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← Science — all chaptersMagnetic Effects of Electric Current
Start here: why this chapter matters
In simple wordsSwitch on a current and a compass needle beside the wire twitches: the wire has become a magnet. That one fact explains electromagnets in cranes and doorbells, the force that spins every electric motor and fan, the speaker in your phone — and the colour-coded wires and fuses that keep a house safe. This chapter is the bridge from electricity to magnetism.
This is Chapter 12 of NCERT's Class 10 Science textbook (pages 195–207 of the 2026-27 reprint; the contents page and the PDF agree). In this rationalised edition it has four parts — magnetic field and field lines (12.1), the field of a current in a straight wire, a loop and a solenoid (12.2), the force on a current-carrying conductor (12.3) and domestic electric circuits (12.4) — with 7 activities, Figures 12.1–12.15, 2 worked examples, a box on Oersted, a ‘More to Know!’ box on magnetism in medicine, five ‘Question(s)’ boxes and 9 exercise questions. The older sections on the electric motor, electromagnetic induction and the generator are no longer in the book (see the rationalisation note below). Every field-line pattern on this page is computed rather than sketched, and every direction is worked out as a vector and checked in the page's source.
- Neighbouring chapters: Electricity (Chapter 11 of this book) gives current, resistance, parallel circuits, Joule heating and the fuse, all used here. Light – Reflection and Refraction (Chapter 9) set the physics conventions used on this page.
- Prelims — science: UPSC has asked about soft iron in electromagnets (1979), domestic wiring in parallel (1996), the frequency of household AC (2004) and how the Earth's magnetic field turns aside fast charged particles from space (2012).
- GS3 — science, technology and energy: electric motors and fans (efficiency), electromagnets in industry and transport, MRI in health care, and electrical safety at home (earthing, fuses, circuit breakers).
| Word | What it means here |
|---|---|
| Magnetic field | The region round a magnet (or a current) where its magnetic force can be detected; it has direction and magnitude |
| Field line | The path a free north pole would follow; its tangent gives the field's direction |
| Solenoid | A long coil of many closely wound turns of insulated wire, shaped like a cylinder |
| Electromagnet | A soft-iron core magnetised by a current-carrying coil — a magnet you can switch on and off |
| Right-hand thumb rule | Thumb along the current, fingers curl along the field of a straight wire |
| Fleming's left-hand rule | Field, current and force (motion) along three mutually perpendicular fingers of the left hand |
| Live / neutral / earth | Red supply wire / black return wire (220 V between them) / green safety wire to the ground |
| Short circuit / overload | Live touching neutral / too much current drawn — both make the current rise sharply |
Reading NCERT's chapter opening and Activity 12.1 with Figure 12.1 (page 195)The opener shows a small photograph of iron filings round a red bar magnet and a QR code labelled 1064CH13 (from when this was Chapter 13). Activity 12.1 puts a thick copper wire XY, perpendicular to the paper, through a board with a compass near it, in a circuit with a resistor, an ammeter, a cell and a key; Figure 12.1 shows the set-up. Closing the key deflects the needle. The point: a current produces a magnetic effect — electricity and magnetism are linked.
Reading NCERT's box on Hans Christian Oersted (page 195)A portrait and a short note: Oersted (1777–1851) found by accident in 1820 that a current in a nearby wire deflects a compass, showing that electricity and magnetism are related; the CGS unit called the oersted is named after him. The point: the magnetic effect of current is Oersted's discovery. (The box says his research ‘later created’ radio, television and fibre optics — read this as ‘opened the way to’; those technologies came from much later work by many scientists.)
Common confusions
- Magnetic field lines are not the path of the current: round a straight wire the current runs along the wire and the field circles round it.
- Right-hand thumb rule gives a field; Fleming's left-hand rule gives a force.
- Rationalisation gap: the motor, electromagnetic induction, the generator and Fleming's right-hand rule have been dropped, yet the opening still promises an ‘electric effect of moving magnets’, the summary still mentions 50 Hz AC, and exercise 7(iii) still asks for the rule for an induced current. For exams, know that the induced current direction is given by Fleming's right-hand rule (Beyond this edition).
At a glance — the six ideas to carry away
- Compass = a small bar magnet; its north-seeking end is the N pole. Like poles repel, unlike attract. Magnetic field: the region where a magnet's force can be detected — it has direction and magnitude.
- Field lines: leave N, enter S outside, run S → N inside — closed curves; crowded = stronger; never cross (a compass can't point two ways). Direction = where a free north pole would move.
- Oersted (1820): a current deflects a compass. Straight wire: field lines are concentric circles; right-hand thumb rule (thumb = current, fingers = field; also Maxwell's corkscrew rule). Field rises with current, falls with distance.
- Loop: near-straight field at the centre; n turns → n times the field. Solenoid: uniform field inside, bar-magnet pattern outside (one end N, one S). Soft iron core → electromagnet.
- Force on a current in a field (Ampère's suggestion; Activity 12.7): largest when current ⟂ field; reverses if current or field reverses. Fleming's left-hand rule: First finger Field, seCond finger Current, thuMb Motion. Used in motors, loudspeakers, meters.
- Home supply: 220 V AC, 50 Hz; live red, neutral black, earth green. Separate 15 A (geyser, cooler) and 5 A (bulbs, fans) circuits; appliances in parallel. Earthing keeps metal bodies at earth potential; fuse stops short-circuit and overload currents.
1. Magnets, magnetic field and field lines
In simple wordsA magnet reaches out into the space around it — bring a pin near and it gets pulled before it touches. That invisible reach is the magnetic field. Sprinkle iron filings and they line up to show its shape; a small compass moved bit by bit traces the same lines, like dots joined in a colouring book.
A compass needle is itself a small bar magnet. Its end that points roughly north is the north-seeking or north pole; the other is the south pole. Like poles repel and unlike poles attract — which is why a compass is deflected near a magnet. In Activity 12.2 iron filings sprinkled round a bar magnet on a board and tapped gently arrange themselves in a pattern (Fig. 12.2): the magnet exerts a force in the region round it. That region is its magnetic field, and the lines along which the filings line up are magnetic field lines. In Activity 12.3 a compass placed near the N pole is marked end to end, moved so its S end sits where its N end was, and so on until the S pole is reached; joining the marks gives one field line, and repeating gives the full pattern (Figs 12.3–12.4); the needle deflects more near the poles. A magnetic field has both direction and magnitude. Its direction is the way a north pole of a compass moves in it, so by convention field lines emerge from the N pole and merge at the S pole; inside the magnet they run from S to N, so field lines are closed curves. Where lines are crowded the field is stronger. No two field lines cross: at a crossing a compass would have to point two ways at once.
⤵ Activity 12.3: drawing one field line with a compass
Place the compass near the N poleits S end points towards the magnet's N pole
Mark both ends of the needletwo dots on the paper
Move the compass onits S end now sits on the dot where its N end was
Repeat step by stepuntil you reach the S pole
Join the dots smoothlythat curve is one field line; repeat for many lines
Reading NCERT's ‘Question’ box and Activity 12.2 with Figure 12.2 (page 196)The single question asks why a compass is deflected near a bar magnet. Activity 12.2 sprinkles iron filings round a bar magnet on a drawing board; Figure 12.2 is a photograph of the filings fanning out from the ends of a red bar marked N and S. The point: a magnet's force fills the space round it, and the filings show its shape.
Reading NCERT's Activity 12.3, Figures 12.3 and 12.4 (pages 196–197)Figure 12.3 shows a row of small compasses arching from the N end to the S end of a bar magnet, each needle along the curve. Figure 12.4 shows the finished pattern: closed loops from N round to S on both sides, with arrows, crowded near the poles. The point: field lines run N → S outside, are crowded near the poles, and never cross.
Common confusions
- Outside the magnet lines go N → S; inside they go S → N. They do not ‘end’ at the poles — they are closed.
- The number of lines drawn is a choice; their closeness shows strength.
- The direction of the field is where a north pole would move — not a south pole.
- Field lines never cross; two lines meeting would mean two field directions at one point.
- The Earth's geographic north has a magnetic south-type pole beneath it — which is why a compass's N end points there (Beyond NCERT).
2. The field of a straight current: Oersted and the right-hand thumb rule
In simple wordsGrab a straight wire with your right hand, thumb pointing the way the current flows: your curled fingers show how the invisible field circles the wire. More current means a stronger field; step farther away and it fades — like the warmth around a heater.
In Activity 12.4 a straight copper wire is stretched over a compass needle, parallel to it, in a circuit with two or three 1.5 V cells and a key. With the current flowing north to south the needle's north pole moves east; reversing the current (south to north) makes it move west — the direction of the field reverses with the current. In Activity 12.5 a thick copper wire passes vertically through the centre of a cardboard, in series with a 12 V battery, a rheostat, an ammeter and a key; iron filings on the card form concentric circles round the wire — the field lines (Fig. 12.6). A compass at a point P shows their direction, which reverses when the current does. Increasing the current increases the deflection: the field at a point grows with the current. Moving the compass farther out (to Q) reduces the deflection: the field weakens with distance, and the circles grow larger as you move away. The right-hand thumb rule (Fig. 12.7) gives the direction: hold the wire in your right hand with the thumb along the current; your fingers wrap round in the direction of the field. It is also called Maxwell's corkscrew rule: drive a corkscrew along the current and its turning gives the field. In Example 12.1, for a power line carrying current from east to west, the field below and above it circles clockwise seen from the east end and anticlockwise seen from the west end.
Reading NCERT's Activity 12.4 and Figure 12.5 (page 198)Two drawings (a) and (b) of a circuit with a key and cells whose sloping wire runs over a compass; a small compass rose shows N, E, S, W. In (a) the current runs north to south and an arrow marks the needle swinging east; in (b) the cells are reversed, the current runs south to north and the needle swings west. The point: reverse the current, reverse the field.
Reading NCERT's Activity 12.5 and Figure 12.6 (pages 198–199)Figure 12.6(a) shows a vertical wire XY through a card with dotted circles, a compass at P and a point Q farther out, in a circuit with a variable resistance, an ammeter, a battery and a key; (b) is a close-up photograph of iron filings forming rings round the wire. The point: circles round the wire, stronger close in and with more current.
Reading NCERT's Figure 12.7, Example 12.1, the footnote and the ‘Questions’ box (page 200)Figure 12.7 shows a right hand gripping a vertical wire, thumb pointing down along the current, with an elliptical field line and arrows. Example 12.1 applies the rule to an east–west power line. A footnote names it Maxwell's corkscrew rule. The box asks you to draw the field lines of a bar magnet, list their properties, and explain why two field lines never intersect. The point: thumb = current, curled fingers = field.
Common confusions
- Round a straight wire the field lines are concentric circles in planes perpendicular to the wire — not lines along it, and not radial lines (exercise 1: (d)).
- The field is stronger closer to the wire and with more current; it does not depend on the wire's colour or insulation.
- Use your right hand: thumb = current, fingers = field. With the current towards you (⊙) the field is anticlockwise; away (⊗) it is clockwise.
- Activity 12.4: the wire is over the needle. With the wire under the needle, the swing would be the other way (Beyond NCERT).
Beyond NCERT
- For a long straight wire the field falls off exactly as 1/distance: B = μ₀I/(2πr). The page's top-view circles are drawn from that rule.
- The SI unit of magnetic field is the tesla (T); the oersted named in the book's box is an older CGS unit.
3. A circular loop and a coil
In simple wordsBend the straight wire into a ring. Each little piece still makes its own circles of field, but now every piece pushes the field through the middle of the ring the same way. So the centre gets a strong, straight-through field — and stacking many turns stacks up the field.
If the straight wire is bent into a circular loop, the circles of field round each part of the wire get bigger farther from the wire; by the time they reach the centre of the loop, the arcs look like straight lines (Fig. 12.8). By the right-hand rule, every section of the loop sends its field the same way through the inside. The field at a point depends directly on the current, so a coil of n turns produces a field n times that of one turn — the currents in all turns flow the same way and their fields add. In Activity 12.6 a coil of many turns passes through two holes in a card, in series with a battery, a key and a rheostat; iron filings show the pattern (Fig. 12.9).
Reading NCERT's Figure 12.8 (page 200)A tilted circular loop with + and − leads; small circles of field round the wire at the top and bottom, and nearly straight lines through the middle running from S (right) to N (left). The point: the loop's field is concentrated and straight at its centre.
Reading NCERT's Activity 12.6 and Figure 12.9 (page 201)A many-turn circular coil standing through holes A and B in a horizontal card, connected to cells, a key and a rheostat; the card shows filings in circles round A and B and straighter lines between them. The point: more turns, stronger field; the same pattern as a single loop.
Common confusions
- At the centre of a loop the field lines are nearly straight and the field is along the axis (exercise 3(a) is true).
- Clockwise current seen from above → field into the table inside the loop; anticlockwise → out of it. (Quick check: grip the wire with your right hand and follow the fingers through the middle.)
- n turns give n times the field only because all the turns carry current the same way.
4. The solenoid and the electromagnet
In simple wordsStack many loops into a long spring-shaped coil and their fields line up into a neat, even ‘tunnel’ of field inside. Outside, the coil looks exactly like a bar magnet with an N and an S end. Slide an iron bar inside and you get a magnet you can switch on and off — an electromagnet.
A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is a solenoid. Its field pattern (Fig. 12.10) is like a bar magnet's (Fig. 12.4): one end behaves as a north pole, the other as a south pole. Inside, the field lines are parallel straight lines: the field is the same at all points inside — uniform. This strong inside field can magnetise a piece of magnetic material such as soft iron placed in the coil (Fig. 12.11); the magnet so formed is an electromagnet.
⤵ From one wire to an electromagnet
Straight wirefield lines are circles round it (right-hand thumb rule)
Bend it into a loopall parts send the field the same way through the centre
Coil of n turnsn times the field of one turn
Long closely wound coil: a solenoiduniform field inside; N and S ends like a bar magnet
Put soft iron insidean electromagnet: strong while the current flows
Reading NCERT's Figures 12.10 and 12.11 and the ‘Questions’ box (pages 201–202)Figure 12.10 shows a solenoid in a shaded region, with parallel lines through it and loops outside from one end round to the other, connected to cells and a key. Figure 12.11 shows a coil wound on a rod, connected to cells and a key. The box asks about a clockwise loop on a table, a diagram of a uniform field, and whether the field inside a long solenoid is zero, falls or rises towards its ends, or is the same everywhere (answer: the same at all points). The point: a solenoid is a switchable bar magnet. (An inconsistency: the text says the core is soft iron, but Figure 12.11's caption says a steel rod. Soft iron makes a temporary electromagnet; steel keeps its magnetism and is used for permanent magnets — go by the text and the summary, which say soft iron.)
Common confusions
- Inside a long solenoid the field is uniform — the same at all points, not zero and not stronger at the ends (page 202 Q3: (d)).
- A solenoid has N and S ends like a bar magnet; reversing the current swaps them.
- Electromagnet core: soft iron (UPSC 1979) — it magnetises strongly and loses magnetism when the current stops. Steel is for permanent magnets.
- The solenoid's wire must be insulated, or the current would short across the turns instead of going round them.
Beyond NCERT
- Uses of electromagnets: cranes that lift scrap iron, electric bells, relays and circuit breakers, and the very strong (often superconducting) coils of MRI scanners and maglev trains.
- An easy pole check for a solenoid: looking at one end, if the current goes round anticlockwise that end is N; clockwise, it is S.
5. Force on a current in a magnetic field: Fleming's left-hand rule
In simple wordsIf a current can push a magnet, a magnet can push a current. Hang a short metal rod between the poles of a horseshoe magnet and switch on: the rod jumps sideways — not along the current, not along the field, but at right angles to both. That sideways kick is what turns the shaft of every electric fan and motor.
A current's field exerts a force on a nearby magnet, so — as André-Marie Ampère (1775–1836) suggested — the magnet must exert an equal and opposite force on the current-carrying conductor. In Activity 12.7 a small aluminium rod AB (about 5 cm) hangs horizontally between the poles of a strong horseshoe magnet, N pole below and S pole above, so the field is upward; it is connected to a battery, a key and a rheostat. With current from B to A the rod is pushed to the left; reversing the current pushes it to the right; reversing the field (swapping the poles) also reverses the force. So the force depends on the directions of both the current and the field. The displacement — and so the force — is largest when the current is at right angles to the field, and then the force is perpendicular to both. Fleming's left-hand rule gives its direction: stretch the thumb, forefinger and middle finger of the left hand mutually perpendicular; if the first finger points along the field and the second finger along the current, the thumb points along the motion (force). Devices using this include the electric motor, electric generator, loudspeakers, microphones and measuring instruments. In Example 12.2 an electron enters a field at right angles; since current is opposite to electron motion, the rule gives a force into the page.
⤵ Using Fleming's left-hand rule
Use your LEFT handthumb, first and second fingers at right angles
First finger along the fieldfrom N to S
Second finger along the currentconventional current; for electrons point it opposite to their motion
Thumb gives the forcethe direction the conductor moves
Checkreverse current or field and the force reverses; reverse both and it does not
Reading NCERT's Activity 12.7 and Figure 12.12 (pages 202–203)A stand holds a short rod AB on two wires between the arms of a red horseshoe magnet (S above, N below), wired to a cell, a key and a rheostat; a small blue arrow at A marks the push. The point: a current in a field feels a force at right angles to both, reversing with either.
Reading NCERT's Figure 12.13, Example 12.2 with Figure 12.14 (page 203)Figure 12.13 draws a left hand with the forefinger up (field), the middle finger across (current) and the thumb sideways (motion), beside a three-arrow sketch. Figure 12.14 shows three field arrows pointing right and an electron moving down into them. The point: for an electron, point the ‘current’ finger opposite to its motion — the force is into the page.
Reading NCERT's ‘Questions’ box (pages 203–204)Three questions: which of a proton's mass, speed, velocity and momentum can change as it moves freely in a magnetic field (velocity and momentum — the force only turns it); how the rod's displacement in Activity 12.7 changes with more current, a stronger magnet, or a longer rod (it increases in each case); and the field that deflects a westward-moving alpha particle to the north (upward). The point: the magnetic force changes direction, not speed.
Reading NCERT's ‘More to Know!’ box: Magnetism in medicine (page 204)Every current makes a magnetic field — even the weak ion currents in our nerves; their fields are about one-billionth of the Earth's field, and are significant mainly in the heart and the brain. The box links this to Magnetic Resonance Imaging (MRI), whose images help diagnosis. The point: magnetism has important uses in medicine. (A simplification: MRI scanners use their own very strong external magnetic fields and radio waves to image the body's hydrogen nuclei; they do not read the body's own tiny fields. Measuring those is a separate technique — Beyond NCERT. Quote the box's figures as NCERT's.)
Common confusions
- Force is largest when current ⟂ field and zero when the current runs along the field (exercise 5).
- Left hand for force (motors); the right-hand thumb rule is for the field round a wire; Fleming's right-hand rule (induced current, generators) is no longer in the book.
- Order of fingers: First = Field, seCond = Current, thuMb = Motion.
- For electrons (negative) the current direction is opposite to their motion — the commonest slip (Example 12.2, exercise 6).
- A magnetic force on a free charge changes its direction (velocity, momentum) but not its speed or mass.
Beyond NCERT
- The force on a straight conductor is F = BIL when it is at right angles to the field (in general F = BIL sin θ) — which is why the book's activity finds a bigger push with more current, a stronger magnet or a longer rod.
- An electric motor uses this force on the two sides of a coil (with a split-ring commutator to keep it turning one way); the section on the motor was removed in rationalisation.
- The Earth's magnetic field deflects fast charged particles from the Sun and space, steering many towards the polar regions (auroras) — the same q v × B force (UPSC 2012).
6. Domestic electric circuits: live, neutral, earth, fuses
In simple wordsYour house gets electricity on two main wires — a ‘live’ one that does the pushing and a ‘neutral’ return — plus a green ‘earth’ wire that is purely for safety. Every appliance hangs across live and neutral side by side (in parallel), each with its own switch. If something goes wrong and the current shoots up, a fuse melts first so the wiring doesn't catch fire.
Homes get power from the mains, by overhead poles or underground cables. One wire, usually with red insulation, is the live wire (the book calls it ‘positive’); another, with black insulation, is the neutral wire (‘negative’); in India the potential difference between them is 220 V. At the meter-board they pass through a main fuse into the electricity meter, then through the main switch to the house's line wires, which feed separate circuits — often a 15 A circuit for high-power appliances (geysers, air coolers) and a 5 A circuit for bulbs and fans. The earth wire, with green insulation, is connected to a metal plate deep in the earth near the house. It is a safety measure for appliances with a metal body (electric press, toaster, table fan, refrigerator): the body is connected to the earth wire, a low-resistance path, so any leakage of current to the body keeps it at the earth's potential and the user does not get a severe shock. In each circuit appliances are connected across live and neutral in parallel, each with its own switch, so each gets the same potential difference (Fig. 12.15). The fuse (Section 11.7) guards against overloading. When the live and neutral wires touch directly — damaged insulation or a faulty appliance — the current rises abruptly: a short circuit. Overloading can also come from an accidental rise in the supply voltage or from too many appliances on one socket. Joule heating melts the fuse and breaks the circuit. The summary adds that our supply is AC at 220 V and 50 Hz.
| Wire | Insulation colour (book) | Job |
|---|---|---|
| Live | Red | Carries the supply; 220 V relative to neutral; fuses and switches go in this wire (Beyond NCERT) |
| Neutral | Black | The return path to the supply |
| Earth | Green | Safety: connects metal bodies of appliances to a plate deep in the ground |
⤵ Short circuit or overload: how the fuse saves the wiring
Short circuitlive touches neutral (damaged insulation, faulty appliance)
Overloadtoo many appliances on one socket, or a voltage surge
Current rises sharplyfar above the circuit's rating
Joule heating in the fuse wireH = I squared R t
Fuse meltsthe circuit is broken before the wiring overheats
⤵ How earthing protects you
Faultthe live wire touches an appliance's metal body
Earth wirea low-resistance path from the body to the ground
Body stays at earth potentialtouching it does not drive a big current through you
Large leakage current flows to earthwhich usually blows the fuse too (Beyond NCERT)
Reading NCERT's Figure 12.15 and the ‘Questions’ box (page 205)Figure 12.15: green earth, red live and black neutral wires entering through the Electricity Board's fuse and the electricity meter to a distribution box with a main switch and fuses for each circuit, then three sockets across live and neutral (with 220 V marked) and the earth wire looping to each. The box asks for two safety measures (fuse and earthing), the result of a 2 kW oven on a 5 A, 220 V circuit (it would draw about 9.1 A, so the fuse blows), and precautions against overloading. The point: parallel connection, a fuse for each circuit, and an earth for metal bodies.
Common confusions
- Green = earth, not live (exercise 3(b) is false); red = live, black = neutral in the book's colour scheme.
- At a short circuit the current increases heavily (exercise 2: (c)) because the resistance in the path becomes very small.
- Household appliances are in parallel (UPSC 1996) — every one gets the full 220 V and can be switched on its own.
- India's supply is 50 Hz (book), so the current reverses direction 100 times a second (twice each cycle). UPSC 2004 built an Assertion–Reason on ‘60 times’ and ‘60 Hz’ — by this book both statements are false for India; treat that question as flawed.
- ‘Positive’ and ‘negative’ for live and neutral is the book's informal language: with AC the live wire's voltage swings both ways about the neutral.
- The book counts a short circuit as a kind of overloading; many texts keep them apart (overload = too much load; short circuit = live meets neutral). Both make the fuse blow.
Beyond NCERT
- Newer wiring uses MCBs (miniature circuit breakers) in place of fuse wires, and RCDs/ELCBs (earth-leakage breakers) that trip when current leaks to earth — protecting people as well as wiring.
- Today's standard wire colours in many new installations differ from the book's (for example brown or red for live, blue or black for neutral, green or green-yellow for earth); go by the book's red, black and green in the exam.
- India's nominal supply voltage is now usually quoted as 230 V; the book uses 220 V.
NCERT's in-text questions and activities — short answers
The chapter has five boxes of questions (pages 196, 200, 201–202, 203–204 and 205) and questions inside its activities. Outlines follow; every direction and number is computed and asserted in the page's source.
Page 196, Q1: Why does a compass needle get deflected when brought near a bar magnet?
The needle is itself a small magnet. In the bar magnet's magnetic field its poles feel forces — like poles repel, unlike attract — which turn it to line up along the field.
Page 200, Q1: Draw magnetic field lines around a bar magnet.
Closed curves leaving the N pole and entering the S pole outside (S → N inside), crowded near the poles, none crossing, with arrows from N to S outside (see the computed diagram above).
Page 200, Q2: List the properties of magnetic field lines.
(i) They emerge from the N pole and merge at the S pole outside; (ii) inside the magnet they run S → N, so they are closed curves; (iii) closer lines mean a stronger field; (iv) no two lines cross; (v) the tangent at a point gives the field's direction (where a north pole would move).
Page 200, Q3: Why don't two magnetic field lines intersect each other?
At an intersection the field would have two directions at one point, and a compass there would have to point two ways at once — impossible.
Page 201, Q1: A circular loop lies in the plane of the table with current clockwise. Find the direction of the field inside and outside the loop.
By the right-hand rule, the field is into the table (downward) inside the loop and out of the table (upward) outside it (checked by summing the field of every bit of the loop).
Page 201, Q2: The magnetic field in a region is uniform. Draw a diagram to represent it.
Equally spaced, parallel straight lines with arrows all the same way — like the field inside a long solenoid.
Page 202, Q3: The magnetic field inside a long straight current-carrying solenoid (a) is zero (b) decreases towards its end (c) increases towards its end (d) is the same at all points.
(d) is the same at all points — the field inside is uniform.
Page 203, Q1: Which of a proton's properties can change while it moves freely in a magnetic field: (a) mass (b) speed (c) velocity (d) momentum?
(c) velocity and (d) momentum. The magnetic force is always perpendicular to the motion, so it only turns the proton: the direction changes, the speed and mass do not.
Page 204, Q2: In Activity 12.7, how will the displacement of rod AB change if (i) the current is increased, (ii) a stronger magnet is used, (iii) the rod is made longer?
It increases in all three cases — the force grows with the current, the field and the length of conductor in the field (F = BIL, Beyond NCERT).
Page 204, Q3: An alpha particle (positive) projected towards the west is deflected towards the north. The field is (a) south (b) east (c) downward (d) upward.
(d) upward. Treat the alpha particle as a current towards the west; with the field upward Fleming's left-hand rule (F = q v × B) gives a force to the north.
Page 205, Q1: Name two safety measures commonly used in electric circuits and appliances.
The electric fuse (or a circuit breaker) and earthing of metal-bodied appliances.
Page 205, Q2: A 2 kW electric oven is used on a 220 V domestic circuit with a 5 A rating. What happens?
I = P/V = 2000/220 ≈ 9.09 A, more than 5 A: the circuit is overloaded, so the fuse melts (or the breaker trips) and the oven stops. It should be on the 15 A circuit (which can feed up to 3300 W).
Page 205, Q3: What precaution should be taken to avoid overloading of domestic circuits?
Don't connect too many appliances to one socket; use high-power appliances on the 15 A circuit; use fuses of the correct rating; keep insulation and appliances in good repair.
Activity 12.1 (page 195): What happens to the compass when the key is plugged in, and what does it mean?
The needle is deflected: the current in the wire produces a magnetic field.
Activity 12.2 (page 196): What do you observe when the board with iron filings round the magnet is tapped?
The filings arrange themselves in a pattern of curves from pole to pole — they line up along the field lines.
Activities 12.4 and 12.5 (pages 198–199): Does the field reverse when the current reverses? What do the circles on the card show, and what happens with more current or more distance?
Yes, it reverses (the needle swings west instead of east). The concentric circles are the field lines; more current → stronger field; farther away → weaker.
Activity 12.7 (pages 202–203): Why does the rod get displaced?
A current-carrying conductor in a magnetic field experiences a force, perpendicular to both the current and the field; it reverses if either is reversed.
NCERT exercise — answer outlines
Reading NCERT's ‘What you have learnt’ and Exercises (pages 206–207)Nine summary points: the compass as a magnet; the magnetic field; field lines and their direction and density; the circles round a straight wire (right-hand rule); field shape depending on the conductor, and the solenoid as a bar magnet; the electromagnet (soft iron in an insulated copper coil); the force on a conductor (Fleming's left-hand rule); household AC at 220 V and 50 Hz with red live, black neutral and green earth; and the fuse. Nine exercises follow; the answer key (page 219) gives 1–3, 6 and 7(i)–(ii). The point: three rules and the safety wiring. (Exercise 7(iii) asks for the rule for a current induced in a rotating coil — a topic removed from this edition; the answer key is silent on it.)
Try each question first, then open the outline. Directions below are worked out as vector cross products and checked against the book's answer key.
1. The magnetic field near a long straight wire: (a) straight lines perpendicular to the wire (b) straight lines parallel to the wire (c) radial lines from the wire (d) concentric circles centred on the wire
(d) concentric circles centred on the wire, in planes perpendicular to it.
2. At the time of a short circuit, the current in the circuit (a) reduces substantially (b) does not change (c) increases heavily (d) varies continuously
(c) increases heavily — live meets neutral, the resistance of the path becomes very small.
3. True or false: (a) The field at the centre of a long circular coil carrying current will be parallel straight lines. (b) A wire with green insulation is usually the live wire.
(a) True — near the centre the field lines are parallel and straight. (b) False — green is the earth wire; live is red.
4. List two methods of producing magnetic fields.
(i) A permanent magnet (e.g. a bar magnet). (ii) An electric current — in a straight wire, a loop or a solenoid; with a soft-iron core this becomes an electromagnet.
5. When is the force on a current-carrying conductor in a magnetic field largest?
When the current is at right angles to the field. (It is zero when the conductor lies along the field.)
6. You sit with your back to one wall; an electron beam moving horizontally from the back wall to the front wall is deflected to your right by a strong magnetic field. What is the field's direction?
The electrons move forward, so the current is backward. With the force to your right, Fleming's left-hand rule (F = q v × B with q negative) gives a field vertically downward (book: vertically downwards).
7. State the rule for the direction of (i) the field round a straight current, (ii) the force on a straight current in a perpendicular field, (iii) the current induced in a coil rotating in a field.
(i) Right-hand thumb rule (Maxwell's corkscrew rule). (ii) Fleming's left-hand rule. (iii) Fleming's right-hand rule: first finger along the field, thumb along the motion, second finger gives the induced current — Beyond this edition, which has dropped electromagnetic induction; the answer key gives only (i) and (ii).
8. When does an electric short circuit occur?
When the live and neutral wires come into direct contact — through damaged insulation or a fault in an appliance — so the current rises abruptly.
9. What is the function of an earth wire? Why is it necessary to earth metallic appliances?
The earth wire gives a low-resistance path from an appliance's metal body to the ground. If current leaks to the body, the body stays at earth's potential, so a person touching it does not get a severe shock (and the large leakage current usually blows the fuse — Beyond NCERT).
What to remember
Numbers
Oersted's discovery1820 (Oersted, 1777–1851)
Ampère (force on a current suggested)1775–1836
Mains in India (book)220 V AC, 50 Hz
Current reversals per second at 50 Hz100 (two per cycle; Beyond NCERT)
Household circuits (book)15 A (geysers, coolers) and 5 A (bulbs, fans)
Most a circuit can feed at 220 V5 A → 1100 W; 15 A → 3300 W (computed)
2 kW oven at 220 V≈ 9.09 A → too much for a 5 A circuit
Wire colours (book)live red, neutral black, earth green
n-turn coiln times the field of one turn
Nerve-current fields (book)about one-billionth of the Earth's field
Activity 12.5 battery / ammeter12 V; 0–5 A
Activity 12.7 rodaluminium, about 5 cm
Chapter size7 activities, Figs 12.1–12.15, 2 examples, 9 exercises
One-liners
- A compass needle is a small bar magnet; its north-seeking end is its N pole.
- Field lines leave N, enter S outside, run S → N inside, and are closed curves.
- Crowded field lines mean a stronger field; field lines never cross.
- Oersted (1820): a current deflects a compass — electricity and magnetism are linked.
- Straight wire: concentric circles; right-hand thumb rule gives their direction.
- The field round a wire grows with current and weakens with distance.
- A loop's field is nearly straight at its centre; n turns give n times the field.
- A solenoid's field is uniform inside and like a bar magnet's outside.
- Soft iron inside a solenoid makes an electromagnet.
- A current in a field feels a force, largest when current ⟂ field: Fleming's left-hand rule.
- Household supply: 220 V, 50 Hz AC; red live, black neutral, green earth.
- Appliances are in parallel; fuses guard against short circuits and overloading; earthing guards against shocks.
Traps to avoid
- Right-hand THUMB rule = field of a current; Fleming's LEFT-hand rule = force on a current.
- Inside a magnet field lines run S → N.
- Field inside a long solenoid is UNIFORM — not zero, not stronger at the ends.
- Electromagnet core: SOFT IRON, not steel.
- For electrons, current is OPPOSITE to their motion (Example 12.2, exercise 6).
- A magnetic force changes a charge's direction, not its speed.
- Green = EARTH wire, not live.
- Short circuit → current INCREASES heavily.
- 50 Hz → current reverses 100 times a second, not 50 or 60.
- Motor, induction, generator and Fleming's RIGHT-hand rule are no longer in this edition.
How to strategise
Prelims — what UPSC asks
- Electromagnets and home wiring: soft iron is the electromagnet metal (1979); domestic wiring is a parallel connection (1996, also on the Electricity page). Learn the wire colours, the 5 A / 15 A circuits, and earthing and fuses.
- AC frequency: India's supply is 50 Hz (book), so the current reverses 100 times a second — UPSC 2004 asked an Assertion–Reason on ‘60 times per second’ and ‘60 hertz’, a question this page treats as flawed (both statements are false for India).
- Moving charges in a field: the Earth's magnetic field turns charged particles from space towards the poles (2012) — the same force as Example 12.2. Practise the three rules with ⊙/⊗ diagrams until the electron-sign reversal is automatic.
- Likely next: properties of field lines; solenoid vs bar magnet; uses of electromagnets (cranes, MRI, maglev — Beyond NCERT); fuse vs MCB vs earth-leakage breaker; motor and generator principles (dropped from the book but fair game for UPSC).
Mains — where it appears and how to answer
- No GS Mains question in the bank so far is built on this chapter. Its use in GS3 is as the science line in answers on electric mobility and motors (efficient fans and pumps), medical technology (MRI), and electrical safety and building codes (earthing, circuit breakers).
- Answer frame: one-line principle (force on a current in a field / electromagnet) → the technology → benefit → risks and limits → way forward.
Self-check
Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.
1. Outside a bar magnet, magnetic field lines run:
2. Why can two magnetic field lines never cross?
3. Current flows upward in a vertical wire. Seen from above, the field lines round it are:
4. The field inside a long current-carrying solenoid is:
5. The core of an electromagnet is usually made of:
6. In Fleming's left-hand rule, the thumb shows the direction of:
7. An electron moves down the page in a field pointing to the right. The force on it is:
8. In a house wired as NCERT describes, the green wire is the:
UPSC questions from this chapter
Open any question to attempt it in the Question bank — your attempt is saved to your progress.