NCERT Exemplar Solutions Class 10 Science Magnetic Effects of Electric Current — Short and Long Answer Questions Q13–32

NCERT Exemplar Problems, Class 10 Science — Magnetic Effects of Electric Current, Short Answer Questions 13 to 24 and Long Answer Questions 25 to 32. All 20 questions solved.

This band is almost entirely about drawing and explaining field patterns, and then about the two machines built from them. Four patterns cover everything:

Source	Field pattern
Straight wire	concentric circles round the wire, closer together near it
Circular loop	like a very short bar magnet; nearly straight along the axis at the centre
Solenoid	like a bar magnet; uniform inside, spreading out at the ends
Bar magnet	out of N, round the outside, into S, continuing inside from S to N

And the three hand rules, which between them answer Questions 18, 26 and 28:

• Right-hand thumb rule — thumb along the current, curled fingers give the field.
• Fleming’s left-hand rule — forefinger field, central finger current, thumb force. The motor rule.
• Fleming’s right-hand rule — the same three fingers, but for the induced current. The generator rule.

Key insight. The strength of a magnetic field is shown by how closely the field lines are packed, and almost every “explain the observation” question in this band is that one fact applied somewhere: lines spreading out as you move away from a wire (Question 19), lines diverging at the ends of a solenoid (Question 20), lines crowding near a loop’s centre (Question 27).

Short Answer Questions

Question 13

A magnetic compass needle is placed in the plane of paper near point A as shown in Figure 13.6. In which plane should a straight current carrying conductor be placed so that it passes through A and there is no change in the deflection of the compass? Under what condition is the deflection maximum and why?

Solution. The compass needle is a small magnet pivoted so that it can only swing in the plane of the paper — its axis of rotation is perpendicular to the page.

For no change in deflection, the conductor should be placed in the plane of the paper itself, passing through A.

The reason is worth setting out. The field of a straight conductor consists of circles in the plane perpendicular to it. If the conductor lies in the plane of the paper, then at the point A its field is perpendicular to the paper — that is, vertical, along the needle’s own axis of rotation. A field along the axis about which the needle turns exerts no turning effect in the plane the needle can move in; it could only tip the needle out of the paper, and the pivot prevents that. So the needle’s deflection is unaltered.

(A field of that kind would produce a dip of the needle, and dips occur only when the compass’s axis is horizontal, which it is not here.)

The deflection is maximum when the conductor is placed perpendicular to the plane of the paper at A. Then its circular field lines lie in the plane of the paper — exactly the plane in which the needle is free to turn — so the whole of the field acts to deflect it, and the turning effect is greatest.

The general principle is that only the component of the field lying in the plane of rotation can deflect the needle: it is maximum when the conductor is perpendicular to that plane, and zero when the conductor lies within it.

Question 14

Under what conditions permanent electromagnet is obtained if a current carrying solenoid is used? Support your answer with the help of a labelled circuit diagram.

Solution. Two conditions must both be met.

(i) The current through the solenoid must be direct current (DC). An alternating current reverses direction a hundred times a second, and the magnetism it induces would reverse with it, so the rod would never settle to a fixed polarity. Only a steady, one-way current gives a steady field with a fixed N and S.

(ii) The rod placed inside must be of a hard magnetic material such as steel, not soft iron.

The second condition is the substance of the answer. Soft iron is easy to magnetise but loses its magnetism as soon as the current stops — which is exactly why it is used for an ordinary electromagnet, where switchable magnetism is the point. Steel is harder to magnetise, but once magnetised it retains its magnetism after the current is switched off. That retention is what makes the magnet permanent.

So: pass a direct current through a solenoid wound on a steel rod, leave it for a time, then switch off and remove the rod. It is now a permanent magnet.

The circuit is a solenoid in series with a battery (for DC), a key and a variable resistance to control the current, with the steel rod inside the coil.

The contrast is worth keeping as a pair: soft iron for temporary magnets, steel for permanent ones — the same distinction that decides what the core of a motor or a transformer is made of.

Question 15

AB is a current carrying conductor in the plane of the paper as shown in Figure 13.7. What are the directions of magnetic fields produced by it at points P and Q? Given $r_1 > r_2$, where will the strength of the magnetic field be larger?

Solution. The field of a long straight conductor consists of concentric circles in the plane perpendicular to it. Since the conductor lies in the plane of the paper, those circles are perpendicular to the paper — so at any point in the plane of the paper the field is either into or out of the page.

Apply the right-hand thumb rule: point the right thumb along the current, from A to B (upwards), and the fingers curl round the wire. On the right of the wire the fingers go into the page; on the left they come out of it.

• P lies to the right of the conductor, so the field there is into the plane of the paper.
• Q lies to the left, so the field there is out of the plane of the paper.

Where is the field stronger? The strength of the field of a straight conductor falls off with distance:

$$B \propto \frac{1}{r}$$

Since $r_1 > r_2$, the point Q — which is the nearer one, at the smaller distance $r_2$ — has the larger field.

The same conclusion can be read from a field-line diagram without any formula, and that is the point of Question 19: the circles are packed closer together near the wire, and closer lines mean a stronger field.

Question 16

A magnetic compass shows a deflection when placed near a current carrying wire. How will the deflection of the compass get affected if the current in the wire is increased? Support your answer with a reason.

Solution. The deflection increases.

The reason is that the magnetic field produced by a straight current-carrying conductor is directly proportional to the current through it:

$$B \propto I$$

A larger current therefore produces a stronger field at the compass’s position. The needle is a small magnet, and the turning effect on it grows with the strength of the field it sits in — so it swings further from its rest position and the deflection is larger.

It is worth adding what the field also depends on, since the two factors are usually tested together. The field is inversely proportional to the distance from the wire,

$$B \propto \frac{I}{r}$$

so the deflection could equally be increased by bringing the compass closer to the wire, and decreased by moving it away — which is the content of Questions 15 and 19.

This is precisely the observation behind Oersted’s experiment, the discovery that started the whole subject: a compass placed near a wire deflects when a current flows, showing that an electric current produces a magnetic field. Increasing the current makes the effect more pronounced, confirming that the field is caused by the current rather than merely accompanying it.

Question 17

It is established that an electric current through a metallic conductor produces a magnetic field around it. Is there a similar magnetic field produced around a thin beam of moving (i) alpha particles, (ii) neutrons? Justify your answer.

Solution. The governing principle is that a magnetic field is produced by moving charge — an electric current is simply charge in motion, and the conductor is incidental. So the question for each beam is whether it carries charge.

(i) Alpha particles — yes, a field is produced. An alpha particle is a helium nucleus, carrying a charge of $+2e$. A beam of them moving in one direction is therefore a flow of positive charge — that is, an electric current, in the direction of motion. Being a current, it produces a magnetic field around itself in exactly the same way a current in a wire does, with circular field lines given by the right-hand thumb rule.

(ii) Neutrons — no field is produced. A neutron is electrically neutral, carrying no charge at all. A beam of neutrons, however fast it moves, transports no charge, so it constitutes no current and produces no magnetic field.

The comparison makes the principle sharp. Both beams involve particles in motion, and both carry energy and momentum; only the alpha beam carries charge, and only the alpha beam is magnetic. Motion alone is not enough — it must be motion of charge.

Note also that the direction matters: because the alpha particles are positive, the conventional current is in the same direction as their motion. A beam of electrons would constitute a current in the opposite direction to its motion.

Question 18

What does the direction of thumb indicate in the right-hand thumb rule. In what way this rule is different from Fleming’s left-hand rule?

Solution.

In the right-hand thumb rule, the thumb indicates the direction of the current in the straight conductor. Hold the conductor in your right hand with the thumb pointing along the current, and the curled fingers then give the direction of the magnetic field lines encircling it.

The difference from Fleming’s left-hand rule is that the two rules answer completely different questions.

	Right-hand thumb rule	Fleming’s left-hand rule
Question answered	What is the direction of the field produced by a current?	What is the direction of the force on a current?
Situation	a current-carrying conductor, on its own	a current-carrying conductor placed in an external magnetic field
Thumb gives	the current	the force (motion)
Fingers give	the field, by curling	forefinger the field, central finger the current

So the right-hand thumb rule is about a current creating a field; Fleming’s left-hand rule is about a field pushing a current-carrying conductor. The first needs only the conductor, the second needs an external magnet as well.

A third rule completes the set and is easy to confuse with the second: Fleming’s right-hand rule uses the same three perpendicular fingers but gives the direction of the induced current in a conductor moved through a field — the generator rule, as against the left hand’s motor rule.

Question 19

Meena draws magnetic field lines of field close to the axis of a current carrying circular loop. As she moves away from the centre of the circular loop she observes that the lines keep on diverging. How will you explain her observation.

Solution. The diverging of the lines means that the magnetic field is getting weaker as she moves away from the centre of the loop.

The explanation rests on the convention for drawing field lines: the closeness of the lines represents the strength of the field. Where lines are packed tightly the field is strong; where they spread apart it is weak. So lines that “keep on diverging” are a picture of a field that keeps on falling.

Physically this is what happens. Every small element of the current-carrying loop contributes to the field, and the contribution of each falls off with distance. Near the centre the whole loop is close by and the contributions add up to a large field, so the lines are crowded and nearly straight. Move away along the axis and every part of the loop is further off, so every contribution is smaller and the total field falls — and the lines spread out to show it.

The same picture explains the shape as well as the spacing. At the centre of the loop the lines are nearly straight and parallel, because the contributions from opposite sides of the loop reinforce along the axis; further away they curve outward to loop round and return, giving the pattern of a short bar magnet.

The identical reasoning, applied to a solenoid instead of a single loop, is Question 20.

Question 20

What does the divergence of magnetic field lines near the ends of a current carrying straight solenoid indicate?

Solution. It indicates that the magnetic field becomes weaker near and beyond the ends of the solenoid.

Inside a long solenoid, well away from the ends, the field lines are straight, parallel and equally spaced, which means the field there is uniform — the same in strength and direction at every point, as Question 10 of the multiple-choice band establishes.

Near the ends that regularity breaks down. The lines begin to spread apart as they leave the coil and curve round the outside to return to the other end. Since the closeness of the lines measures the field’s strength, lines that are spreading out mean a field that is falling in strength.

The physical reason is the same as in Question 19. A point deep inside the solenoid has many turns of the coil on both sides of it, all contributing to the field. A point at the end has turns on one side only, so roughly half the contribution is missing — and the field there is about half its value at the centre. Beyond the end it falls off faster still.

This is why a solenoid is described as being like a bar magnet: the field is strong and uniform inside, emerges from one end as from a north pole, spreads out through the surrounding space, and re-enters at the other end.

It is also why the uniform-field claim always carries the word long: in a short coil the end regions are a large fraction of the whole, and there is little uniform interior left.

Question 21

Name four appliances wherein an electric motor, a rotating device that converts electrical energy to mechanical energy, is used as an important component. In what respect motors are different from generators?

Solution.

Four appliances that contain an electric motor: electric fan, mixer or food processor, washing machine, and computer disc drive. Others would do equally well — a refrigerator compressor, a water pump, an electric drill, a hair dryer.

The difference between motors and generators is a matter of which way the energy conversion runs:

	Converts	Driven by	Rule that applies
Motor	electrical → mechanical	an electric current supplied to it	Fleming’s left hand
Generator	mechanical → electrical	a mechanical rotation supplied to it	Fleming’s right hand

A motor is given a current and produces rotation: the current-carrying coil placed in a magnetic field experiences a force, and the coil turns. A generator is given rotation and produces a current: the coil is turned in a magnetic field, and the changing field through it induces an e.m.f.

The striking fact is that the two machines have essentially the same parts — a coil, a magnet, and a commutator or slip rings. What differs is which end you supply and which end you take from. A motor driven mechanically will in fact act as a generator, and a generator supplied with current will turn like a motor.

Question 22

What is the role of the two conducting stationary brushes in a simple electric motor?

Solution. The brushes are the sliding contacts that connect the stationary external circuit to the rotating coil.

They are fixed in place — which is what “stationary” means — and they press against the outer side of the two halves of the split ring (the commutator). The inner sides of those half-rings are insulated from each other and are attached to the axle, so they rotate with the coil.

That arrangement does two things at once.

It carries current into and out of a rotating coil. Wires could not simply be attached to a coil that spins, since they would twist and break. A sliding contact between fixed brushes and a rotating ring solves that.

It reverses the current in the coil every half turn. As the coil rotates, the two halves of the split ring exchange brushes. The current through the coil therefore reverses direction at exactly the moment the coil passes the vertical, which is what keeps the force on each side acting in the same rotational sense — and so keeps the motor turning continuously in one direction instead of oscillating.

Without that reversal the coil would turn half a revolution, come to rest and swing back. The brushes and split ring together are what make continuous rotation possible, and it is the same commutator that turns an AC generator into a DC one in Question 11.

Question 23

What is the difference between a direct current and an alternating current? How many times does AC used in India change direction in one second?

Solution.

The difference. A direct current (DC) always flows in one direction; its magnitude may vary but its sense does not. An alternating current (AC) reverses its direction periodically, flowing one way and then the other, over and over.

A cell or battery supplies DC; the mains supply is AC.

How many times per second. The frequency of AC in India is 50 Hz, meaning the current completes 50 complete cycles every second.

In each cycle the current reverses direction twice — once as it falls through zero from positive to negative, and once as it rises back through zero from negative to positive. So

$$\text{number of direction changes per second} = 2 \times 50 = \mathbf{100}$$

and the interval between successive reversals is $\frac{1}{100}$ second.

This is the calculation behind Question 8 of the multiple-choice band, where the statement “AC changes direction after every $\frac{1}{50}$ second” is the incorrect one. The $\frac{1}{50}$ second is the period of a full cycle, which contains two reversals, not one.

The practical advantage of AC is worth a sentence, since it explains why the mains uses it: AC voltages can be stepped up and down efficiently with transformers, so power can be transmitted over long distances at high voltage and low current, which greatly reduces the $I^2R$ losses in the cables.

Question 24

What is the role of fuse, used in series with any electrical appliance? Why should a fuse with defined rating not be replaced by one with a larger rating?

Solution.

The role of the fuse. A fuse is a short length of wire of high resistivity and low melting point, connected in series with the appliance, so that the whole of the appliance’s current passes through it.

It protects against short-circuiting and overloading. If a fault makes the current exceed the fuse’s rated value, the heat produced in it — $H = I^2Rt$ — raises the wire to its melting point. It melts, breaking the circuit, and the current stops before the appliance or the house wiring can be damaged or set on fire.

Why not use a larger rating? Because the fuse would then no longer do its job.

A fuse rated well above the appliance’s safe current will simply not melt when a dangerous current flows. The excessive current continues, and the appliance is damaged — or the wiring overheats — while the fuse sits intact. The protecting device has been defeated by being made too strong.

The point is that the fuse is deliberately designed to be the weakest link in the circuit: it is meant to fail first, so that nothing more expensive or more dangerous fails instead. Replacing it with a higher-rated one moves the weakest link somewhere you do not want it.

So a blown fuse must always be replaced by one of identical rating — never a larger one, and never a piece of ordinary wire or foil, which has no defined rating at all.

Long Answer Questions

Question 25

Why does a magnetic compass needle pointing North and South in the absence of a nearby magnet get deflected when a bar magnet or a current carrying loop is brought near it. Describe some salient features of magnetic lines of field concept.

Solution.

Why it deflects. A compass needle is itself a small magnet, and in the absence of anything else it aligns with the only field present — the Earth’s magnetic field — so it settles along the north–south direction.

Bringing a bar magnet near it adds a second, much stronger field in that region. Bringing a current-carrying loop near it does the same thing, because a current-carrying loop behaves like a short bar magnet and has its own associated field. Either way the field at the needle’s position is now the sum of the Earth’s field and the new one, and the needle turns to align with that resultant — which is a deflection away from north–south.

Salient features of magnetic field lines.

• They show direction. The tangent to a field line at any point gives the direction of the field there — the direction in which the north pole of a compass needle placed there would point.
• They show strength. The degree of closeness of the lines represents the magnitude of the field: closely packed lines mean a strong field, widely spaced lines a weak one. Parallel, equally spaced lines mean a uniform field.
• They emerge from N and enter S. Outside a magnet the lines run from the north pole to the south pole; inside the magnet they continue from S to N, so every line is a closed curve.
• They never cross one another. If two lines crossed, the field at the crossing point would have two different directions at once, which is impossible — at any point the field has one unique value and one unique direction.

That last feature is the one most often asked for a reason, and the reason is exactly that: a single point cannot have two net field values.

Question 26

With the help of a labelled circuit diagram illustrate the pattern of field lines of the magnetic field around a current carrying straight long conducting wire. How is the right hand thumb rule useful to find direction of magnetic field associated with a current carrying conductor?

Solution.

The pattern. Pass a straight wire vertically through a hole in a horizontal card, and connect it in series with a battery, a key, a variable resistance and an ammeter. Sprinkle iron filings on the card and tap it gently, or move a small compass about on it.

The filings arrange themselves into concentric circles centred on the wire, lying in the plane of the card — the plane perpendicular to the conductor. Two features of the pattern matter:

• The circles are closest together near the wire and spread further apart with distance, showing that the field is strongest near the conductor and weakens as $1/r$ — the point of Questions 15 and 19.
• Increasing the current with the variable resistance makes the pattern more pronounced, because $B \propto I$ — the point of Question 16.

The right-hand thumb rule. It gives the direction in which those circles are traced:

Hold the current-carrying straight conductor in your right hand with the thumb pointing along the direction of the current. Then the fingers wrap around the conductor in the direction of the magnetic field lines.

So for a current flowing upwards through the card, the field lines run anticlockwise seen from above; reverse the current and they run clockwise. Reversing the current reverses the field, which is easily checked with the compass.

The rule is what makes the pattern usable: the filings show the shape of the field but not its sense, and the thumb rule supplies the sense.

Question 27

Explain with the help of a labelled diagram the distribution of magnetic field due to a current through a circular loop. Why is it that if a current carrying coil has $n$ turns the field produced at any point is $n$ times as large as that produced by a single turn?

Solution.

The distribution. Every small element of the loop produces its own circular field. Adding them up gives a pattern with three recognisable features:

• Close to the wire of the loop, the lines are nearly circles round the wire, just as for a straight conductor, and they are tightly packed — the field is strongest there.
• At the centre of the loop, the contributions from every element point the same way, along the axis, so they reinforce. The lines there are straight and perpendicular to the plane of the loop.
• Further along the axis, the lines diverge and curve round to return through the outside, so the field weakens — which is Meena’s observation in Question 19. The whole pattern is that of a very short bar magnet, with one face a north pole and the other a south pole.

Why $n$ turns give $n$ times the field. A coil of $n$ turns is $n$ circular loops of the same radius, placed so close together that they are effectively coincident — they all lie at essentially the same distance from any given point on the axis, and all carry the same current in the same direction.

Each turn therefore produces its own magnetic field at that point, and all $n$ of those fields point in the same direction. Magnetic fields add as vectors, and vectors in the same direction simply add in magnitude:

$$B_{\text{total}} = B_1 + B_2 + \cdots + B_n = nB_1$$

So the field is $n$ times that of a single turn. The essential conditions are that the turns be close together, of the same radius, and carry the same current the same way round — if any turn were wound in reverse it would subtract instead of add.

This is why coils are wound with many turns wherever a strong field is wanted: in an electromagnet, in the armature of a motor (Question 6), and in a solenoid.

Question 28

Describe the activity that shows that a current-carrying conductor experiences a force perpendicular to its length and the external magnetic field. How does Fleming’s left-hand rule help us to find the direction of the force acting on the current carrying conductor?

Solution.

The activity.

1. Take a small aluminium rod AB and suspend it horizontally, by two connecting wires, from a stand.
2. Place a strong horseshoe magnet so that the rod lies between its poles, with the north pole vertically below the rod and the south pole above it — so the magnetic field is vertical, perpendicular to the rod.
3. Connect the rod through a key and a battery to complete the circuit.
4. Close the key. The rod is displaced — it swings, in a direction perpendicular both to its own length and to the field.
5. Reverse the current and close the key again: the rod is displaced in the opposite direction.
6. Reverse the magnet instead, keeping the current as it was: again the displacement reverses.

The displacement shows that a current-carrying conductor placed in a magnetic field experiences a force, and its direction shows that the force is perpendicular to both the conductor and the field. The two reversal tests show that the force direction depends on the direction of the current and of the field.

Fleming’s left-hand rule turns those observations into a usable rule:

Stretch the thumb, forefinger and central finger of the left hand so that they are mutually perpendicular. If the forefinger points in the direction of the magnetic field and the central finger in the direction of the current, then the thumb points in the direction of the force — that is, of the motion of the conductor.

The mnemonic is that the three fingers stand for Field, Current and Force in the order fore, central, thumb.

This is the principle of the electric motor: the force on the current-carrying coil is what turns it, which is Question 29.

Question 29

Draw a labelled circuit diagram of a simple electric motor and explain its working. In what way these simple electric motors are different from commercial motors?

Solution.

Working. A rectangular coil is placed between the poles of a magnet, with its ends connected to the two halves of a split ring, which press against two stationary brushes joined to a battery.

When the key is closed, current flows through the coil. The two sides of the coil carry current in opposite directions, and by Fleming’s left-hand rule the force on one side is upward while the force on the other is downward. That pair of opposite forces is a couple, and it makes the coil rotate.

After half a revolution the two sides have exchanged places, and the forces would now turn the coil back again. This is where the split ring acts: as the coil passes the vertical, the two halves of the ring change brushes, so the current through the coil reverses. Each side is now carrying current the other way, the forces are again in the sense that continues the rotation, and the coil keeps turning in one direction.

The split ring is therefore the essential component — without it the coil would merely oscillate through half a turn and stop.

How commercial motors differ. A simple demonstration motor uses a permanent magnet, a single loop of wire and no core. A commercial motor is built for real torque and differs in three ways, all of which increase the force on the coil:

• it uses an electromagnet in place of the permanent magnet, giving a much stronger field;
• it has a large number of turns of conducting wire in the coil, since the force is proportional to the number of turns;
• the coil is wound on a soft iron core, which concentrates the field through it. The coil and core together are called the armature.

Question 30

Explain the phenomenon of electromagnetic induction. Describe an experiment to show that a current is set up in a closed loop when an external magnetic field passing through the loop increases or decreases.

Solution.

The phenomenon. Electromagnetic induction is the production of an electric current in a conductor by a changing magnetic field. When the magnetic field passing through a closed loop changes — whether it grows or dies away — an e.m.f. is induced in the loop, and if the loop is closed a current flows.

The essential word is changing. A steady magnetic field, however strong, induces nothing.

The experiment.

1. Take a coil of many turns — call it the secondary coil — and connect its ends to a galvanometer, forming a closed loop. There is no battery in this circuit.
2. Take a second coil, the primary, and connect it to a battery through a key. Place the two coils close together, or wind both on the same non-conducting rod, so that the field of the primary passes through the secondary.
3. Insert the key. The current in the primary rises from zero, so the magnetic field through the secondary increases. The galvanometer shows a momentary deflection, which dies away once the primary current becomes steady.
4. Leave the key in. The field is now constant, and the galvanometer reads zero — confirming that a steady field induces nothing.
5. Remove the key. The primary current falls to zero, so the field through the secondary decreases. The galvanometer again deflects momentarily, but now in the opposite direction.

The conclusion. A current is set up in the closed loop only while the field through it is changing, and its direction depends on whether the field is increasing or decreasing. This is exactly the observation of Question 7 in the multiple-choice band.

The same effect can be shown even more simply by moving a bar magnet in and out of a coil connected to a galvanometer: the needle deflects while the magnet moves, in opposite directions for pushing in and pulling out, and rests at zero while the magnet is held still.

Electromagnetic induction is the principle of the electric generator, and Fleming’s right-hand rule gives the direction of the induced current.

Question 31

Describe the working of an AC generator with the help of a labelled circuit diagram. What changes must be made in the arrangement to convert it to a DC generator?

Solution.

The arrangement. A rectangular coil ABCD is mounted so that it can be rotated about an axis between the poles of a strong magnet. The two ends of the coil are connected to two separate slip rings, $R_1$ and $R_2$, which rotate with the coil. Two stationary brushes press against these rings and lead to the external circuit through a galvanometer.

Working. The coil is rotated mechanically. As it turns, the magnetic field passing through it changes continuously, and by electromagnetic induction a current is induced in the coil. Fleming’s right-hand rule gives its direction.

Consider the coil starting horizontal. Side AB moves upwards through the field while CD moves downwards, so the induced currents in the two sides drive current round the coil in one sense. After half a revolution AB is moving downwards and CD upwards — the two sides have exchanged roles, so the induced current in the coil reverses.

Because each end of the coil stays permanently attached to its own slip ring, that reversal is carried straight out to the external circuit. The current in the external circuit therefore changes direction every half rotation: it is alternating current, and the galvanometer needle swings to and fro.

To convert it into a DC generator. Replace the two slip rings by a single split-ring commutator — one ring cut into two halves, each connected to one end of the coil.

Now, at the very instant the current in the coil reverses, the two halves of the ring change brushes. The reversal inside the coil is cancelled by a reversal of the external connections, so each brush is always in contact with whichever side of the coil is moving the same way. The current in the external circuit then always flows in the same direction — direct current.

That is the only change required; the coil, magnet and brushes are otherwise the same. It is the same commutator that keeps the motor of Question 29 rotating one way.

Question 32

Draw an appropriate schematic diagram showing common domestic circuits and discuss the importance of fuse. Why is it that a burnt out fuse should be replaced by another fuse of identical rating?

Solution.

The domestic circuit. Power reaches a house through three wires:

• the live wire, usually with red insulation, at 220 V;
• the neutral wire, usually black, at zero potential;
• the earth wire, usually green, connected to a metal plate buried in the ground.

These pass through the electricity meter and then a main fuse to the distribution box. From there the house is divided into separate circuits — typically a 15 A circuit for heavy appliances such as geysers and air conditioners, and a 5 A circuit for lights and fans — each with its own fuse.

Within each circuit the appliances are connected in parallel, so that every one receives the full 220 V and can be switched independently, as Question 27 of the Electricity unit explains. The earth wire is connected to the metal body of each appliance.

The importance of the fuse. The fuse is a wire of low melting point placed in series with the circuit. If a short circuit or an overload makes the current exceed its rating, the heat produced ($H = I^2Rt$) melts the fuse and breaks the circuit, cutting off the current before the wiring can overheat and start a fire, or the appliances be damaged.

It is worth distinguishing it from earthing, which is often confused with it: the fuse protects the circuit and the appliances from excess current, while the earth wire protects the user from a shock if a live wire touches an appliance’s metal casing.

Why the rating must be identical. A fuse of larger rating will not melt at the current the circuit was designed to be protected against. The excess current would continue to flow, and the wiring or the appliance would be damaged while the fuse remained intact — the protection would have been silently removed.

A fuse of smaller rating is no good either: it would melt under normal load and interrupt the supply constantly.

The fuse is deliberately the weakest link in the circuit, chosen to fail before anything else does. Replacing it with anything other than an identical rating moves that weakest link somewhere far more expensive.

Common mistakes

• Getting Question 13 the wrong way round. The conductor lies in the plane of the paper for no change, because its field at A is then perpendicular to the paper and along the needle’s axis of rotation. Perpendicular to the paper gives maximum deflection.
• Using soft iron for a permanent magnet, in Question 14. Soft iron loses its magnetism as soon as the current stops; steel retains it. Soft iron is for electromagnets, steel for permanent magnets.
• Halving the AC reversals, in Question 23. 50 Hz means 50 cycles per second and two reversals per cycle, so 100 changes of direction per second.
• Saying a neutron beam produces a field, in Question 17. A magnetic field needs moving charge. Neutrons are neutral, so however fast they move they carry no current.
• Confusing the right-hand thumb rule with Fleming’s left-hand rule, in Question 18. The first gives the field produced by a current; the second gives the force on a current in an external field.
• Omitting the split ring from the motor’s explanation, in Questions 22 and 29. Without the commutator the coil turns half a revolution and stops. The reversal every half turn is what makes rotation continuous.
• Claiming a steady field induces a current, in Question 30. Induction needs the field to be changing; a constant field, however strong, induces nothing.
• Treating earthing and the fuse as the same safeguard, in Question 32. The fuse protects the appliance from excess current; the earth wire protects the user from shock.

Practise next

• Exemplar Multiple Choice Questions — twelve questions on the same field patterns and rules in recognition form.
• NCERT textbook Chapter 12 — Magnetic Effects of Electric Current for the end-of-chapter exercise on field patterns, the motor and induction.
• Exemplar Electricity — the previous unit, whose circuits and fuses Questions 24 and 32 above build on.
• Textbook activities for the iron-filing patterns of Questions 26 and 27 and the force experiment of Question 28.