NCERT Exemplar Solutions Class 10 Science Chapter 12

Read the spacing, then read the option. I teach students to look first at what the line spacing is telling them, because three of the four options are textbook facts and only one bends a rule.

Concept used. The number of lines crossing a unit area gives the field strength, so spacing is a strength map. Even spacing is a special case where the strength is constant, which we call a uniform field. Closed-loop shape and the tangent-direction rule are the other two defining properties of field lines.

1. Statement (a) is the working definition of field direction using a compass, so it stands.
2. Statement (b) is true because field lines have no free ends; they close on themselves through the magnet.
3. Statement (d) is true because the degree of closeness is exactly how we judge whether a field is strong or weak.
4. Statement (c) misreads equal spacing as zero field. Equal spacing is the picture of a uniform field, like the field inside a long solenoid. The field there is steady and certainly not zero.

Why this matters. The same uniform-field picture comes back when students draw the field inside a solenoid in later questions, so spotting that equal spacing means ``uniform'' saves them from the same trap twice.

(c) is the incorrect statement; parallel, equidistant lines depict a uniform field of non-zero, constant strength.

No current means no current-field. I always pin down the source of every field line before naming its shape, and here the source is the deciding clue.

Concept used. Around a straight current-carrying wire the field lines are concentric circles in a plane at right angles to the wire. But that pattern needs a live current. Remove the current and that pattern vanishes, leaving only the background Earth's field, which is uniform over a small region.

1. Identify the switch state: the key is out, so the circuit is open and the conductor carries no current.
2. Drop the concentric-circle pattern, because it belongs to a live wire, not a dead one.
3. Recognise the surviving field: the Earth's magnetic field passes through ABCD and is uniform across this small plane.
4. A uniform field is drawn as straight, equally spaced, parallel lines, so option (c) is correct.

Why this matters. This question quietly tests whether students know that the magnetic effect needs a moving charge. The same idea decides whether a compass near a wire deflects or not in the short-answer questions later.

(c): an open circuit kills the wire's circular field, so the plane shows only the Earth's straight, parallel field lines.

Name each face from its own viewpoint. The whole question turns on reading the current sense from the correct side, so I label both faces before choosing.

Concept used. A circular loop is a magnetic dipole. The polarity follows the anticlockwise-North, clockwise-South convention, and the direction the field points must agree with field lines running North to South outside the loop.

1. Stand at A and look along the axis: the current is anticlockwise, which marks the A-face as North.
2. Stand at B and look the other way: the current is clockwise, which marks the B-face as South.
3. Check consistency with the given line direction. Inside the loop, lines run from the South face to the North face, i.e. from B toward A, exactly as stated.
4. Therefore the resultant North pole sits on the face close to A. Options (c) and (d) are wrong because the polarity of a loop does not flip with the size of the current; only reversing the current direction would swap the poles.

Why this matters. The clock-face rule reappears when students decide which end of a solenoid is North, so getting the viewpoint right here builds the habit they need for the solenoid questions.

(a): the A-face is North because the current is anticlockwise when viewed from A; loop polarity does not depend on current magnitude.

Compare the picture to a bar magnet. I ask students to recall the standard solenoid diagram and lay it next to a bar magnet, because the question is really testing that comparison.

Concept used. The defining property of a solenoid is that its overall field, inside and out, copies that of a bar magnet: uniform straight lines inside, looping lines outside from North to South. Reversing the current reverses the poles, and the strong inner field can magnetise soft iron.

1. Confirm (a): inside, the lines run straight and evenly spaced, the signature of a uniform field.
2. Confirm (b): that same strong inner field aligns the domains of a soft-iron rod, magnetising it, the basis of an electromagnet.
3. Confirm (d): swap the current direction and the curl of the field reverses, so North and South ends trade places.
4. Reject (c): the solenoid pattern is not different from a bar magnet's, it is the same, so (c) is the false statement and the required answer.

Why this matters. Knowing that a solenoid mimics a bar magnet is the bridge to electromagnets and to the long-answer question on field lines around a circular loop, where the same dipole picture is used.

(c): the solenoid's field pattern is identical to a bar magnet's, so the statement calling it different is the incorrect one.

Convert charge motion into a current first. The single skill being tested is handling a negative charge correctly, so I always state the conventional current before touching the hand rule.

Concept used. The magnetic force on a charge depends on the conventional current direction and the field. Fleming's left-hand rule gives the force: forefinger along field, central finger along current, thumb gives force. For the electron the current opposes the motion.

1. Set the field: left to right across the page for both particles.
2. Proton up: current up. Fleming's left hand gives thrust into the page.
3. Electron down: motion down, so conventional current is up (opposite to the electron). Fleming's left hand again gives thrust into the page.
4. Because the proton's current-up and the electron's current-up are the same, and the field is common, both forces land in the same sense, into the page. That rules out (b), (c) and (d).

Why this matters. This is the exact reasoning behind the force on a current-carrying wire and the turning of a motor coil, where many electrons drift together and their forces add up.

(a): the proton's upward current and the electron's effective upward current both give a force into the plane of paper.

List the three upgrades, then pick the odd one. The question is a direct memory check on how a commercial motor differs from a simple one, so I write the upgrade list first.

Concept used. The turning force on a motor coil grows with field strength, number of turns and core material. Commercial motors push all three: electromagnet for a strong field, many turns for a large torque, and a soft-iron core to concentrate the field.

1. Recall the upgrade list: electromagnet, large number of turns, soft-iron core.
2. Match (a), (b) and (d) to the list; all three appear, so all are used.
3. The leftover option is (c), the permanent magnet. A permanent magnet gives a weak, fixed field, which is why commercial motors replace it with an electromagnet.
4. Hence the feature a commercial motor does not use is the permanent magnet.

Why this matters. The same three differences are asked again in the long-answer question that compares a simple motor with a commercial one, so this list is worth memorising once and reusing.

(c): a permanent magnet is the one item commercial motors avoid; they use an electromagnet instead.

Track the field at three moments. I split the action into key-out, key-in and key-removed, because the induced current depends only on how the field is changing at each moment.

Concept used. An induced current flows only while the magnetic flux linked with a coil changes. An increasing flux and a decreasing flux drive the induced current in opposite directions, which is the heart of Faraday's idea.

1. Key out: no current, no flux, no change, so the galvanometer shows zero.
2. Key in: the flux through the second coil rises from zero to steady. Only during the rise is there a deflection, which then settles back to zero.
3. Key removed: the flux falls from steady back to zero. This opposite change gives a deflection in the opposite direction, which also dies out.
4. Two opposite, short-lived deflections match option (d). Option (b) is wrong because removing the key does cause a deflection; (c) is wrong because the two deflections are opposite, not the same.

Why this matters. This two-coil set-up is the seed of the transformer and of the generator's working, both of which rely on a changing field inducing a current in a nearby coil.

(d): inserting and removing the key change the flux in opposite ways, giving two opposite momentary deflections.

Separate one cycle from one reversal. The error in option (d) hides in confusing the cycle time with the time between reversals, so I compute both.

Concept used. Frequency is cycles per second, and the time period is its reciprocal. Within a single AC cycle the current goes positive, returns, goes negative and returns, reversing direction twice.

1. Check (a), (b), (c): each is a standard definition and is correct.
2. Find the time period of Indian AC. T = 1f = 150 = 0.02 s
3. Count reversals per cycle: two. So the time between successive reversals is T2 = 0.022 = 0.01 s = 1100 s.
4. Option (d) says 150 s, which is the whole cycle time, not the reversal time. Hence (d) is the wrong statement.

Why this matters. This 100-changes-per-second figure is asked again as a short-answer question, so a clear grip on cycle versus reversal pays off twice in this very chapter.

(d): the reversal time is 1100 s; the figure 150 s is the full cycle, so the statement is incorrect.

Curl the right hand around the wire. The cleanest path is to apply the thumb rule directly and read off the field at the point below the wire.

Concept used. For a straight current-carrying wire the field is a set of circles in planes perpendicular to the wire, with direction given by the right-hand thumb rule. The field at any point is tangent to the circle through that point.

1. Set the current direction: East to West, so the thumb of the right hand points West.
2. The curled fingers give the circulation: looking from the East end toward the West, the field circles clockwise around the wire.
3. Read the tangent at the point directly below the wire: it points toward the South, that is, North to South. So (b) fits.
4. Reject (c) and (d): points in the plane of the paper to the north or south of the wire lie on the wire's own line of circles only edge-on, and there the field is into or out of the paper, not North to South.

Why this matters. The right-hand thumb rule for a straight wire is the most-used tool in this chapter; the same grip decides the compass deflection in the short-answer questions.

(b): the right-hand thumb rule places a North-to-South field directly below the West-flowing current.

Picture the inner field lines. A single look at the inside field diagram settles this, so I describe what those lines look like.

Concept used. The inside of a long solenoid carries a uniform field, drawn as straight, parallel, equispaced lines. Uniformity means constant magnitude and constant direction everywhere inside.

1. Recall the diagram: inside the coil the lines are straight and run parallel to the axis.
2. Note the equal spacing of these lines, the visual marker of constant strength.
3. Conclude that the field magnitude does not vary along or across the inside of the solenoid.
4. So the field is the same at all interior points, option (c); the other options wrongly suggest the field changes inside.

Why this matters. This uniform inner field is exactly what magnetises a soft-iron rod evenly, the principle behind a strong electromagnet used in commercial motors and many machines.

(c): straight, equally spaced inner field lines show a uniform field, the same at all points inside.

Change the contact device, nothing else. The single change that matters is at the rings, so I focus on what slip rings do versus a split ring.

Concept used. The induced EMF in a rotating coil is naturally alternating. What the output looks like depends only on how the coil connects to the brushes: slip rings pass the alternation through, while a split-ring commutator reverses the connection at the right instant to produce a one-way current.

1. Identify the AC source: a rotating coil in a field always generates alternating EMF.
2. See why slip rings give AC: each ring stays joined to the same coil end, so the reversal reaches the circuit unchanged.
3. Replace them with a split ring: its two halves swap brushes every half-turn, cancelling the reversal in the external circuit.
4. Hence option (a) is correct. Options (c) and (d) only change the size of the EMF or the shape of the loop, not whether the output is AC or DC, so they cannot do the conversion.

Why this matters. This is the precise change asked in the long-answer question on generators, so understanding the split ring here answers that question too.

(a): only a split-ring commutator, in place of slip rings, converts the alternating output into direct current.

Pair each device with the hazard it handles. Three of the four options do useful jobs, so I match every option to a specific danger to find the right one.

Concept used. Protection against an excessive current relies on a series element that breaks the circuit when the current crosses a rated value. The fuse does this by melting; the other listed devices serve different purposes.

1. State the hazard named: short circuiting and overloading, both of which mean too much current.
2. Match the fuse: placed in series, it melts at a set current and cuts the circuit, directly stopping the overcurrent.
3. Rule out earthing: it diverts leakage current from a metal casing to the ground, guarding the person, not the appliance, against overload.
4. Rule out stabiliser and meter: one controls voltage swings, the other counts energy, and neither breaks the circuit on overcurrent. So the fuse, option (b), is the most important safety method here.

Why this matters. The fuse and its correct rating come up again in two short-answer questions and a long-answer domestic-circuit question, so anchoring its job now supports all of them.

(b): the fuse is the series device that melts and disconnects the circuit during a short circuit or overload.

Ask where the wire's field points at A. The deflection depends entirely on the direction of the wire's field at the needle, so I find that direction for each placement.

Concept used. The field of a straight wire circles it in perpendicular planes. Only the component of this field lying in the horizontal plane of the compass can rotate the needle; a vertical component cannot.

1. Lay the wire in the plane of the paper through A. Its field there is directed vertically (out of or into the horizontal plane), so it has no in-plane part to turn the needle, and the deflection stays unchanged.
2. Now stand the wire perpendicular to the paper through A. Its circular field lines then lie flat in the plane of the paper, the same plane as the needle.
3. This in-plane field is what rotates the needle, and being close to the wire it is strong, so the deflection is the largest possible.
4. Hence: in-plane wire gives no change, perpendicular wire gives maximum deflection.

Why this matters. This is Oersted's experiment read backwards: instead of asking ``does the compass move'', students reason about which wire orientation makes the field cut through the needle's plane.

Wire in the plane of paper: field vertical at A, no change. Wire perpendicular to the paper: circular field lies in the needle's plane, deflection maximum.

Two conditions, then the diagram. A permanent magnet from a solenoid needs the right current and the right core, so I state both before sketching the circuit.

Concept used. The inside field of a solenoid aligns the magnetic domains of a core placed in it. A hard magnetic material such as steel locks those domains in place, so it stays magnetised; a steady DC supply ensures the field points one way while it magnetises.

1. Choose DC: an alternating current would keep flipping the field and would not leave a fixed magnetisation, so direct current is required.
2. Choose a steel core: steel has high retentivity, so it holds its magnetism after the current is removed, unlike soft iron which loses it.
3. Connect the circuit: the solenoid (with the steel rod inside) goes in series with a battery, a key and a rheostat, as labelled in the diagram above.
4. With DC flowing, the steel becomes magnetised; switch off and it remains a permanent magnet.

Why this matters. This contrast between steel and soft iron explains why an electric bell uses soft iron (it must lose magnetism instantly) while a compass needle uses steel (it must keep it).

A permanent magnet results when a steel core is magnetised inside a solenoid carrying direct current, drawn with a battery, key and rheostat in series.

Direction first, then strength. I split the question into two clean parts, finding the field direction at each point and then comparing magnitudes by distance.

Concept used. For a straight wire, the field direction follows the right-hand thumb rule and lies tangent to the circle through the point, while the field strength decreases as the distance from the wire increases.

1. Set the current upward (A to B) and grip the wire with the right hand, thumb up.
2. Read the curl: field is into the page on the right side and out of the page on the left side of the wire.
3. Assign directions: P (right, distance r₁) has the field into the page; Q (left, distance r₂) has the field out of the page.
4. Compare strengths using distance: the field falls off with distance, and since r₂ < r₁, point Q is nearer the wire, so the field at Q is the larger of the two.

Why this matters. The dependence of field strength on distance is what makes field lines crowd near the wire and spread out away from it, the visual cue used to compare field strength throughout this chapter.

P: field into the page; Q: field out of the page; with r₁ > r₂, the nearer point Q has the stronger field.

Link current to field to deflection. The answer is a short chain of cause and effect, so I make each link explicit.

Concept used. The field of a straight wire grows in step with the current. The compass needle aligns with the net field, so a bigger wire-field pulls the needle further from its rest position, increasing the deflection.

1. Increase the current in the wire.
2. The wire's magnetic field at the compass rises in direct proportion to the current.
3. The needle feels a stronger sideways field and turns through a larger angle.
4. Therefore the deflection increases, and the reason is the direct proportion between field strength and current.

Why this matters. This proportionality is the working principle of a galvanometer, where a larger current gives a larger needle deflection, letting the meter measure current.

Deflection increases; the wire's field is directly proportional to the current, so more current means a stronger field and a larger turn of the needle.

Check the charge of each beam. The deciding factor is whether each particle is charged, so I test both cases against that single rule.

Concept used. An electric current is a flow of charge, and only an electric current produces a magnetic field. A moving beam behaves like a current exactly when its particles are charged.

1. Recall the rule: moving charge equals current equals a magnetic field.
2. Test alpha particles: they carry positive charge (charge +2e each), so a moving beam is a real current and does produce a magnetic field.
3. Test neutrons: their net charge is zero, so a moving neutron beam carries no current and produces no magnetic field.
4. Conclude: yes for alpha particles, no for neutrons, decided purely by charge.

Why this matters. This is why charged-particle beams in accelerators can be steered with magnets while neutron beams cannot, a direct extension of the chapter's central idea.

Alpha particles (charged) give a magnetic field; neutrons (neutral) do not, because only moving charge forms a current that produces a field.

State the input and output of each rule. The clearest way to contrast the rules is to say what each one is given and what each one returns.

Concept used. The right-hand thumb rule maps a current to the field it creates. Fleming's left-hand rule maps a current and an external field to the mechanical force on the conductor. Same physics family, different questions.

1. Right-hand thumb rule input: the current direction in a straight wire (the thumb). Output: the direction of the circular field around the wire (the fingers).
2. So the thumb stands for the current direction.
3. Fleming's left-hand rule input: an external field (forefinger) and a current (central finger). Output: the force or motion of the conductor (thumb).
4. The difference: the first rule has no external field and gives a field; the second rule needs an external field and gives a force.

Why this matters. The right-hand thumb rule sets up the fields in this chapter, while Fleming's left-hand rule drives the electric motor, so knowing which rule to reach for is half the battle in numerical and diagram questions.

Thumb = current direction. The right-hand thumb rule finds the field of a current; Fleming's left-hand rule finds the force on a current in an external field.

Translate the picture into strength. Meena is reading a strength map, so I convert ``lines diverge'' into ``field weakens''.

Concept used. Field strength is encoded in line spacing. Equal, close spacing means a strong field; widening spacing means a falling field. The loop's field is largest at the centre and decreases outward.

1. Start at the centre: the loop's field is strongest there, so the lines lie close together.
2. Move outward: the loop's field grows weaker with distance from the centre.
3. Read the spacing: a weaker field is drawn with lines further apart, so the lines diverge.
4. Conclude: the diverging lines are the visual sign that the field strength is decreasing as Meena moves away.

Why this matters. Reading line spacing as field strength is exactly how students compare the field at two points or judge where a field is strongest, a skill reused in the solenoid divergence question that follows.

Diverging lines show a weakening field: as Meena moves from the centre, the loop's field decreases, so its lines spread apart.

Compare the inside with the ends. The contrast between the crowded inside lines and the spread-out end lines tells the whole story, so I set them side by side.

Concept used. Line spacing is a strength gauge. Inside the solenoid the lines are tight and parallel (strong, uniform field); at the ends they fan out, signalling a drop in strength.

1. Look inside: the lines are close and parallel, marking a strong uniform field.
2. Look at the ends: the lines spread apart and curve away from the axis.
3. Apply the spacing rule: wider spacing means a weaker field.
4. Conclude: the divergence at the ends indicates that the field strength decreases near and beyond the ends of the solenoid.

Why this matters. This explains why a soft-iron rod is magnetised most strongly inside a solenoid and why the field is weak just outside its ends, useful when designing an electromagnet.

Diverging end lines indicate a fall in field strength near and beyond the solenoid's ends, in contrast with the strong uniform field inside.

Spot the spinning part, then compare conversions. Any appliance with a spinning part driven by electricity uses a motor, so I list such appliances and then state the motor-versus-generator contrast.

Concept used. A motor is an energy converter: electrical in, mechanical (rotation) out. A generator is the same machine run backwards: mechanical in, electrical out. Recognising the conversion direction separates the two.

1. List appliances with electrically driven rotation: electric fan, mixer or grinder, washing machine, and computer disk drive.
2. Note that in each the motor's rotation is the useful output, confirming a motor is present.
3. State the motor's conversion: electrical energy to mechanical energy.
4. Contrast the generator: it converts mechanical energy to electrical energy, the opposite direction, which is the respect in which they differ.

Why this matters. Realising that a motor and a generator are the same machine run in opposite directions makes the later generator question easier, since its diagram is just a motor's diagram driven by hand instead of by current.

Fan, mixer, washing machine and computer drive use motors. A motor converts electrical to mechanical energy; a generator does the reverse.

Trace the current from battery to coil. The brushes' job is best seen by following the current path, so I trace it from the fixed supply into the moving coil.

Concept used. The brushes are the sliding contacts that link a stationary battery to a rotating coil, and together with the split ring they flip the coil current each half-turn so the turning effect always pushes the coil the same way.

1. Connect the brushes to the battery: each brush is wired to one battery terminal and presses on the outer face of one half of the split ring.
2. Pass current to the coil: through this sliding contact, current flows from the battery into the spinning coil.
3. Note the insulation: the inner sides of the split-ring halves are insulated and fixed to the axle, so the contact between a given brush and a given half changes every half-turn.
4. Reverse the current: this change swaps the current direction in the coil each half-rotation, keeping the rotation continuous and one-way.

Why this matters. Brushes and a split ring are what let a DC motor spin steadily; the same parts, arranged as a split-ring commutator, turn a generator's AC output into DC, linking this answer to the generator question.

The brushes carry current from the battery to the rotating coil via the split ring and, by contacting alternate halves each half-turn, reverse the coil current to sustain continuous rotation.

Define, then count. The question has a definition part and a counting part, so I handle the difference first and then compute the reversals.

Concept used. DC keeps a single direction; AC alternates with a fixed frequency. Each AC cycle contains two reversals, so multiplying the frequency by two gives the reversals per second.

1. State the difference: DC has a constant direction of flow, while AC reverses direction periodically.
2. Note the Indian frequency: f = 50 Hz, i.e. 50 cycles each second.
3. Count reversals per cycle: two (once on each half of the cycle).
4. Multiply: 2 × 50 = 100 direction changes per second.

Why this matters. This 100-per-second figure is a favourite one-mark fact in board exams, and it ties directly to why option (d) in the MCQ section was wrong.

DC: one fixed direction; AC: periodic reversal. Indian AC (50 Hz, two reversals per cycle) changes direction 100 times per second.

Tie the rating to the safe current. The whole answer hinges on what ``rating'' means, so I anchor it to the maximum safe current of the circuit.

Concept used. A fuse in series protects by melting at its rated current. The rating must match the safe current of the appliance, so a fuse that melts at a higher current can no longer guard against a fault.

1. Place the fuse in series so it carries the same current as the appliance.
2. During a short circuit or overload, the current shoots up; the correctly rated fuse melts and disconnects the circuit before harm is done.
3. Replace it with a higher-rated fuse: now the fuse tolerates a much larger current and stays intact during the fault.
4. As a result, the excess current flows on through the appliance and can damage it, defeating the protection, which is why a larger rating must not be used.

Why this matters. This rule explains a common household hazard: ``upgrading'' a blown fuse to a thicker wire removes the safety margin and risks fire, the practical lesson of the chapter's domestic-circuit topic.

A fuse melts at its rated current to break the circuit during a fault. A larger-rated fuse passes a harmful current without blowing, leaving the appliance exposed, so the rating must not be increased.

Explain the turn, then list the rules. The question has two parts, so I first explain the deflection as a resultant-field effect and then set down the field-line properties cleanly.

Concept used. A compass needle settles along the net field where it sits. Adding the field of a bar magnet or a loop (which acts as a magnet) shifts that net field, turning the needle. Field lines obey a fixed set of rules that let us read off direction and strength.

1. With no magnet, the needle reads the Earth's field and points North–South.
2. Recognise that a current loop is a magnetic dipole, just like a bar magnet, with its own field lines.
3. Bring it close: its field adds to the Earth's. The needle aligns with the combined field, so it deflects.
4. List the salient features: field lines go N to S outside the magnet; they are closed curves; their closeness measures strength; they never intersect because a point has a single field direction; and equally spaced parallel lines depict a uniform field.

Why this matters. This is the deeper version of Oersted's experiment, and the same closed-loop, no-crossing rules govern every field diagram students will draw for wires, loops and solenoids in this chapter.

Bringing a magnet or loop near adds its field to the Earth's; the needle aligns with the resultant and deflects. Field lines: N to S outside, closed loops, never cross, closeness = strength, equal spacing = uniform field.

Draw the circles, then orient them. The pattern and the rule are two halves of one answer, so I describe the concentric-circle picture and then use the thumb rule to point the arrows.

Concept used. The field of a straight wire forms concentric circles in perpendicular planes, with strength shown by spacing and direction given by the right-hand thumb rule.

1. Sketch the field: concentric circles around the wire, in planes at right angles to it, as in the diagram.
2. Read the spacing: close circles near the wire mean a strong field; widely spaced circles far away mean a weak field.
3. Apply the thumb rule: grip the wire with the right hand, thumb along the current; the fingers curl in the field's direction.
4. Use it at any point: the rule instantly tells whether the field there is into or out of the plane, or clockwise or anticlockwise around the wire.

Why this matters. This single rule fixes the field direction for every straight-wire problem in the chapter, including the compass-deflection and force-on-conductor questions.

Concentric circles around the wire, close-in strong and far-out weak, with directions set by the right-hand thumb rule (thumb = current, fingers = field).

Build the coil from single loops. The factor of n is easiest to justify by stacking identical loops, so I describe the field of one loop and then add n of them.

Concept used. A circular loop gives circular lines near its wire and a straight axial field at its centre. A coil of n turns is n such loops together, and since the fields of identical, co-directed currents add, the centre field scales with n.

1. Describe one loop: near the wire the lines are small circles, and at the centre the field is straight and perpendicular to the loop's plane.
2. Stack n loops close together, all carrying the same current in the same direction.
3. Note that each loop's field at the centre points the same way, so the fields are in the same direction.
4. Add them: n equal, same-direction contributions give a total field n times that of a single turn.

Why this matters. This adding-up of turns is exactly why a many-turn coil makes a strong electromagnet and why commercial motors use a large number of turns, linking this question to the motor questions.

One loop: circular lines near the wire, straight axial field at the centre. For n turns with the same current in the same direction, the equal fields add, giving n times the single-turn field.

Show the push, then point the force. The activity proves a force exists, and Fleming's rule names its direction, so I cover both in turn.

Concept used. A current in an external field feels a force perpendicular to both. The activity show this by the visible displacement of a free rod, and Fleming's left-hand rule fixes the direction of that displacement.

1. Set up the activity: suspend an aluminium rod between the poles of a horseshoe magnet, with the field across the rod, and connect the rod to a battery through a key.
2. Switch on the current: the rod is pushed and swings out, showing a force acts on it.
3. Confirm the geometry: the push is perpendicular to both the current along the rod and the field across it; reversing either reverses the push.
4. Apply Fleming's left-hand rule: forefinger along the field, central finger along the current, thumb then gives the force, matching the observed direction of motion.

Why this matters. This force on a current-carrying conductor is precisely what makes the coil of an electric motor turn, so this activity is the foundation of the motor question that follows.

The suspended rod is pushed perpendicular to both current and field when the current is switched on; Fleming's left-hand rule (Field, Current, Force on fore-, central-finger and thumb) gives that force direction.

Force on the sides, split ring keeps it going. The working is a two-step story, so I explain the initial turn and then how the split ring sustains it, before listing the upgrades to a commercial motor.

Concept used. A current-carrying coil in a magnetic field feels opposite forces on its two sides (Fleming's left-hand rule), producing a turning effect. The split ring reverses the current every half-turn so the turning effect never reverses.

1. Set up the coil ABCD between magnet poles, ends joined to a split ring contacted by two brushes wired to a battery, as in the diagram.
2. Send current through: the two arms carry current in opposite directions, so one is pushed up and the other down, and the coil starts to rotate.
3. Reach the half-turn: the split-ring halves change brushes, reversing the coil current at exactly the moment needed.
4. Sustain rotation: the reversed current keeps the forces turning the coil the same way, giving continuous rotation. A commercial version then swaps the permanent magnet for an electromagnet, adds many more turns, and winds the coil on a soft-iron core for greater power.

Why this matters. The three upgrades named here are the same ones tested in the commercial-motor MCQ, so this answer ties the diagram, the working and that MCQ together.

The coil rotates because its current-carrying sides feel opposite forces; the split ring reverses the current each half-turn for continuous spin. Commercial motors add an electromagnet, many turns and a soft-iron core.

Define, then show. The answer needs a clear definition and a clean experiment, so I give the idea first and then a step-by-step demonstration.

Concept used. A current is induced in a coil only while the magnetic flux through it changes. An increasing and a decreasing flux drive the induced current in opposite directions, and no change means no current.

1. State the phenomenon: a changing magnetic field through a closed coil induces a current in it, called electromagnetic induction.
2. Build the set-up: join a coil to a galvanometer so the circuit is closed, and take a bar magnet.
3. Push the magnet in: the flux through the coil rises, and the galvanometer deflects, showing an induced current.
4. Pull the magnet out: the flux falls, and the galvanometer deflects the opposite way; hold the magnet still and the needle stays at zero, proving the current appears only when the field changes.

Why this matters. This is the exact principle behind the generator in the next question, where a coil spinning in a field continually changes the flux and so produces a steady induced current.

A changing field through a coil induces a current (electromagnetic induction). A magnet pushed toward a galvanometer-coil deflects the needle one way, pulled away the other way, and held still gives no deflection.

Spin the coil, then choose the rings. The working depends on induction, and the AC-versus-DC choice depends only on the rings, so I separate these two ideas.

Concept used. Rotating a coil in a magnetic field continuously changes the flux, inducing an alternating EMF by electromagnetic induction. Slip rings pass this alternation to the load (AC), while a split-ring commutator reverses the connection each half-turn to deliver a one-way current (DC).

1. Rotate the coil ABCD between the magnet poles; its arms cut the field and an EMF is induced (Fleming's right-hand rule).
2. See the reversal: each half-turn, the arms swap sides of the field, so the induced current in the coil reverses.
3. Use slip rings: since each ring keeps the same coil end, the reversal reaches the external circuit, producing AC.
4. Convert to DC: swap the slip rings for a split-ring commutator, whose two halves change brushes each half-turn, cancelling the reversal outside and giving direct current.

Why this matters. This mirrors the AC-to-DC generator MCQ in this chapter and shows that a generator and a motor share the same hardware, differing only in whether you supply current or extract it.

A coil rotated in a field induces an alternating current delivered as AC through slip rings; replacing the slip rings with a split-ring commutator converts the output to DC.

Draw the circuit, then justify the rating. The answer has a diagram part and a reasoning part, so I lay out the domestic circuit and then explain the identical-rating rule.

Concept used. A domestic circuit feeds appliances in parallel through live and neutral wires, with a fuse of fixed rating in the live wire. The fuse melts at its rated current, so its rating must match the safe current of the circuit.

1. Trace the supply: live and neutral wires enter through a main fuse and switch, then branch to separate parallel circuits, with an earth wire for the metal bodies.
2. Place the fuse in each live branch: it carries the branch current and melts if a short circuit or overload pushes the current too high, breaking the circuit and protecting the appliances.
3. Replace with identical rating: the new fuse must melt at the same safe current, so it gives the same protection as the original.
4. Rule out other ratings: a higher-rated fuse would allow a damaging current to flow without blowing, and a lower-rated one would blow under normal load, so only an identical rating is correct.

Why this matters. This explains a real safety rule at home: never replace a blown fuse with a thicker wire or a higher-rated one, the practical message of this whole topic.

The domestic circuit feeds parallel appliances through live and neutral wires with a fuse in the live wire that melts during a fault. The replacement fuse must have an identical rating so it blows at the same safe current; a higher rating fails to protect and a lower one blows needlessly.