BharatNotes Why this chapter matters for UPSC: This is Chapter 12, "Magnetic Effects of Electric Current", of NCERT's Class X Science (Reprint 2026-27, pp. 195-207). It starts from Oersted's observation that a current deflects a compass needle, maps the magnetic field of a bar magnet, a straight wire, a loop and a solenoid, and then finds the force that a magnetic field exerts on a current. It ends inside the home: live, neutral and earth wires, separate 15 A and 5 A circuits, the fuse, the short circuit and overloading. For Prelims general science the chapter supplies the direction rules and the safety logic of home wiring; GS3 meets the same physics in power generation, electric motors and medical imaging.
Contemporary hook: On 31 August 2026 India's installed capacity included 2,24,408 MW of coal, 52,065 MW of hydro (including pumped storage) and 8,780 MW of nuclear plant, according to the Central Electricity Authority. Every one of these stations makes its electricity in a generator, which NCERT's Class XII Physics (Reprint 2026-27) sorts into hydro-electric, thermal and nuclear by what supplies the mechanical energy. The current Class X book no longer teaches how a generator works. The 2020-21 edition did, and this page keeps that part in a retained block.
🧠 First Principles — Read This First
- A current makes a magnetic field. In 1820 Oersted saw a compass needle deflect when a current passed through a wire nearby. Electricity and magnetism are linked (p. 195).
- Field lines are a map of the field. Outside a magnet they emerge from the north pole and merge at the south pole; inside, they run from south to north, so they are closed curves. They crowd where the field is strong, and no two of them ever cross (p. 197).
- The shape of the conductor sets the pattern. A straight wire gives concentric circles, stronger with more current and weaker farther away. A circular loop gives nearly straight lines at its centre. A solenoid behaves like a bar magnet and its field inside is uniform (pp. 198-201).
- Two rules, two hands. The right-hand thumb rule gives the direction of the field around a current. Fleming's left-hand rule gives the direction of the force on a current placed in a field. The force is largest when the current is at right angles to the field (pp. 200-203).
- A magnet pushes back on a current. Ampere suggested that if a current exerts a force on a magnet, the magnet must exert an equal and opposite force on the current. Electric motors, generators, loudspeakers, microphones and measuring instruments all use a current and a magnetic field together (pp. 202-203).
- Home wiring is built around safety. Three wires (live, neutral, earth), appliances in parallel, a fuse that melts when the current becomes unduly high, and an earth wire that holds a leaking metal body at the potential of the earth (pp. 204-205).
PART 1 — Quick Reference
Table 1: Magnets, magnetic field and field lines (12.1)
| Idea | What NCERT says |
|---|---|
| Compass needle | A small bar magnet. Its ends point approximately north and south; the north-seeking end is the north pole, the south-seeking end the south pole |
| Poles | Like poles repel; unlike poles attract |
| Magnetic field | The region around a magnet in which the force of the magnet can be detected. It has both direction and magnitude |
| Direction of the field | The direction in which the north pole of a compass needle moves inside the field |
| Field lines | The lines along which iron filings align (Activity 12.2), or curves traced point by point with a compass (Activity 12.3). The summary defines a field line as the path along which a hypothetical free north pole would tend to move |
| Direction of field lines | Outside the magnet, from the north pole to the south pole; inside, from the south pole to the north pole. So field lines are closed curves |
| Strength | Shown by how close the lines are: the field is stronger where they are crowded, as near the poles |
| No crossing | If two lines crossed, a compass needle at that point would have to point in two directions, which is not possible |
Source: NCERT, Science Class X, ch. 12, Reprint 2026-27, section 12.1, Activities 12.2-12.3 and summary (pp. 196-197, 206).
Table 2: The magnetic field of a current (12.2)
| Conductor | Pattern of field lines | What sets the strength | Where in the book |
|---|---|---|---|
| Straight wire | Concentric circles centred on the wire, in a plane perpendicular to it; they grow larger with distance from the wire | Rises as the current rises; falls as the distance from the wire increases (the book: it "depends inversely on the distance") | Activities 12.4-12.5, Fig. 12.6 (pp. 198-200) |
| Circular loop | Near the wire, concentric circles; by the centre of the loop the arcs of these large circles appear as straight lines | Every section of the wire adds field in the same direction inside the loop; a coil of n turns gives n times the field of a single turn | Section 12.2.3, Fig. 12.8, Activity 12.6 (pp. 200-201) |
| Solenoid (a coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder) | Similar to a bar magnet: one end behaves as a north pole, the other as a south pole; inside, parallel straight lines | The field is the same at all points inside: it is uniform | Section 12.2.4, Fig. 12.10 (p. 201) |
| Electromagnet | A piece of magnetic material such as soft iron, placed inside a solenoid, is magnetised by the strong field there | The summary defines it as a core of soft iron wrapped with a coil of insulated copper wire | Fig. 12.11 (p. 201); summary (p. 206) |
Source: NCERT, Science Class X, ch. 12, Reprint 2026-27, section 12.2 and the summary (pp. 197-201, 206).
Table 3: The direction rules
| Rule | Hand | What it gives | How to apply it |
|---|---|---|---|
| Right-hand thumb rule (also called Maxwell's corkscrew rule) | Right | Direction of the magnetic field around a current-carrying straight conductor | Hold the conductor in the right hand with the thumb along the current; the fingers wrap around it in the direction of the field lines. Corkscrew form: drive a corkscrew in the direction of the current, and its rotation gives the direction of the field |
| Fleming's left-hand rule | Left | Direction of the force (and motion) on a current-carrying conductor in a magnetic field perpendicular to it | Stretch thumb, forefinger and middle finger so that they are mutually perpendicular. First finger along the field, second finger along the current: the thumb gives the motion or force |
| Fleming's right-hand rule (2020-21 edition only) | Right | Direction of the current induced in a conductor moving in a magnetic field | Forefinger along the field, thumb along the motion of the conductor: the middle finger gives the induced current |
Source: NCERT, Science Class X, ch. 12, Reprint 2026-27, sections 12.2.2 and 12.3 (pp. 200, 203); 2020-21 edition, ch. 13, section 13.5 (p. 235).
Table 4: The activities
| Activity | Set-up | What it shows |
|---|---|---|
| 12.1 (p. 195) | A thick copper wire XY, perpendicular to the plane of the paper, in a circuit; a small compass placed horizontally near it | The needle is deflected when the current flows: a current has a magnetic effect |
| 12.2 (p. 196) | A bar magnet on white paper; iron filings sprinkled round it; the board tapped | The filings align along the field lines |
| 12.3 (pp. 196-197) | A compass moved step by step from the north pole of a bar magnet towards its south pole, the needle's ends marked each time | A field line drawn by joining the points; the deflection increases as the needle nears the poles |
| 12.4 (p. 198) | A long straight copper wire placed parallel to and over a compass needle; two or three 1.5 V cells and a plug key | With the current from north to south, the north pole of the needle moves east; with the current reversed, west. Reversing the current reverses the field |
| 12.5 (pp. 198-199) | A thick copper wire through the centre of a cardboard, normal to its plane; a 12 V battery, a rheostat and an ammeter (0-5 A); iron filings on the cardboard | Concentric circles round the wire. A larger current gives a larger deflection at a point; at a farther point the deflection is smaller |
| 12.6 (p. 201) | A circular coil of many turns passed through two holes in a cardboard; iron filings | The field pattern of a current-carrying coil |
| 12.7 (pp. 202-203) | An aluminium rod AB about 5 cm long, hung horizontally between the poles of a strong horse-shoe magnet, north pole below and south pole above (field upwards) | With the current from B to A the rod is displaced to the left; reversed, to the right; with the poles interchanged, the force reverses again. The force depends on the directions of both the current and the field |
Source: NCERT, Science Class X, ch. 12, Reprint 2026-27, Activities 12.1-12.7 (pp. 195-203).
Table 5: Worked examples and in-text questions
NCERT prints answers for the worked examples only. For the in-text questions the answers below follow from the chapter's own rules.
| Item | Question | Answer |
|---|---|---|
| Example 12.1 (p. 200) | A current flows east to west in a horizontal power line. What is the field at a point directly below it and directly above it? | The book: the field turns clockwise in a plane perpendicular to the wire when viewed from the east end, and anti-clockwise when viewed from the west end. As compass directions, by the same rule: towards the south at the point below, towards the north at the point above |
| Example 12.2 (p. 203) | An electron enters a magnetic field at right angles to it (Fig. 12.14). Direction of the force? | (d) into the page. The current is taken opposite to the motion of electrons; Fleming's left-hand rule then gives the force |
| In-text, p. 201 | A circular loop lies on a table and carries a clockwise current. Field inside and outside? | By the right-hand rule: inside the loop the field points into the table; outside it, out of the table |
| In-text, p. 202 | The field inside a long straight current-carrying solenoid | (d) is the same at all points (the text: uniform inside) |
| In-text, p. 203 | Which property of a proton can change as it moves freely in a magnetic field: mass, speed, velocity, momentum? | Velocity and momentum. The force acts at right angles to the motion, so it turns the proton without changing its speed; the mass does not change |
| In-text, p. 204 | How does the displacement in Activity 12.7 change if (i) the current is increased, (ii) a stronger magnet is used, (iii) the rod is made longer? | (i) and (ii) increase the force, so the displacement increases. (iii) increases it too, as long as the added length lies in the field between the poles |
| In-text, p. 204 | An alpha particle (positive) moving west is deflected north. Direction of the field? | (d) upward. Take the current along the particle's motion (west), the force north, and apply Fleming's left-hand rule |
| In-text, p. 205 | A 2 kW oven is run on a 220 V circuit rated 5 A. What happens? | It draws I = P/V = 2000/220 ≈ 9.1 A, far above the rating (arithmetic, P = VI from Chapter 11). The circuit is overloaded and its fuse should melt and break the circuit |
Source: NCERT, Science Class X, ch. 12, Reprint 2026-27, Examples 12.1-12.2 and in-text questions (pp. 200-205).
Table 6: Domestic electric circuits (12.4)
| Element | What NCERT says |
|---|---|
| Mains | Power reaches the home through overhead poles or underground cables |
| Live wire | Usually red insulation (NCERT adds "or positive": see Table 8) |
| Neutral wire | Black insulation (NCERT adds "or negative") |
| Potential difference | 220 V between live and neutral, in India |
| Meter board | The wires pass through a main fuse into the electricity meter, then through the main switch to the line wires of the house |
| Separate circuits | Often two: one of 15 A rating for appliances of higher power such as geysers and air coolers; one of 5 A for bulbs, fans and the like |
| Earth wire | Green insulation; usually connected to a metal plate deep in the earth near the house. Used for appliances with a metallic body (electric press, toaster, table fan, refrigerator) |
| How earthing protects | The metallic body is connected to the earth wire, which provides a low-resistance conducting path. Any leakage of current to the body keeps it at the potential of the earth, so the user may not get a severe shock |
| Appliances | Connected across live and neutral in parallel, each with its own switch, so that each has an equal potential difference (Fig. 12.15) |
| Fuse | Joule heating melts it and breaks the circuit when the current becomes unduly high (principle in section 11.7 of Chapter 11) |
| Short circuit | Live and neutral come into direct contact, when insulation is damaged or an appliance is faulty; the current rises abruptly |
| Other causes of overloading | An accidental hike in the supply voltage; too many appliances on a single socket |
| Supply | AC, 220 V, 50 Hz (stated in the summary) |
Source: NCERT, Science Class X, ch. 12, Reprint 2026-27, section 12.4 and summary (pp. 204-206).
Table 7: Exercises and NCERT's answers
| Exercise | Answer | Where it comes from |
|---|---|---|
| 1. Field near a long straight wire | (d) Concentric circles centred on the wire | NCERT Answers, p. 219 |
| 2. Current at the time of a short circuit | (c) Increases heavily | NCERT Answers, p. 219 |
| 3. True or false: (a) the field at the centre of a long circular coil is parallel straight lines; (b) a wire with green insulation is usually the live wire | (a) True; (b) False (green is the earth wire; live is red) | NCERT Answers, p. 219 |
| 4. Two methods of producing magnetic fields | A magnet, and an electric current through a conductor (straight wire, loop, solenoid or electromagnet) | No printed answer; sections 12.1-12.2 |
| 5. When is the force on a current in a field largest? | When the current is at right angles to the field | No printed answer; p. 203 |
| 6. Electron beam moving from the back wall to the front wall, deflected to your right: direction of the field? | Vertically downwards | NCERT Answers, p. 219 |
| 7. Rules for (i) the field around a straight current, (ii) the force on a current in a perpendicular field, (iii) the current induced in a coil rotating in a field | (i) Right-hand thumb rule; (ii) Fleming's left-hand rule. Part (iii) is not answered in the 2026 key: it is Fleming's right-hand rule | NCERT Answers, p. 219; 2020-21 Answers, p. 282 (old Exercise 15) |
| 8. When does a short circuit occur? | When the live wire and the neutral wire come into direct contact | No printed answer; p. 205 |
| 9. Function of the earth wire | A low-resistance path to earth that keeps a leaking metallic body at earth potential, so the user does not get a severe shock | No printed answer; p. 204 |
Source: NCERT, Science Class X, ch. 12, Reprint 2026-27, Exercises (p. 207) and Answers (p. 219); 2020-21 edition, Answers (p. 282).
Table 8: NCERT lines to read with care
| Where | What the book prints | How to read it |
|---|---|---|
| Opening (p. 195) | Asks about "the reverse possibility of an electric effect of moving magnets" and promises to study "such electromagnetic effects" | The 2026 chapter never returns to the question. Its answer, electromagnetic induction, was in the sections cut from the book; it is kept in the retained block below |
| Oersted box (p. 195) | "His research later created technologies such as the radio, television and fiber optics." | The box documents only the 1820 observation and gives no account of how these technologies followed from it. Read the line as a broad tribute (they grew from the wider science of electromagnetism), not as an exam fact |
| Oersted box (p. 195) | "The unit of magnetic field strength is named the oersted in his honor." | A question on the SI unit of the magnetic field wants the tesla. Class XII Physics gives the tesla (T) as the unit of the magnetic field B and names the gauss (10⁻⁴ T) as a smaller non-SI unit |
| Fig. 12.11 caption (p. 201) | A solenoid used to "magnetise steel rod" as an electromagnet | The text on the same page names soft iron, and the summary defines an electromagnet as "a core of soft iron". Use soft iron in an answer |
| Section 12.4 (p. 204) | Live wire "(or positive)"; neutral wire "(or negative)" | The home supply is AC (the summary says 220 V at 50 Hz), and an alternating current reverses its direction periodically: in India after every 1/100 second, as the 2020-21 edition put it. Neither wire stays positive or negative. Read live and neutral as names, not polarities |
| Section 12.4 (p. 205) | "Overloading can occur when the live wire and the neutral wire come into direct contact ... This is called short-circuiting." | The paragraph folds the short circuit into overloading. The summary (the fuse protects against "short-circuiting or overloading") and Exercises 2 and 8 treat the short circuit on its own, and answers should too: a short circuit is live touching neutral; overloading in the narrower sense is a circuit made to carry more current than it is rated for, such as too many appliances on one socket or the 2 kW oven on a 5 A circuit. Both make the current unduly high, and the fuse guards against both |
| Summary (p. 206) | "AC electric power of 220 V with a frequency of 50 Hz" | Alternating current and frequency are no longer explained in the chapter. The 2020-21 edition explained both in its generator section (retained below) |
| Exercise 7(iii) (p. 207) | Asks for the rule giving the current induced in a coil rotating in a magnetic field | The 2026 text does not teach induction, and its key answers only (i) and (ii). The rule is Fleming's right-hand rule (2020-21 edition, p. 235) |
| Magnetism in medicine box (p. 204) | "The magnetic field inside the body forms the basis of obtaining the images of different body parts." | The box itself puts the body's fields at about one-billionth of the earth's. MRI works differently: per the US National Institute of Biomedical Imaging and Bioengineering, powerful magnets make a strong field that aligns protons in the body, radiofrequency pulses then disturb them, and the scanner detects the energy they release as they realign. Read the box as saying only that MRI rests on magnetism |
Source: NCERT, Science Class X, ch. 12, Reprint 2026-27 (pp. 195-207) and Answers (p. 219); 2020-21 edition, ch. 13 (pp. 235, 237); NCERT, Physics Class XII Part I, ch. 4, Reprint 2026-27, section 4.2.2 (p. 110); NIBIB, "Magnetic Resonance Imaging (MRI)".
PART 2 — Concepts & Narrative
Oersted's needle (Activity 12.1)
The chapter opens where the previous one ended. Chapter 11 studied the heating effect of a current; this one studies its magnetic effect. In Activity 12.1 a compass placed near a straight copper wire is deflected as soon as a current flows. NCERT's box tells the story behind it: in 1820 Hans Christian Oersted (1777–1851) accidentally discovered that a compass needle got deflected when an electric current passed through a metallic wire placed nearby, and so showed that electricity and magnetism are related phenomena.
The book then asks the natural next question, whether a moving magnet can produce an electric effect. The 2026 text does not answer it. The 2020-21 edition did, with Faraday's experiments of 1831; they are kept in the retained block below because the generator in every power station rests on them.
Magnetic field and field lines (12.1)
A compass needle is itself a small bar magnet, which is why it turns near another magnet: like poles repel and unlike poles attract. Sprinkle iron filings round a bar magnet and tap the board (Activity 12.2), and the filings settle into a pattern. The magnet exerts its influence in the space around it, the filings feel a force there, and the lines they settle along represent the magnetic field lines. The same lines can be drawn with a compass alone (Activity 12.3): mark the two ends of the needle, move it so that its south pole sits where its north pole was, and repeat until you reach the other pole of the magnet.
A field has both direction and magnitude. Its direction at a point is the way a compass north pole would move there, which fixes the convention that outside a magnet the lines run from the north pole to the south pole. Inside the magnet they continue from south to north, so every field line closes on itself. The closeness of the lines shows the strength: they crowd near the poles, where the force on another magnet is greatest.
Why field lines never cross. At any point the field has one direction, the way a compass needle placed there would point. If two lines crossed, the needle at the crossing would have to point two ways at once, which is impossible. This is NCERT's own argument (p. 197) and the answer to in-text question 3 on p. 200.
The field of a straight wire (12.2.1-12.2.2)
Activity 12.4 lays a long straight wire over a compass needle. With the current flowing from north to south the north pole of the needle swings east; reverse the current and it swings west. The field of a current has a direction, and it reverses with the current.
Activity 12.5 shows the shape. A thick wire is pushed through a cardboard, normal to its plane, and iron filings are sprinkled round it: they form concentric circles. A compass on one of those circles points along it. Raising the current increases the deflection at a given point; moving the compass farther from the wire, at the same current, reduces it. The field of a straight wire therefore grows with the current and weakens with distance.
The right-hand thumb rule gives the direction without a compass. Hold the wire in the right hand with the thumb along the current, and the curled fingers show the direction of the field lines. NCERT's footnote gives the same rule as Maxwell's corkscrew rule: drive a corkscrew in the direction of the current, and its rotation is the direction of the field.
Example 12.1 worked through. A horizontal power line carries current from east to west. Point the right thumb west. Seen from the east end, looking along the current, the fingers curl clockwise; seen from the west end they curl anti-clockwise. That is the book's answer. To turn it into compass directions, stand at the east end facing west: on a clockwise circle the top moves to your right, which is north, and the bottom moves to your left, which is south. So the field points north at a point directly above the line and south at a point directly below it. Check it against Activity 12.4: a current from north to south over a compass pushes the needle's north pole east, and the same rule gives east for a point below a southward current.
Loop, coil and solenoid (12.2.3-12.2.4)
Bend the straight wire into a circular loop. Near the wire the field is still a set of concentric circles, but they grow larger farther from the wire, and by the centre of the loop their arcs look like straight lines. Applying the right-hand rule to each part of the loop shows that every section of the wire sends its field through the inside of the loop in the same direction, so the contributions add. With n turns carrying the same current, the field is n times that of one turn (Activity 12.6 shows the pattern with iron filings).
Wind many turns of insulated copper wire closely into a cylinder and you have a solenoid. Its field outside looks like a bar magnet's: one end acts as a north pole, the other as a south pole. Inside, the lines are parallel and straight, which means the field is the same at all points there: it is uniform. That strong field can magnetise a piece of magnetic material such as soft iron placed inside the coil, and the result is an electromagnet.
Which end of a solenoid is north? Apply the right-hand rule to the turns: curl the fingers of the right hand along the current in the turns, and the thumb points along the field inside the coil. Inside a magnet the field runs from south to north, so the thumb points to the north end. Equivalently, the end at which the current is seen to flow anti-clockwise is the north pole. The 2020-21 edition asked students to find the poles with a bar magnet instead (old Exercise 8, which its key does not answer). Since like poles repel, the end that repels the north pole of a bar magnet is the north pole.
The force on a current in a magnetic field (12.3)
If a current pushes on a magnet, does the magnet push back? The French scientist Andre Marie Ampere (1775–1836) suggested that it must, with an equal and opposite force on the conductor. Activity 12.7 tests it. A short aluminium rod hangs between the poles of a horse-shoe magnet, the north pole below it, so the field points upward. Pass a current from B to A and the rod moves to the left; reverse the current and it moves to the right; swap the poles and the force reverses again. The force depends on the direction of the current and the direction of the field, and the displacement is largest when the two are at right angles. In that position the force is perpendicular to both, and Fleming's left-hand rule names its direction: with the thumb, forefinger and middle finger of the left hand mutually perpendicular, the first finger along the field and the second along the current, the thumb points along the force.
NCERT lists the devices that use current-carrying conductors and magnetic fields: the electric motor, the electric generator, loudspeakers, microphones and measuring instruments. The 2026 book names the motor and the generator here but no longer explains how either works.
Moving charges and the left-hand rule. The rule is stated for a current, and current is taken in the direction positive charges move. For an electron, point the second finger opposite to its motion; this is how Example 12.2 gets "into the page". Exercise 6 works the same way. An electron beam moves from the back wall to the front wall, so the current runs from front to back, towards you. The beam is pushed to your right. With the thumb to the right and the second finger pointing back towards you, the first finger points vertically downwards, which is NCERT's key. For a positive alpha particle moving west and deflected north (in-text question, p. 204), the current is along the motion, and the field comes out vertically upward. Because the force is always at right angles to the motion, it changes a charge's direction, and so its velocity and momentum, but not its speed.
Domestic electric circuits (12.4)
Power reaches a house through the mains, by overhead poles or underground cables. NCERT describes three wires: live (usually red), neutral (black) and earth (green), with 220 V between live and neutral. At the meter board the supply passes through a main fuse into the electricity meter, and through the main switch to the line wires of the house. These feed separate circuits, often one of 15 A for appliances of higher power such as geysers and air coolers and one of 5 A for bulbs and fans.
Every appliance sits across the live and neutral wires with its own switch, and the appliances are connected in parallel so that each has the same potential difference across it (Fig. 12.15). That is the household form of the parallel circuit of Chapter 11.
Why earthing protects. Appliances with a metallic body, such as an electric press, a toaster, a table fan or a refrigerator, have the body connected to the earth wire, which runs to a metal plate deep in the earth near the house. The earth wire provides "a low-resistance conducting path for the current". If current leaks to the metal body, the body is held at the potential of the earth, and a user who touches it may not get a severe shock. Earthing and the fuse are the two safety measures of in-text question 1 on p. 205.
The fuse, whose working the previous chapter explained, protects the circuit and the appliances from unduly high currents: the Joule heating in the fuse wire melts it and breaks the circuit. The book lists three ways the current can rise that far: live and neutral touching, an accidental hike in the supply voltage, and too many appliances on one socket.
Short circuit vs overloading. A short circuit happens when the live wire and the neutral wire come into direct contact, when insulation is damaged or an appliance is faulty; the current in the circuit rises abruptly. Overloading, in the narrower sense, is a circuit carrying more current than it is rated for: too many appliances on one socket, or a 2 kW oven on a 5 A circuit, which would draw about 9.1 A at 220 V. NCERT's paragraph calls the first a case of the second; its summary and exercises keep them apart, and so should an answer. The precaution against overloading (in-text question 3, p. 205) follows: do not connect too many appliances to one socket, and run high-power appliances on the circuit rated for them.
Magnetism in medicine (More to Know, p. 204)
Every current makes a magnetic field, including the weak ion currents that travel along nerve cells. When we touch something, the nerves carry an electric impulse to the muscles, and the impulse produces a temporary field, about one-billionth of the earth's. The heart and the brain are the two organs where the body's own field is significant. The box ends by linking magnetism to Magnetic Resonance Imaging; as Table 8 explains, MRI does not image these weak internal fields but uses a strong external one.
Did the 2020-21 edition differ?
Yes, substantially. The chapter was Chapter 13 (pp. 223-241) in the 2020-21 edition and is Chapter 12 (pp. 195-207) now. Three sections have been cut: 13.4 Electric Motor, 13.5 Electromagnetic Induction (with Activities 13.8 and 13.9, the Faraday box and Fleming's right-hand rule) and 13.6 Electric Generator, together on pp. 232-237. The opening paragraph lost its references to electric motors and generators, and the summary lost its lines on them. The exercises fell from 18 to 9: gone are the questions on induction, the generator, AC and DC generators, the motor, the poles of a solenoid, a magnet moved in a coil and two neighbouring coils, and the two motor and generator statements in the true-or-false question. What remains is unchanged in wording, including the domestic circuits section (old 13.7, now 12.4, which now refers to section 11.7 for the fuse), the Magnetism in medicine box and the summary's line on AC at 50 Hz.
Source: NCERT, Science Class X, 2020-21 edition (whole-book zip, Wayback Machine capture of 9 October 2021), compared with Reprint 2026-27.
Retained content: the electric motor, electromagnetic induction and the electric generator (2020-21 edition, ch. 13, pp. 232-237; not in Reprint 2026-27)
The electric motor (13.4). NCERT defined it as "a rotating device that converts electrical energy to mechanical energy", used in electric fans, refrigerators, mixers, washing machines, computers and MP3 players. A rectangular coil ABCD of insulated copper wire sits between the poles of a magnet, with the arms AB and CD perpendicular to the field. The ends of the coil connect to the two halves, P and Q, of a split ring, which turn with the axle and touch two stationary brushes, X and Y. Current enters through brush X and flows from A to B in one arm and from C to D in the other, opposite directions. By Fleming's left-hand rule the force pushes AB down and CD up, so the coil turns anti-clockwise. After half a rotation, Q touches X and P touches Y: the current in the coil reverses to DCBA, the forces reverse, and the coil keeps turning the same way. "A device that reverses the direction of flow of current through a circuit is called a commutator. In electric motors, the split ring acts as a commutator." Commercial motors use an electromagnet instead of a permanent magnet, many turns of wire, and a soft iron core; the core and the coil together are the armature, which enhances the power of the motor.
Electromagnetic induction (13.5). In 1831 Michael Faraday discovered how a moving magnet can be used to generate electric currents. In Activity 13.8 a coil with many turns is connected to a galvanometer, an instrument whose pointer rests at zero, the centre of its scale, and swings left or right according to the direction of a current. Move the north pole of a bar magnet towards the coil and the needle deflects momentarily, say to the right; it returns to zero when the magnet stops. Withdraw the magnet and it deflects to the left. Moving the coil instead of the magnet gives the same result, a south pole reverses each deflection, and when coil and magnet are both still there is no current. Relative motion of a magnet and a coil induces a potential difference, which sets up an induced current.
Activity 13.9 replaces the magnet with a second coil. Two coils (say of 50 and 100 turns) sit on a thick paper roll. Coil-1, with more turns, is connected to a battery and key; coil-2 to a galvanometer. When the key is plugged in, the needle jumps to one side and returns to zero; when coil-1 is disconnected it jumps the other way; while the current in coil-1 is steady or zero, nothing happens. Coil-1 is the primary, coil-2 the secondary, and a changing current in the primary changes the field lines around the secondary. "This process, by which a changing magnetic field in a conductor induces a current in another conductor, is called electromagnetic induction." The induced current is highest when the coil moves at right angles to the field, and Fleming's right-hand rule then gives its direction: forefinger along the field, thumb along the motion, and the middle finger shows the induced current.
NCERT's box on Faraday (1791–1867) described an experimental physicist with no formal education who worked in a book-binding shop and read the books that came in for binding. He attended Humphrey Davy's public lectures at the Royal Institute, sent Davy his careful notes, and was made an assistant in Davy's laboratory. His discoveries include electromagnetic induction and the laws of electrolysis, and he turned down the honorary degrees several universities conferred on him.
The electric generator (13.6). A generator uses mechanical energy to rotate a conductor in a magnetic field. A rectangular coil ABCD turns between the poles of a permanent magnet. Its two ends connect to two rings, R1 and R2, and two stationary brushes, B1 and B2, press on them and lead to the external circuit. As the coil turns with AB moving up and CD moving down, Fleming's right-hand rule gives induced currents along AB and CD, so current flows round ABCD and through the external circuit from B2 to B1. After half a rotation the arms swap roles and the current flows the other way, DCBA, from B1 to B2. "Such a current, which changes direction after equal intervals of time, is called an alternating current (abbreviated as AC)", and the device is an AC generator. Replace the two rings with a split-ring commutator and one brush always touches the arm moving up, the other the arm moving down: the current in the external circuit then flows one way, and the device is a DC generator.
AC and DC, in the 2020-21 text. Direct current always flows in one direction; alternating current reverses its direction periodically. Most power stations, the book said, produce AC. "In India, the AC changes direction after every 1/100 second, that is, the frequency of AC is 50 Hz." So 50 complete cycles a second means 100 reversals a second. NCERT gave AC's advantage as the ability to transmit electric power over long distances without much loss of energy.
The 2020-21 key settles the retained questions. The device used for producing electric current is a generator (old Exercise 3, answer (a)), and electromagnetic induction is producing an induced current in a coil by relative motion between a magnet and the coil (old Exercise 2, answer (c)). The essential difference between an AC and a DC generator is that the AC generator has slip rings (the rings R1 and R2) while the DC generator has a commutator (old Exercise 4, answer (d)). "An electric motor converts mechanical energy into electrical energy" is false, and "an electric generator works on the principle of electromagnetic induction" is true (old Exercise 6). A bar magnet pushed into a coil connected to a galvanometer moves the needle momentarily one way; withdrawn, momentarily the other way; held stationary inside the coil, it gives no deflection (old Exercise 13). The rule for the current induced in a rotating coil is Fleming's right-hand rule (old Exercise 15). The in-text question on how often a rotating coil's current changes direction has the answer half a revolution, from the text's own description.
Source: NCERT, Science Class X, 2020-21 edition, ch. 13, sections 13.4-13.6 (pp. 232-237), Exercises (pp. 240-241) and Answers (p. 282).
Beyond the textbook: generators in power stations, the unit of the field, India's magnetic observatories, and how MRI works
- Generators in power stations (Class XII). NCERT's Class XII Physics (Reprint 2026-27) sorts commercial generators by what supplies the mechanical energy: water falling from a height, as from dams (hydro-electric generators); steam at high pressure, raised with coal or other sources (thermal generators); and nuclear fuel in place of coal (nuclear power generators). It adds: "In most generators, the coils are held stationary and it is the electromagnets which are rotated." The frequency is 50 Hz in India and 60 Hz in certain countries such as the USA.
- The unit of the magnetic field. The same book's chapter on moving charges gives the SI unit of the magnetic field B as the tesla (T), named after Nikola Tesla (1856–1943), and the gauss (10⁻⁴ T) as a smaller non-SI unit. It puts the earth's magnetic field at about 3.6 × 10⁻⁵ T.
- India's magnetic observatories. According to the Indian Institute of Geomagnetism (IIG), the first magnetic observations in India were started at Madras in 1822, followed by recordings at Simla, Trivandrum and Colaba, all in 1841. Of these, "only Colaba observatory continued uninterrupted since 1841". The Alibag observatory in Maharashtra, IIG's Prime Magnetic Observatory, was "established in April 1904 as a successor to Colaba Magnetic Observatory, Bombay" and has been an INTERMAGNET observatory since 1997. IIG was given a full-scale mandate for geomagnetic and allied research in 1971 and now functions directly under the Department of Science and Technology.
- How MRI works. The US National Institute of Biomedical Imaging and Bioengineering (NIBIB) explains that "MRIs employ powerful magnets which produce a strong magnetic field that forces protons in the body to align with that field." A pulsed radiofrequency current then pushes the protons out of equilibrium, and when it is switched off the scanner detects the energy they release as they realign. The time this takes, and the energy released, vary with the environment and chemical nature of the molecules, which lets physicians tell tissues apart. MRI scanners "do not use the damaging ionizing radiation of x-rays", so MRI is the modality of choice when frequent imaging is required, especially in the brain, though it is more expensive than x-ray imaging or CT scanning. The strong field exerts very powerful forces on objects of iron, and people with implants such as pacemakers should not enter an MRI machine.
Source: NCERT, Physics Class XII Part I, Reprint 2026-27, ch. 4, section 4.2.2 (p. 110) and ch. 6 (pp. 171-172); Indian Institute of Geomagnetism, "About us" and "Magnetic Observatory Alibag" pages (accessed 7 October 2026); NIBIB, "Magnetic Resonance Imaging (MRI)" (accessed 7 October 2026).
PART 3 — UPSC Integration
Cross-paper relevance
- Prelims (general science) — Properties of field lines; the right-hand thumb rule against Fleming's two rules; the solenoid and the electromagnet; motor against generator; the earth wire, the fuse, the short circuit and overloading; AC at 50 Hz; the tesla as the unit of the magnetic field.
- GS3 (energy and infrastructure) — How hydro-electric, thermal and nuclear stations generate electricity; AC for long-distance transmission; electrical safety in homes.
- GS3 (science and technology) — Electromagnets and motors in everyday devices; MRI as an imaging technique without ionising radiation.
Past questions on these themes: No past question in the bank is set directly on this chapter.
Frames for Mains Answers
1. How a power station makes electricity. Begin with induction: a conductor moving in a magnetic field, or a field changing around a conductor, sets up a current (Faraday, 1831). In a station, falling water or steam raised by coal or nuclear fuel supplies the mechanical energy that turns the generator (Class XII Physics). Then anchor it in data: coal, hydro and nuclear capacity on 31 August 2026 (CEA). Close with the motor as the same physics run the other way, electrical energy in and motion out.
2. Electrical safety in homes. Three wires with distinct roles; appliances in parallel so that each gets the full voltage; separate 15 A and 5 A circuits; the fuse as a deliberate weak link that melts by Joule heating; earthing that keeps a faulty metal body at earth potential. Distinguish the short circuit from overloading, and give the precaution for each.
3. MRI in public health. MRI images soft tissue using a strong magnetic field and radio pulses, without ionising radiation (NIBIB). That makes it suitable for repeated imaging, especially of the brain, but it is costlier than x-ray imaging or CT, and it is unsafe for people with implants such as pacemakers. Those three points carry an answer on diagnostic access.
Exam Strategy
Prelims fact-traps:
- Right-hand thumb rule: the field around a current. Fleming's left-hand rule: the force on a current (motor). Fleming's right-hand rule: the induced current (generator).
- In both Fleming rules the forefinger is the field. In the left-hand rule the second finger is the current and the thumb the motion; in the right-hand rule the thumb is the motion and the middle finger the induced current.
- Field lines run north to south outside a magnet and south to north inside; they are closed curves and never cross.
- The field inside a long solenoid is uniform; an electromagnet's core is soft iron.
- For electrons, take the current opposite to their motion.
- The force on a current is largest when the current is at right angles to the field.
- Red live, black neutral, green earth; 220 V; 15 A and 5 A circuits; appliances in parallel.
- AC in India: 50 Hz, changing direction after every 1/100 second.
- A motor turns electrical energy into mechanical energy; a generator does the reverse. The split ring is the commutator.
Mains: Use the chapter as the science line in a GS3 energy or health answer: one principle (induction, or the force on a current), one dated figure, then the application.
Practice Questions
Questions 1-3 are NCERT exercise items with answers in NCERT's key (question 3 is Exercise 6 given options). Practice (UPSC-pattern, not past papers): questions 4-11.
1. Which of the following correctly describes the magnetic field near a long straight wire?
(a) The field consists of straight lines perpendicular to the wire.
(b) The field consists of straight lines parallel to the wire.
(c) The field consists of radial lines originating from the wire.
(d) The field consists of concentric circles centred on the wire.
Answer: (d) (NCERT Answers, p. 219).
2. At the time of short circuit, the current in the circuit
(a) reduces substantially.
(b) does not change.
(c) increases heavily.
(d) vary continuously.
Answer: (c) (NCERT Answers, p. 219).
3. You sit in a chamber with your back to one wall. An electron beam moving horizontally from the back wall towards the front wall is deflected by a strong magnetic field to your right. The direction of the magnetic field is
(a) vertically upwards
(b) vertically downwards
(c) towards the front wall
(d) towards the back wall
Answer: (b) (NCERT Answers, p. 219). The current runs opposite to the electrons, from the front wall to the back.
4. Consider the following statements about magnetic field lines:
1. Outside a magnet they emerge from the north pole and merge at the south pole.
2. Inside a magnet they run from the north pole to the south pole.
3. Two field lines can cross where two magnets are placed close together.
Which of the statements given above is/are correct?
(a) 1 only
(b) 1 and 2 only
(c) 2 and 3 only
(d) 1, 2 and 3
Answer: (a). Inside a magnet the lines run from south to north, and field lines never cross (p. 197).
5. Consider the following pairs:
1. Right-hand thumb rule : direction of the force on a current in a magnetic field
2. Fleming's left-hand rule : direction of the force on a current-carrying conductor in a magnetic field
3. Fleming's right-hand rule : direction of the current induced in a conductor moving in a magnetic field
Which of the pairs given above is/are correctly matched?
(a) 1 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1, 2 and 3
Answer: (b). The right-hand thumb rule gives the direction of the field around a current (p. 200; 2020-21 edition, p. 235).
6. Consider the following statements:
1. The magnetic field inside a long current-carrying solenoid is uniform.
2. An electromagnet consists of a core of soft iron wrapped with a coil of insulated copper wire.
3. The field of a current-carrying solenoid is similar to that of a bar magnet.
Which of the statements given above is/are correct?
(a) 1 only
(b) 1 and 2 only
(c) 2 and 3 only
(d) 1, 2 and 3
Answer: (d) (pp. 201, 206).
7. Consider the following statements about domestic wiring in NCERT's account:
1. Appliances are connected in series so that each gets an equal potential difference.
2. The earth wire keeps the metallic body of a leaking appliance at the potential of the earth.
3. Higher-power appliances such as geysers are run on the 15 A circuit.
Which of the statements given above is/are correct?
(a) 1 and 2 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1, 2 and 3
Answer: (b). Appliances are connected in parallel (pp. 204-205).
8. An electric oven rated 2 kW is operated on a 220 V domestic circuit with a current rating of 5 A. The current it draws is about
(a) 0.11 A
(b) 4.5 A
(c) 9.1 A
(d) 440 A
Answer: (c). I = P/V = 2000/220 ≈ 9.1 A, well above 5 A, so the circuit is overloaded (arithmetic, on the in-text question of p. 205).
9. A proton moves freely in a magnetic field. Which of the following can change?
1. Its mass
2. Its speed
3. Its velocity
4. Its momentum
Select the correct answer using the code given below:
(a) 1 and 2 only
(b) 3 and 4 only
(c) 2, 3 and 4 only
(d) 1, 2, 3 and 4
Answer: (b). The force is at right angles to the motion, so it changes the direction but not the speed (NCERT in-text question, p. 203; NCERT prints no answer).
10. Consider the following statements from the 2020-21 edition of the chapter:
1. In an electric motor, the split ring acts as a commutator.
2. An AC generator has slip rings, while a DC generator has a commutator.
3. In India, AC changes direction after every 1/50 second.
Which of the statements given above is/are correct?
(a) 1 and 2 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1, 2 and 3
Answer: (a). AC changes direction after every 1/100 second, a frequency of 50 Hz (2020-21 edition, pp. 233, 237; Answers, p. 282).
11. Consider the following statements about Magnetic Resonance Imaging (MRI):
1. Like CT, it uses ionising x-rays.
2. It uses a strong magnetic field that aligns protons in the body.
3. People with implants such as pacemakers should not enter an MRI machine.
Which of the statements given above is/are correct?
(a) 1 and 2 only
(b) 2 and 3 only
(c) 3 only
(d) 1, 2 and 3
Answer: (b). MRI does not use the ionising radiation of x-rays (NIBIB).
📦 Revision Capsule
Hard Facts
- Oersted (1777–1851): a current deflects a compass needle, 1820. The oersted is named after him.
- Ampere (1775–1836): a magnet exerts an equal and opposite force on a current-carrying conductor.
- Faraday (1791–1867): a moving magnet generates current, 1831 (2020-21 edition).
- Home supply: 220 V; red live, black neutral, green earth; 15 A and 5 A circuits.
- AC in India: 50 Hz, changing direction after every 1/100 second (2020-21 edition, p. 237).
- SI unit of the magnetic field: tesla; gauss = 10⁻⁴ T (Class XII).
Core Concepts
- A current produces a magnetic field, and a magnetic field exerts a force on a current.
- The pattern of the field depends on the shape of the conductor.
- A straight wire's field rises with the current and falls with distance; n turns give n times the field.
- The force is largest when current and field are at right angles.
- Earthing and the fuse are the two safety measures.
- Relative motion of a coil and a magnet, or a changing current in a nearby coil, induces a current (2020-21 edition).
Confused Pairs
- Right-hand thumb rule (field around a current) vs Fleming's left-hand rule (force on a current) vs Fleming's right-hand rule (induced current).
- Motor (electrical to mechanical) vs generator (mechanical to electrical).
- Split-ring commutator (motor, DC generator) vs slip rings (AC generator).
- Short circuit (live touches neutral) vs overloading (more current than the circuit's rating).
- Soft iron (electromagnet core, text and summary) vs steel rod (Fig. 12.11 caption).
- Primary coil vs secondary coil (Activity 13.9, 2020-21 edition).
Data Points
- CEA, 31 August 2026: coal 2,24,408 MW; hydro including pumped storage 52,065 MW; nuclear 8,780 MW.
- IIG: Madras 1822; Simla, Trivandrum and Colaba 1841; Alibag April 1904; INTERMAGNET since 1997; full-scale mandate 1971; under DST.
- Earth's magnetic field about 3.6 × 10⁻⁵ T (Class XII); the body's own fields about one-billionth of the earth's (NCERT box).
PYQ Pattern
- No past question in the bank matches this chapter directly.
Sources
- NCERT, Science, Textbook for Class X, ch. 12 "Magnetic Effects of Electric Current", Reprint 2026-27 — ncert.nic.in PDF.
- NCERT, Science, Class X, Answers, Reprint 2026-27 — ncert.nic.in PDF.
- NCERT, Science, Class X, 2020-21 edition (whole-book zip), as archived on 9 October 2021, ch. 13 "Magnetic Effects of Electric Current" — Wayback Machine.
- NCERT, Physics Part I, Textbook for Class XII, ch. 4 "Moving Charges and Magnetism", Reprint 2026-27 — ncert.nic.in PDF.
- NCERT, Physics Part I, Textbook for Class XII, ch. 6 "Electromagnetic Induction", Reprint 2026-27 — ncert.nic.in PDF.
- Central Electricity Authority, "Installed Capacity (in MW) of the country as on 31.08.2026" — cea.nic.in PDF.
- Indian Institute of Geomagnetism, "About us" — iigm.res.in; "Magnetic Observatory Alibag" — iigm.res.in (both accessed 7 October 2026).
- National Institute of Biomedical Imaging and Bioengineering (US National Institutes of Health), "Magnetic Resonance Imaging (MRI)" — nibib.nih.gov (accessed 7 October 2026).
