Class 9 Science Chapter 12 Magnetic Effects of Electric Current: 13 Solved Previous Year Questions

Reading your NCERT textbook teaches you *what* electromagnetic induction is; solving past papers teaches you *how* examiners ask about it. Every CBSE paper selects questions from a fixed conceptual pool: (1) magnetic field patterns around a straight wire and solenoid, (2) Fleming's left-hand and right-hand rules, (3) principles of electromagnetic induction and Lenz's law, (4) construction and function of AC generators and DC motors. When you solve 3–5 previous papers, you stop fearing 'new' questions because you recognize the underlying concept. Past papers also reveal time-management secrets: which questions demand full diagrams (5-mark), which need only symbolic answers (1-mark), and which tricks make students lose marks (e.g., confusing the direction of induced current with the direction of motion). Practising these questions under timed conditions—typically 45 seconds per 1-mark, 2 minutes per 3-mark, 4 minutes per 5-mark—builds the automaticity that earns you 35+ out of 40 in the Science paper. Start a 3-day free trial at cbsetutor.ai to access video walkthroughs of these exact questions.

**Q1: The magnetic field due to a long straight current-carrying wire is found to be proportional to (a) I/r (b) I/r² (c) I × r (d) I × r²** *Answer: (a) I/r* The magnetic field B around a straight wire is given by B = μ₀I/(2πr), where I is current and r is distance. Thus B ∝ I/r. This is a 1-mark recall question that appears almost every year. **Q2: Which end of a solenoid acts as the North Pole when current enters from the left end?** *Answer: The right end (apply the right-hand thumb rule: curl fingers in direction of current, thumb points to N-pole).* **Q3: State Lenz's Law in one sentence.** *Answer: The direction of the induced current is such that it opposes the change in magnetic flux that produced it.* **Q4: An electric motor converts _____ energy into _____ energy. (a) thermal, mechanical (b) electrical, mechanical (c) mechanical, electrical (d) thermal, electrical** *Answer: (b) electrical, mechanical* A motor uses electromagnetic force (Lorentz force on a current-carrying conductor in a magnetic field) to rotate the coil. **Q5: The device used to convert mechanical energy into electrical energy is called a/an _____.** *Answer: Generator (or alternator, or AC generator).* While a motor uses current to create motion, a generator uses motion (mechanical rotation) to induce current via Faraday's law.

**Q1: Describe the construction of an electric motor with a neat labelled diagram. State the function of the commutator.** *Solution:* An electric motor consists of: (1) a rectangular coil ABCD suspended between two magnetic poles (N and S), (2) two carbon brushes in contact with a split-ring commutator, (3) an external battery/power supply. The commutator is a split ring that reverses the direction of current in the coil every half rotation, ensuring the coil rotates continuously in the same direction. Without the commutator, the force would reverse and the coil would oscillate, not rotate. **Q2: State Fleming's Left-Hand Rule and apply it to find the direction of force on a current-carrying conductor AB placed perpendicular to a magnetic field.** *Solution:* Fleming's Left-Hand Rule: Stretch the thumb, forefinger, and middle finger of your left hand mutually perpendicular. Thumb = direction of force, Forefinger = direction of magnetic field (N to S), Middle finger = direction of current. If current flows from A to B and the field points into the page, the force points upward (perpendicular to both). **Q3: How is the direction of induced current determined by Lenz's Law? Explain with an example.** *Solution:* When a bar magnet is pushed into a solenoid, the magnetic flux through it increases. By Lenz's law, the induced current creates a magnetic field opposing this increase—i.e., the induced field repels the approaching magnet. The direction of induced current is such that its field points opposite to the increasing external field. This can be determined using the right-hand rule: if the induced field must point North inside the solenoid, curl your fingers in the direction of current; your thumb points North. **Q4: Distinguish between an AC generator and a DC motor in terms of (a) function, (b) commutator type, (c) output.** *Solution:* (a) Function: AC generator converts mechanical energy to alternating electrical energy; DC motor converts electrical energy to mechanical energy. (b) Commutator: AC generator has a slip ring (full ring); DC motor has a split-ring commutator. (c) Output: AC generator produces alternating current (voltage changes direction); DC motor receives direct current and produces continuous rotation. **Q5: A rectangular coil is rotated in a uniform magnetic field. Why does the induced EMF vary sinusoidally?** *Solution:* As the coil rotates, the angle θ between the normal to the coil and the magnetic field changes. The magnetic flux Φ = B·A·cos(θ) varies as cosine. By Faraday's law, induced EMF = −dΦ/dt ∝ sin(θ). Since θ = ωt (ω = angular velocity), EMF = EMF₀·sin(ωt), a sinusoidal function. Maximum EMF occurs when the coil is perpendicular to B (θ = 90°), and zero EMF when parallel.

**Q1: (a) Explain the principle of an AC generator with a diagram. (b) Derive an expression for the induced EMF. (c) Why is the output AC and not DC?** *Solution:* (a) Principle: An AC generator rotates a rectangular coil in a uniform magnetic field. As the coil rotates, the magnetic flux through it changes periodically. By Faraday's law of electromagnetic induction, a changing flux induces an EMF, which drives current through an external circuit. (b) When the coil rotates with angular velocity ω, the flux Φ(t) = B·A·cos(ωt). Induced EMF: ε = −dΦ/dt = B·A·ω·sin(ωt) = ε₀·sin(ωt), where ε₀ = B·A·ω is the peak EMF, and A is the area of the coil. (c) The output is AC because the coil rotates continuously, and the flux alternately increases and decreases. When the plane of the coil is parallel to B, flux is maximum and EMF is zero; when perpendicular, flux is zero and EMF is maximum. This oscillation produces alternating current. The slip ring commutator does not reverse current direction, so AC output results. **Q2: (a) Explain Faraday's Law of electromagnetic induction. (b) A solenoid with 500 turns has a cross-sectional area of 0.04 m². The magnetic field through it changes from 0.1 T to 0.3 T in 0.02 s. Calculate the induced EMF. (c) State Lenz's Law and explain how it determines the direction of induced current.** *Solution:* (a) Faraday's Law: The magnitude of induced EMF in a coil is equal to the rate of change of magnetic flux through it: ε = −N·(dΦ/dt), where N is the number of turns and Φ is the magnetic flux through one turn. (b) Given: N = 500, A = 0.04 m², ΔB = 0.3 − 0.1 = 0.2 T, Δt = 0.02 s. Change in flux: ΔΦ = A·ΔB = 0.04 × 0.2 = 0.008 Wb. Induced EMF: ε = N·ΔΦ/Δt = 500 × 0.008/0.02 = 500 × 0.4 = 200 V. (c) Lenz's Law: The direction of induced current is such that the magnetic field it produces opposes the change in the original magnetic flux. If flux is increasing into the page, the induced current creates a field out of the page to oppose the increase. This is determined using the right-hand rule: curl fingers in the direction of current, thumb points in the direction of the induced magnetic field. **Q3: (a) Draw a neat labelled diagram of a DC electric motor. (b) Explain how a DC motor works. (c) Why is a split-ring commutator essential in a DC motor?** *Solution:* (a) [Diagram should show: rectangular coil ABCD, north and south magnetic poles, carbon brushes touching the commutator, battery, and arrows indicating current flow and rotation.] (b) Working of DC motor: When current flows through the coil in the magnetic field, each current-carrying side experiences a force given by F = B·I·L (Fleming's Left-Hand Rule). Sides AB and CD experience forces in opposite directions, creating a torque that rotates the coil. As the coil rotates, it cuts magnetic field lines, which would induce a back EMF opposing the applied voltage. However, the commutator continuously reverses the current direction every half rotation, so the torque always acts in the same direction, causing continuous rotation. (c) The split-ring commutator reverses the current direction in the coil every half rotation. This ensures that the sides of the coil always experience forces in the same rotational direction. Without the commutator, the current would remain in the same direction relative to the external circuit, but the coil's orientation would reverse, making the force reverse—the coil would oscillate instead of rotating. The commutator is thus essential for sustained rotation.

The CBSE Class 9 Science paper has been rationalized, and Chapter 12 now emphasizes *application and integration* over memorization. Here's what changed: (1) **Reduced 1-mark factual recall:** Fewer standalone definition questions; more conceptual recognition embedded in circuit/diagram contexts. (2) **Increased case-based questions:** Expect multi-part scenarios describing a real-world setup (e.g., 'A turbine generates 50 Hz AC power; calculate the number of pole pairs'), requiring students to link theory to engineering. (3) **Focus on energy flow:** Questions now emphasize energy conversion efficiency and losses (friction, resistance). (4) **Diagram literacy:** Labelled circuit diagrams for motors/generators are no longer *asked to be drawn*; instead, students must *interpret* unlabelled diagrams and predict what happens when a variable changes (e.g., 'If the speed of the coil increases, what happens to the peak EMF?'). (5) **Cross-chapter integration:** Expect connections to Chapter 11 (Electric Current) and Chapter 13 (Sound) in multi-chapter questions. Practice drawing *quick, schematic* diagrams rather than art-class-level neat ones; examiners value clarity of labels over artistic quality. The 2026–27 pattern rewards students who can *reason* why a law exists (e.g., why Lenz's law makes physical sense via energy conservation) rather than students who simply state it.

**Time management:** On exam day, allocate 30 seconds per 1-mark, 1.5 minutes per 3-mark, and 3–4 minutes per 5-mark. In Chapter 12, 1-mark questions are fastest if you've memorized the three rules (Fleming's Left/Right-Hand, right-hand thumb rule for solenoids, Lenz's Law statement). Flag any question that asks you to *apply* these rules with a specific scenario; don't rush into an answer. **Question-reading trick:** Before attempting, underline key words: "increases," "decreases," "opposes," "perpendicular," "parallel." These words determine the direction (Lenz's law, Fleming's rule). Missing one word flips your answer. **Diagram strategy:** For motor/generator 5-mark questions, sketching takes 1.5 minutes if you know the standard layout. Label the coil, poles, commutator/slip-ring, and brushes clearly. If asked to explain function, use arrows to show current direction and use the phrase *'by Fleming's Left-Hand Rule'* to justify force direction. **Common marks-killer:** Forgetting to state that induced EMF *opposes* the applied voltage (back EMF in a motor). Always mention back EMF when explaining why a motor doesn't draw infinite current. **Formula safety:** Write Faraday's law as ε = −N·(dΦ/dt) and the magnetic field as B = μ₀I/(2πr) *once* in the margin of your answer sheet at the start. Referencing written formulas reduces careless algebra errors. **Final review tip:** Spend 2 minutes on questions about *direction* (Fleming, Lenz, right-hand rule). Spend 3 minutes on *calculation* questions (EMF, current, field strength). Spend 1 minute on definition questions. This balanced distribution maximizes marks per minute.