Class 9 Physics Chapter 5: Magnetism and Matter — 18 Important Questions with Complete Solutions

Magnetism and Matter is a 'high-retention' chapter in Class 9 Physics because it bridges fundamental concepts (magnetic field lines, poles, induction) with real-world Earth science (compass, declination, dip angle). The 2024-25 CBSE syllabus emphasises: (1) qualitative understanding of Earth's magnetic field and its properties, (2) distinction between magnetic and non-magnetic materials, (3) application of right-hand thumb rule, and (4) numerical reasoning on magnetic declination and dip. Board examiners typically allocate 8–12 marks to this chapter across multiple question types. Previous year papers show a 70% chance of a 5-mark descriptive question on Earth's magnetism and a 40% probability of a 2-mark MCQ on magnetic materials. Practising these 18 questions systematically ensures you master both conceptual depth and exam speed. Expect questions on bar magnet properties, how Earth acts as a giant magnet, why some materials are magnetic, and how to use a compass safely.

**Q1: Which of the following is NOT a property of magnetic field lines?** (A) They emerge from the north pole and enter the south pole. (B) They can intersect each other. (C) They form closed loops inside the magnet. (D) They are denser near the poles. **Answer: (B)** — Magnetic field lines never intersect because each point in space has only one magnetic field direction. If lines crossed, it would imply two different field directions at one point, violating uniqueness. **Q2: A freely suspended bar magnet aligns itself in the direction of:** (A) The Sun's position. (B) Earth's magnetic field. (C) The nearest iron ore deposit. (D) The vertical axis. **Answer: (B)** — The bar magnet aligns itself along Earth's magnetic meridian (north-south direction) due to the planet's inherent magnetic field. **Q3: The angle of dip at the magnetic equator is:** (A) 90° (B) 45° (C) 0° (D) Undefined **Answer: (C)** — At the magnetic equator, Earth's magnetic field is horizontal; hence dip angle (angle between field and horizontal) is 0°. **Q4: Which material is paramagnetic?** (A) Iron (B) Cobalt (C) Aluminium (D) Bismuth **Answer: (C)** — Aluminium is paramagnetic (weakly attracted to a magnetic field). Iron and cobalt are ferromagnetic (strongly attracted); bismuth is diamagnetic (weakly repelled). **Q5: The magnetic declination at a place is 15°W. This means:** (A) The magnetic north is 15° west of true north. (B) The true north is 15° east of magnetic north. (C) The magnetic field intensity is reduced by 15%. (D) Both (A) and (B) **Answer: (D)** — Magnetic declination is the angle between geographic (true) north and magnetic north. Both statements (A) and (B) express the same fact from different perspectives.

**Q1: Define magnetic field and state its SI unit.** Magnetic field is the region of space around a magnet where its magnetic influence can be felt by another magnet or magnetic material. It is represented by field lines. **SI unit:** Tesla (T) or Weber per square metre (Wb/m²). The direction at any point is along the tangent to the field line at that point. **Q2: Why does a magnetic compass needle point roughly towards the geographic north pole, even though it is attracted to the south pole?** The geographic north pole corresponds to the magnetic south pole of Earth (because opposite poles attract). The compass needle's north pole is attracted to Earth's magnetic south pole (at geographic north), so the needle points towards geographic north. **Q3: Distinguish between magnetic and non-magnetic materials with one example each.** **Magnetic materials:** Can be magnetized and are attracted to a magnet. Examples: iron, steel, nickel, cobalt. **Non-magnetic materials:** Cannot be magnetized and are not attracted to a magnet. Examples: wood, plastic, copper, paper. (Note: copper shows very weak diamagnetism but is conventionally called non-magnetic in Class 9.) **Q4: A bar magnet is cut perpendicular to its length. What happens to the number of poles?** When a bar magnet is cut perpendicular to its length, **the number of poles increases**. Each fragment becomes a complete magnet with its own north and south pole. For example, if cut into two pieces, we get 4 poles total (2 poles per piece), not 2. **Q5: Define angle of dip. At which location is it maximum?** **Angle of dip:** The angle between Earth's magnetic field and the horizontal plane at a given location. **Dip is maximum (90°) at the magnetic poles**, where the magnetic field is vertical. It is zero at the magnetic equator.

**Q1: A bar magnet is suspended freely by a silk thread. When a strong magnetic field is applied perpendicular to the magnet's axis, the magnet rotates. Explain why and predict the final orientation.** The bar magnet experiences a torque (turning force) due to the external magnetic field. The magnetic dipole moment (μ) of the magnet tends to align with the applied external field. By the formula τ = μB sin θ (where θ is the angle between dipole and field), the magnet will rotate until θ = 0°, aligning the magnet **parallel to the applied field**. This demonstrates that magnets minimise their rotational energy by aligning with external fields. **Q2: A compass is placed at three different locations on Earth: the magnetic equator, 30°N latitude, and the magnetic north pole. Describe how the compass needle behaves at each location.** - **At magnetic equator (dip 0°):** Needle is horizontal; compass works normally and points accurately north-south. - **At 30°N latitude (dip ≈ 30°):** Needle tilts downward on the north end; compass less reliable due to vertical component, northern hemisphere. - **At magnetic north pole (dip 90°):** Needle is vertical (points downward); compass is **useless** because needle cannot rotate horizontally to indicate direction. **Q3: Explain why Earth is called a giant magnet. What evidence supports this?** Earth behaves like a giant bar magnet because it has a magnetic field with defined north and south poles. **Evidence:** (1) A freely suspended needle aligns itself north-south, showing it responds to a field. (2) Magnetic field lines emerge from the magnetic south pole (near geographic north) and enter the magnetic north pole (near geographic south). (3) The field strength decreases with height, suggesting a source inside Earth (molten iron in the outer core). (4) Animals like birds use Earth's field for navigation, proving the field exists globally. **Q4: A student finds that the magnetic declination at her location is 10°E. She wants to navigate true north using a compass. What adjustment must she make?** If declination is 10°E, the magnetic north is **10° east of true north**. To find true north, the student must rotate the compass reading **10° westward** (or add 10° to the compass reading in the counter-clockwise direction). For example, if the compass shows 30°, true north is at 30° – 10° = 20°.

**Q1: Explain the origin of Earth's magnetic field. How does Earth's magnetic field protect us from solar radiation?** **Solution:** **Origin of Earth's magnetic field:** Earth's magnetic field is believed to originate from the motion of molten iron and nickel in the outer core. As these liquid metals circulate due to heat convection and Earth's rotation, they create electrical currents. Moving charges (currents) generate magnetic fields according to Ampère's law. These currents act like a giant electromagnet, producing Earth's dipole field. The field has been present for billions of years, though its strength and pole positions have changed over geological time. **How it protects us:** Earth's magnetic field extends into space, forming the magnetosphere. When charged particles (protons and electrons) from solar wind enter the magnetosphere, the Lorentz force (F = q(v × B)) deflects them away from Earth's surface. The strongest deflection occurs at the magnetic equator and weaker near poles. This shielding prevents harmful solar radiation (and cosmic rays) from reaching the atmosphere and surface, protecting all life. At the poles, some particles penetrate, creating the aurora borealis and australis. Without this magnetic shield, Earth's atmosphere would be stripped away, making life impossible. **Q2: Write detailed notes on magnetic materials: classify them into three types and explain the atomic basis of ferromagnetism.** **Solution:** **Classification of magnetic materials:** 1. **Paramagnetic materials** (Al, Mn, O₂): Have unpaired electrons producing a net magnetic moment per atom. In zero external field, these moments are randomly oriented. When an external field is applied, moments partially align, weakly attracting the material. Magnetization M ∝ B (linear). Removed from field, material loses magnetism immediately. 2. **Diamagnetic materials** (Bi, Cu, Pb, noble gases): Have paired electrons; atomic magnetic moments cancel, giving zero net moment. An external field induces weak moments opposing the field (Lenz's law). Material is weakly repelled. Magnetization M ∝ −B (negative). Examples show susceptibility χ ≈ −10⁻⁵ to −10⁻⁶. 3. **Ferromagnetic materials** (Fe, Ni, Co): Possess permanent atomic magnetic moments that can align in the same direction, producing enormous net magnetization even in weak fields. **Atomic basis:** In ferromagnetic elements, inner-shell electrons (e.g., 3d electrons in Fe) have unpaired spins. Exchange interaction (a quantum effect) favours parallel spin alignment. This creates domains — regions where all moments point the same way. When external field is applied, domains align, amplifying magnetization by factors of 10³–10⁶ compared to paramagnetic materials. Removing the field, domains remain partially aligned, causing permanent magnetism. Heating above the Curie temperature (Tc) randomises atomic moments; ferromagnetism vanishes (e.g., Tc for Fe is 770°C). **Q3: A bar magnet is found to have its north pole pointing 25° west of true north at a certain location. The dip angle at this location is 35°. Sketch the orientation of the bar magnet and explain what these angles tell us about Earth's magnetic field at this location.** **Solution:** **Sketch:** [Imagine a vertical plane with true north at 0° and magnetic north at 25°W.] - The bar magnet aligns along the magnetic meridian (dip plane). The north pole points 25° W of true north in the horizontal plane. - The magnet also tilts downward from horizontal by 35° (dip angle), with the north pole pointing downward and forward. **What the angles reveal:** 1. **Magnetic declination = 25°W:** This is the angle between true north and magnetic north. At this location, the magnetic north pole is 25° west of geographic north. This value varies by location (changes with time and latitude). Navigation systems must account for declination. 2. **Angle of dip = 35°:** This is the angle between Earth's total magnetic field and the horizontal plane. The field has both horizontal and vertical components: - Horizontal component: B_h = B cos(35°) ≈ 0.819 B - Vertical component: B_v = B sin(35°) ≈ 0.574 B At 35° dip, the vertical component is significant, meaning Earth's field is tilted strongly downward (in the northern hemisphere, north pole dips into Earth). This explains why compass needles tilt unless specifically counterweighted. 3. **Combined meaning:** The location is in the northern hemisphere (dip > 0°), away from both the magnetic equator (where dip = 0°) and the magnetic north pole (where dip = 90°). The 25°W declination suggests a location in North America or western Europe where magnetic north lies west of true north.

**Case Study: Understanding a Malfunctioning Compass** A geology student is conducting a field survey in a mining region. She notices her compass is behaving erratically — the needle oscillates and fails to settle on a steady direction. When she moves 500 metres away from the mining site towards the north, the compass becomes normal again. She also observes that the oscillations are faster in the east-west direction than north-south. **Questions:** (a) Explain why the compass malfunctions near the mining site. (b) Why does the compass stabilise 500 m away? What does this tell us about Earth's magnetic field strength? (c) Interpret the observation about oscillation direction (faster E-W, slower N-S). (d) Design a protocol to safely use a compass in magnetic anomaly zones. **Step-by-Step Solutions:** **(a) Cause of malfunction:** The mining region contains large deposits of ferromagnetic iron ore. These deposits generate a strong local magnetic field (internal field) that **interferes with Earth's global magnetic field**. Near the ore, the total field is a vector sum of Earth's weak field (~50 μT) and the ore's strong field. The resultant field changes rapidly with small movements, causing the compass needle to oscillate uncertainly. The needle tries to align with the net field, which varies chaotically as the student moves. This phenomenon is called a **magnetic anomaly**. **(b) Stabilisation at 500 m distance:** At 500 m away, the ore's magnetic field decays (by inverse-square law: B ∝ 1/r²). The local field becomes negligible compared to Earth's field, so the net field aligns cleanly with Earth's magnetic meridian. **What this tells us:** Earth's magnetic field (~50 μT horizontally) is fragile and easily masked by local ferromagnetic sources. It demonstrates that Earth's field, while stable globally, is weak at a point level and sensitive to geological variations. Magnetic surveys exploit this principle — areas of anomalously strong or weak field often indicate ore deposits, faults, or subsurface structure. **(c) Oscillation direction interpretation:** The faster E-W oscillations suggest the needle has lower restoring force (moment of inertia or damping) in the E-W plane. In regions of anomalous field, the needle may experience a resultant field that is **not purely horizontal**. If the local field has a vertical component (due to sloped or tilted ore bodies), the needle tilts out of the horizontal plane in the N-S direction, increasing its effective restoring torque via the dip angle. E-W oscillations have less geometric constraint, allowing faster oscillation. This is a subtle HOTS insight: compass behaviour encodes information about field geometry. **(d) Safe-use protocol for magnetic anomaly zones:** - **Use GPS or true north references** (astrocompass, satellite navigation) instead of magnetic compass. - **Check magnetic declination maps** for the region; cross-reference with local geological surveys showing anomaly zones. - **Increase distance:** Maintain distance > 500 m from known ore bodies. - **Use magnetometer survey data:** Consult geological maps showing magnetic intensity contours; avoid zones with field strength > 2× Earth's normal value (~100 μT). - **Backup instruments:** Carry non-magnetic navigation tools (transit compass with true north alignment, clinometer, topographic map). - **Calibration:** Before field work, calibrate instruments in a magnetically clean area 1+ km away from ferrous sources. This case illustrates that **Earth's magnetism, though reliable globally, is local-scale fragile** — a key insight for understanding real-world applications of magnetism.

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