Class 9 Science Chapter 12 Some Natural Phenomena Previous Year Questions (2020–2025)

Reading your NCERT textbook a third time yields diminishing returns. Past papers, however, serve three critical functions: (1) **Pattern Recognition** — You see which sub-topics examiners prioritize. For example, "definition of electric charge" appears as a 1-mark question almost every year, while "explain lightning formation" is the classic 5-mark setup. (2) **Terminology Precision** — Exam answers demand exact NCERT phrasing. Saying "electrons move" is vague; saying "rapid movement of electrons from cloud to ground or vice versa" earns full marks. (3) **Time Management** — Solving 13 questions under timed conditions trains you to spot which questions you can answer in 90 seconds (1-mark) versus which need 4 minutes (3-mark). Students who ignore PYQs often run out of time in the actual exam, even though they know the content. By contrast, those who solve at least two years of papers typically complete the paper with 5–10 minutes to spare.

**Question 1: Define electric charge. What is its SI unit?** Answer: Electric charge is the physical property of matter that causes it to experience a force in an electric or magnetic field. The SI unit of electric charge is the coulomb (C). One coulomb is the charge carried by 6.25 × 10¹⁸ electrons (or protons). **Question 2: State the law of conservation of charge.** Answer: The total electric charge in an isolated system remains constant. Charge cannot be created or destroyed; it can only be transferred from one object to another. **Question 3: What is meant by "charging by friction"?** Answer: Charging by friction is the process by which two uncharged objects become electrically charged when rubbed together. Electrons transfer from one material to the other, leaving one positively charged and the other negatively charged. Example: rubbing a glass rod with silk. **Question 4: Define an electric field.** Answer: An electric field is the region around a charged object where another charged object experiences a force. It is a vector quantity, represented by field lines that always point away from positive charges and towards negative charges. **Question 5: What is the cause of lightning?** Answer: Lightning is caused by a sudden and violent discharge of electrical energy between clouds (or between a cloud and the Earth's surface), resulting from the accumulation of large amounts of electrical charge in storm clouds. This discharge occurs when the electric field strength exceeds the breakdown strength of air.

**Question 1: Explain the process of charging by induction with a diagram.** Answer: (i) Charging by induction does not involve direct contact. (ii) When a negatively charged rod is brought near an uncharged metal sphere, electrons in the sphere are repelled and move to the far side. The near side becomes positively charged (deficit of electrons). (iii) If the sphere is grounded (connected to Earth), electrons flow out, leaving the sphere permanently positively charged. (iv) Remove the rod, then remove the ground connection—the sphere now carries a permanent positive charge. [Diagram: Rod (−) near sphere; arrow showing electron movement to far side; then ground symbol with electrons flowing out.] **Question 2: How do lightning rods protect buildings?** Answer: A lightning rod is a metal conductor (usually copper or aluminium) installed on the roof of a building and connected to an underground metal plate. (i) The rod provides a safe path for electrical discharge from the cloud to the ground, bypassing the building structure. (ii) The metal conductor has very low resistance, so lightning follows this path instead of passing through the building. (iii) The rod is thick and pointed to enhance electric field concentration, allowing charge to leak away gradually during a thunderstorm, preventing sudden discharge. **Question 3: State the differences between lightning and electric spark.** Answer: **Lightning**: (i) Occurs between clouds and ground (or cloud to cloud), (ii) Voltage is extremely high (≈ 10⁸ volts), (iii) Current is very large (≈ 20,000–200,000 amperes), (iv) Temperature reaches ≈ 30,000 K, (v) Causes thunder due to heating of air. **Electric spark**: (i) Occurs between two nearby conductors, (ii) Voltage is moderate (≈ 10³–10⁴ volts), (iii) Current is small, (iv) Temperature is lower, (v) No sound. **Question 4: Explain why tall buildings, trees, and isolated structures are more prone to lightning strikes.** Answer: (i) Electric field strength increases near sharp, pointed objects. (ii) Tall structures reduce the distance between the cloud and ground, making the electric field stronger at those points. (iii) Trees and isolated buildings lack grounding protection, so they attract lightning rather than dissipating charge safely. (iv) The electric field becomes strong enough to cause ionization and breakdown of air resistance along these pathways. **Question 5: What is the relationship between earthquake magnitude and damage?** Answer: (i) Earthquake magnitude measures the energy released by seismic waves using the Richter scale (typically 1–9). (ii) Each unit increase in magnitude represents roughly 32 times more energy release (logarithmic scale). (iii) A magnitude 5 earthquake is ≈ 32 times stronger than magnitude 4. (iv) Magnitude ≥ 7 causes severe structural damage; magnitude ≤ 4 is barely felt by people. (v) Damage also depends on distance from epicentre, building design, and soil type, not magnitude alone.

**Question 1: Describe the process of lightning formation and explain why it is dangerous. How can humans protect themselves?** **Full Solution:** 1. **Lightning Formation Process** (2 marks): - Water droplets in storm clouds collide with each other and with ice particles. - Friction during collision causes transfer of electrons between particles. - Lighter ice particles move upward (become positively charged); heavier water droplets sink (become negatively charged). - A charge separation develops: positive charge accumulates at cloud top, negative at cloud base. - When charge separation becomes large enough (≈ 10⁸ volts), the electric field strength exceeds the breakdown potential of air. - Electrons suddenly rush from cloud base to ground (or positive charge flows upward from ground), creating a luminous discharge channel called lightning. 2. **Why Lightning is Dangerous** (1.5 marks): - Lightning current (20,000–200,000 A) causes instantaneous heating of air and anything in its path to ≈ 30,000 K. - This extreme heat burns tissue, causes cardiac arrest, and ignites fires. - The blast wave from superheated air causes shock injuries. - Lightning strike survivors often suffer permanent nerve damage, memory loss, and chronic pain. 3. **Safety Measures** (1.5 marks): - Install lightning rods on buildings connected to underground grounding plates. - During a thunderstorm, stay indoors; if caught outside, crouch low (do not lie flat) in an open area away from trees. - Avoid touching metal objects, water, or electrical appliances during storms. - Wait 30 minutes after the last thunderclap before resuming outdoor activities. **Question 2: Explain the difference between static and current electricity. How does charging by friction produce static electricity?** **Full Solution:** 1. **Static vs. Current Electricity** (2 marks): - **Static Electricity**: Charge accumulates on an insulator or isolated conductor and remains stationary. Electrons do not flow continuously; charge builds up until an external pathway (like lightning) allows sudden discharge. Example: rubbing a balloon makes it stick to a wall. - **Current Electricity**: Charge flows continuously through a conductor in a circuit, driven by a potential difference (voltage). Electrons move from negative to positive terminal through a closed loop. Example: current flowing through a bulb. - Key difference: Static = stationary charge accumulation; Current = continuous charge flow. 2. **Charging by Friction Mechanism** (2 marks): - When two insulating materials are rubbed together, mechanical energy causes electrons in outer shells to be displaced. - The material with weaker electron binding (lower ionization energy) loses electrons; the other gains them. - Example: Rubbing a glass rod with silk—electrons transfer from glass to silk. Glass becomes positively charged (electron deficit), silk becomes negatively charged (electron excess). - These charges remain static on insulators because insulators do not allow easy electron flow. A gradient builds up until the electric field becomes strong enough to cause discharge (e.g., a spark when you touch a charged object). 3. **Why Charge Remains Static** (1 mark): - Insulators have very high electrical resistance; they trap charge on their surface. - Charge cannot easily recombine because electron pathways through the material are blocked by covalent bonds. **Question 3: What is the epicentre and focus of an earthquake? Explain how seismic waves are generated and detected.** **Full Solution:** 1. **Epicentre and Focus** (1 mark each): - **Focus (Hypocenter)**: The point inside the Earth where seismic rupture originates. This is where two tectonic plates suddenly slip past each other, releasing stored elastic energy. - **Epicentre**: The point on Earth's surface directly above the focus. It is the location closest to the ground where earthquake damage is usually most severe. - Epicentre distance from focus = focus depth (typically 10–700 km). 2. **Generation of Seismic Waves** (1.5 marks): - Tectonic plates move continuously; friction at plate boundaries prevents smooth motion, causing stress to accumulate over years or decades. - When accumulated stress exceeds the strength of rock, sudden rupture occurs at the focus. - This rupture releases elastic energy in the form of seismic waves that radiate outward through the Earth in all directions. - Two main wave types: P-waves (primary, longitudinal—fast, ≈ 6 km/s) and S-waves (secondary, transverse—slower, ≈ 4 km/s). 3. **Detection of Seismic Waves** (1.5 marks): - Seismograph (seismometer) detects ground vibrations using a suspended mass attached to a stylus. - As the ground shakes, the mass remains stationary (inertia), and the stylus records the ground motion on a rotating drum or digital sensor. - The time difference between arrival of P-waves and S-waves indicates earthquake distance: Δt × velocity = distance. - Triangulation from three seismograph stations locates the epicentre. - Magnitude is determined from the amplitude of recorded waves (Richter scale).

Analysis of 2020–2025 papers reveals three heavily weighted micro-topics: (1) **Electrostatic Fundamentals** (≈ 40% of marks) — Charging methods, field definitions, and charge conservation appear in every paper. Expect at least one 1-mark definition question, one 3-mark explanation (induction or friction), and one 5-mark problem (lightning + safety). Master precise NCERT definitions here; vague language costs marks. (2) **Lightning Physics & Safety** (≈ 35% of marks) — This is the "real-world" section examiners love. Questions ask you to explain cloud charge separation, describe lightning formation step-by-step, justify why lightning rods work, and identify safety measures. Diagrams are essential; a labelled cloud-ground discharge diagram often earns 1–2 bonus marks. (3) **Earthquakes** (≈ 25% of marks) — Expect one 3-mark question on focus/epicentre definitions, Richter scale interpretation, or seismic wave detection. Fewer questions here, but they test conceptual depth. A common trap: students confuse epicentre with focus or incorrectly link magnitude to damage without mentioning distance/building design. To ace this chapter, memorize the three big diagrams: (a) friction charging, (b) induction charging, (c) lightning formation with cloud layers and charge distribution.

The CBSE has signalled three subtle shifts in Science assessment for 2026–27: (1) **Emphasis on Graphical Interpretation** — Rather than purely descriptive answers, expect questions that ask you to interpret a seismic graph, a Richter scale comparison, or a charge vs. force graph. Example: "A graph shows P-wave and S-wave arrival times from three stations. Mark the epicentre on a given map." Practise reading seismograms and plotting epicentre locations. (2) **Case-Study / Application Focus** — New 5-mark questions link phenomena to real events. Example: "A building in a lightning-prone region experiences three strikes per year. Design a protection system and explain each component." This tests both knowledge and engineering thinking. (3) **Reduced Rote Memorization** — One-word or single-sentence 1-mark answers are less favoured. Even 1-mark questions now ask for one short explanation, not just a name. Example: "Why does a glass rod become positively charged when rubbed with silk? (1 mark)" Answer: "Because electrons transfer from glass to silk." (Mechanics matter as much as terminology.) Start a 3-day free trial at cbsetutor.ai to access interactive 5-mark question walkthroughs and timed mock papers that match the 2026–27 format.

**In the Exam Hall — Spend 18–22 minutes on this chapter (proportional to 5–6% of the Science paper):** (1) **Scan & Triage (1 min)** — Read all questions for this chapter first. Identify which questions you can answer fastest (1-mark definitions), which need medium effort (3-mark explanations), and which require structured reasoning (5-mark problems). (2) **Attack 1-Marks First (3 mins)** — Do all five 1-mark questions back-to-back. These are confidence-builders and quick points: definitions of charge, electric field, epicentre, focus. Do not overwrite or rethink; move fast. (3) **Tackle 3-Marks Next (8 mins)** — Choose the two 3-mark questions you find clearest (usually induction/friction or earthquake Richter scale). Write point-form answers; examiners reward structure over prose. If stuck on one, skip and come back later. (4) **Solve 5-Marks Carefully (6–8 mins)** — This is the high-stakes question. Read it twice. If it asks to explain lightning AND protection, divide your answer into two labelled sections. Draw diagrams (charge distribution, rod setup, wave arrival). Do not rush the conclusion. (5) **Reserve 2 mins for Review** — Check that you have numbered all answers, that diagrams are labelled, and that your 5-mark solution has at least three distinct ideas (definition + mechanism + application). Do not rewrite entire answers unless you spot a factual error; minor grammar mistakes cost no marks.