10th Class Physics Notes 2026 — Chapter 21
Class: 10th
Subject: Physics
Board: Punjab
Chapter: 21
Exercise Solutions
A. Multiple Choice Questions (MCQs)
21.1 What process generates energy in the Sun?
(a) Nuclear fission
(b) Nuclear fusion
(c) Radioactive decay
(d) Chemical reactions
Correct Answer: (b) Nuclear fusion.
21.2 The factor that primarily determines a planet’s surface temperature is:
(a) Its density
(b) Its orbital distance from the Sun
(c) Its gravitational field strength
(d) The number of moons it has
Correct Answer: (b) Its orbital distance from the Sun.
21.3 What is the primary reason cyclones form over warm ocean waters?
(a) The presence of strong winds
(b) High atmospheric pressure
(c) The rising of warm, moist air, creating low pressure
(d) The gravitational pull of the Moon
Correct Answer: (c) The rising of warm, moist air, creating low pressure.
21.4 Which of the following gases contributes the most to global warming?
(a) Oxygen
(b) Carbon dioxide
(c) Nitrogen
(d) Argon
Correct Answer: (b) Carbon dioxide.
21.5 How does the Coriolis effect influence cyclones?
(a) It causes cyclones to spin in opposite directions in different hemispheres.
(b) It strengthens cyclones over land.
(c) It prevents warm air from rising.
(d) It stops the formation of low-pressure areas.
Correct Answer: (a) It causes cyclones to spin in opposite directions in different hemispheres.
21.6 A major cause of rising sea levels is:
(a) Increased volcanic activity
(b) The Earth moving closer to the Sun
(c) Melting glaciers and ice sheets
(d) Decreased solar radiation
Correct Answer: (c) Melting glaciers and ice sheets.
21.7 A significant source of background radiation is:
(a) Oxygen in the air
(b) Cosmic rays from space
(c) Sound waves in the atmosphere
(d) Light from the Sun
Correct Answer: (b) Cosmic rays from space.
B. Short Answer Questions
21.1 What are the two main elements that make up the Sun?
Answer:
The two main elements that compose the Sun are:
Hydrogen: Approximately 73%.
Helium: Approximately 25%.
21.2 What factors are used to compare planets in terms of their characteristics?
Answer:
Planets are primarily compared on the basis of the following factors:
Orbital distance from the Sun.
Mass and size (diameter).
Surface temperature and atmospheric composition.
Density and surface gravity.
21.3 Explain how global warming increases the intensity of hurricanes.
Answer:
Global warming increases ocean surface temperatures. Warmer ocean water evaporates more rapidly, transferring greater thermal energy and moisture into the atmosphere. As moist air rises and water vapour condenses, latent heat is released. This process can fuel stronger winds and heavier rainfall in hurricanes when other atmospheric conditions are favourable.
21.4 What is background radiation, and what are its natural sources?
Answer:
Background radiation is the low-level ionizing radiation that is constantly present in our environment from natural and artificial sources.
Natural sources include:
Cosmic rays from outer space.
Radon gas released from rocks and soil.
Naturally occurring radioactive elements in food, minerals, and living tissues, such as potassium-40.
21.5 Name three safety measures used to handle radioactive materials safely.
Answer:
Shielding: Use lead aprons, thick concrete walls, or lead-lined containers where appropriate.
Distance: Keep as far away from radioactive sources as possible and use remote-handling tools.
Limiting exposure time: Minimize the time spent near radioactive sources.
21.6 Why do cyclones weaken when they move over land?
Answer:
Cyclones weaken over land because they lose their primary energy source: warm ocean water that supplies heat and moisture through evaporation. In addition, greater surface friction over land disrupts their organized wind circulation.
21.7 How does the Coriolis effect influence the movement of cyclones?
Answer:
The Coriolis effect, caused by Earth’s rotation, deflects moving air masses. It causes cyclones to rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The Coriolis effect is very weak near the equator, making cyclone formation difficult there.
21.8 What is the primary cause of rising sea levels due to global warming?
Answer:
The two primary causes of rising sea levels are:
Thermal expansion: Seawater expands as the oceans absorb heat.
Melting land ice: Melting glaciers and land-based ice sheets add water to the oceans.
C. Constructed Response Questions
21.1 Why do gas giants like Jupiter, despite their larger size, have lower densities than rocky planets such as Earth and Mars? What does this tell us about their internal structure?
Answer:
Reason for low density: Gas giants consist predominantly of hydrogen and helium, whereas terrestrial planets are composed mainly of dense metals and silicate rocks.
Internal structure: Their low average density indicates that gas giants contain large amounts of light material and do not have a solid rocky surface like Earth. They have extensive gaseous outer layers surrounding dense interiors, which may include liquid or metallic hydrogen and relatively small, dense cores.
21.2 If two planets receive the same amount of sunlight but have different atmospheric compositions, why might their surface temperatures still differ significantly?
Answer:
Surface temperature depends partly on the greenhouse effect. A planet with an atmosphere rich in greenhouse gases, such as carbon dioxide and water vapour, can trap more outgoing infrared radiation and become warmer than a planet with a thin atmosphere that allows more heat to escape into space.
Therefore, even if two planets receive the same amount of sunlight, differences in their atmospheric composition can cause significant differences in surface temperature.
21.3 How does the Coriolis effect play a role in creating the spiralling motion of cyclones, and what might happen to cyclone formation if the Earth stopped rotating?
Answer:
Spiralling motion: Air flows towards a low-pressure centre. Earth’s rotation causes the moving air to be deflected, helping produce the rotating circulation associated with cyclones.
If Earth stopped rotating: The Coriolis effect would disappear. Air would still move towards low-pressure regions, but the rotation caused by the Coriolis effect would no longer occur. This would make the formation of cyclones with their usual large-scale rotating circulation much more difficult.
21.4 Compare the ways radiation, conduction, and convection each contribute to geothermal activity. Why is convection considered the primary driver of tectonic plate motion?
Answer:
Modes of heat transfer:
Radiation: Transfers energy through electromagnetic waves. Earth’s surface radiates heat into space.
Conduction: Transfers heat through direct contact between particles, such as through solid rocks in Earth’s crust and mantle.
Convection: Transfers heat through the movement of material. Slow convection in Earth’s mantle redistributes internal heat.
Why convection is important: Mantle convection, together with forces such as slab pull and ridge push, contributes to tectonic plate motion. The slow movement of hot and cooler mantle material helps drive the processes that move Earth’s lithospheric plates.
21.5 Explain why background radiation is always present in our environment and how cosmic and terrestrial sources contribute to it.
Answer:
Background radiation is continuously present because natural radioactive substances occur in Earth’s crust, soil, building materials, food, and living tissues. Cosmic rays from outer space also constantly reach Earth.
Cosmic sources include high-energy particles and radiation originating in space.
Terrestrial sources include radon gas, uranium, thorium, and potassium-40 found naturally in the environment.
Human activities, such as medical X-rays, CT scans, and industrial uses of radioactive materials, can add to radiation exposure. However, natural background radiation remains present even without these activities.
D. Comprehensive Questions
21.1 Describe the process of nuclear fusion in the Sun and explain how it generates energy.
Answer:
Process: The core of the Sun has an extremely high temperature of approximately \(15\times10^6\text{ K}\) and very high pressure. Under these conditions, hydrogen nuclei (protons) undergo a series of nuclear reactions known as the proton–proton chain reaction. Ultimately, four hydrogen nuclei are converted into a helium-4 nucleus, with other particles also produced.
Energy generation: The mass of the resulting products is slightly less than the total mass of the original reactants. This difference, called the mass defect, is converted into energy according to Einstein’s equation:
Where:
\(E\) = Energy released
\(\Delta m\) = Mass defect
\(c\) = Speed of light in a vacuum
This process releases vast amounts of energy, initially including gamma-ray photons and neutrinos. The energy ultimately reaches the Sun’s surface and is emitted into space.
21.2 How does a planet’s orbital distance affect its surface temperature and the amount of solar energy it receives?
Answer:
Solar energy intensity: The intensity of solar radiation received by a planet follows the inverse-square law:
Where \(I\) is the solar radiation intensity and \(r\) is the planet’s distance from the Sun.
For example, a planet twice as far from the Sun receives one-fourth as much solar energy per unit area, assuming other conditions are equal.
Surface temperature: Planets closer to the Sun generally receive more solar energy and tend to have higher equilibrium temperatures. More distant planets receive less solar energy and tend to be colder. However, atmospheric composition, greenhouse effects, reflectivity, and internal heat can also influence a planet’s actual temperature.
21.3 Explain the role of convection in the formation and strengthening of cyclones.
Answer:
Convection plays an important role in the formation and strengthening of tropical cyclones.
Heating and evaporation: Warm ocean water heats the air above it and supplies water vapour through evaporation.
Formation of low pressure: Warm, moist air rises because it is less dense than the surrounding air, helping create low pressure near the ocean surface.
Release of latent heat: As the rising moist air cools, water vapour condenses to form clouds and releases latent heat.
Strengthening of the storm: The released heat warms the surrounding air and supports further upward motion. More air flows towards the low-pressure centre, potentially strengthening the cyclone when environmental conditions are favourable.
21.4 What are the major sources of background radiation, and how do human activities contribute to it?
Answer:
Major natural sources:
Radon gas: Produced by the radioactive decay of naturally occurring elements in rocks and soil.
Cosmic radiation: High-energy particles and radiation reaching Earth from outer space.
Radioactive elements in the environment: Uranium, thorium, and their decay products occur naturally in rocks and soil.
Internal radiation: Naturally occurring radioactive isotopes, such as potassium-40, are present in food and human tissues.
Human contributions:
Medical procedures: Diagnostic X-rays, CT scans, and certain nuclear medicine procedures expose people to ionizing radiation.
Nuclear weapons testing: Radioactive fallout from past tests can contribute to environmental radiation.
Industrial activities: Certain industrial applications and nuclear facilities involve radioactive materials and require appropriate controls.
21.5 Explain how radiation exposure can be harmful and describe three key safety measures used to minimize its risks.
Answer:
Harmful effects: Ionizing radiation can remove electrons from atoms and molecules in human cells, damaging chemical bonds and DNA. High doses can cause tissue damage, radiation sickness, and cell death. Exposure can also increase the risk of mutations and cancer.
Three key safety measures:
Time: Minimize the time spent near a radiation source to reduce the total radiation dose.
Distance: Increase the distance from the source. For a point source of radiation, doubling the distance reduces the intensity to approximately one-fourth, following the inverse-square law.
Shielding: Place suitable absorbing materials between the person and the source. Lead, concrete, and other materials may be used depending on the type and energy of radiation.
Notes by Sir Dilawar
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