary Cause of Rising Sea Levels: Due to rising global temperatures:
• Melting of Ice: Polar ice caps and glaciers begin to melt rapidly, adding water to the oceans.
• Thermal Expansion of Seawater: Seawater expands in volume as it warms up. Both effects combine to cause higher sea levels.

3. Six Extreme Weather Impacts of Global Warming:

• Stronger Hurricanes: More heat is absorbed by the ocean, which fuels hurricanes, increasing their intensity with higher wind speeds and heavier rainfall.
• Heat Waves: Heat trapped by greenhouse gases in the atmosphere leads to extended periods of extreme heat.
• Flooding: A warmer atmosphere holds more moisture, producing heavier rainfall and intense inland and coastal flooding.
• Wildfires: High temperatures and long periods of dryness reduce moisture in air and soil, making forests and grasslands easily ignitable.
• Droughts: Higher temperatures cause faster evaporation from land, drying out soil and reducing water for plants, animals, and people.
• Stronger Winter Storms: Because warmer air holds more moisture, it produces heavier snowfall when winter temperatures are cold enough.

1. What is Geothermal Activity? Geothermal activity refers to the inner heat of the Earth (derived from the planet's molten interior and radioactive decay of minerals in the crust/mantle), which causes:

• Volcanic eruptions (molten magma escaping as lava and ash)
• Geysers (jets of boiling water and steam shooting into the air)
• Hot springs (naturally heated groundwater pools)
• Movement of tectonic plates (causing continental drift and earthquakes)

2. Why is Geothermal Energy a Renewable Resource? Geothermal energy is considered a renewable resource of energy due to its continuous sustainability as long as the Earth's internal heat remains active. Unlike fossil fuels, which can be exhausted and emit heavy greenhouse gases, Earth's core heat is continuously replenished for billions of years and can be harnessed to generate clean electricity and heat buildings.

1. Radiation Exposure & Background Radiation: Radiation exposure occurs when someone comes into contact with radiation energy from natural sources (Sun, ground) or man-made sources (X-ray machines, nuclear power plants). Background radiation is the constant, low-level ionizing radiation that exists around us all the time in the environment.

2. Major Natural Sources of Background Radiation:

• Cosmic Rays from Space: The Sun and other stars emit enormous numbers of cosmic rays (including high-speed protons). When they enter Earth's atmosphere and collide with air molecules, they produce gamma (γ) radiation.
• Rocks & Buildings: Heavy radioactive elements such as uranium and thorium occur naturally in underground rocks, decorative stones, and bricks.
• Radon Gas: Uranium in rocks decays into radon gas, which is a tasteless, colourless, and odourless gas. Radon is an alpha (α) emitter and is particularly dangerous if inhaled into the lungs in large quantities.
• Food & Drink: Food and water contain trace amounts of naturally occurring radioactive isotopes such as Potassium-40 (⁴⁰K) and Carbon-14 (¹⁴C), absorbed by plants and animals and ingested by humans.

3. Health & Environmental Hazards of Radiation:

• Short-term High Exposure: Causes radiation sickness (nausea, vomiting, fatigue, organ damage).
• Long-term Low Exposure: Increases the risk of cancer (especially lungs, thyroid, and bones).
• Genetic Effects: Damages DNA, causing genetic mutations affecting future generations.
• Internal & Environmental Contamination: Inhaling radioactive dust or drinking contaminated water keeps radioactive particles inside the body; improper storage or nuclear plant accidents cause leaks, spills, explosions, or large-scale disasters.

4. Three Essential Radiation Safety Measures:
(i) Wearing protective wear (lead aprons/gloves/shielding),
(ii) Ensuring limited exposure (minimizing time & maximizing distance), and
(iii) Storing and handling radioactive materials safely (in sealed lead containers with proper waste management).