Chapter 13 · Magnetic Effects of Electric Current

NCERT Exemplar Class 10 Science Chapter 13 Short Answer Questions

All 12 questions of NCERT Exemplar Class 10 Science Chapter 13 Short Answer Questions (Magnetic Effects of Electric Current), in the book's order, each with its answer and a step-by-step solution.

1. Q13
   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?

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   The conductor must lie in the plane of the paper for no change; the deflection is maximum when it is perpendicular to the paper.

   The conductor must be placed in the plane of the paper, that is, in the same plane as the compass needle. Its field at the needle is then perpendicular to the paper, and a needle free to swing only within the paper feels no turning effect from it, so the deflection is unchanged. The deflection is maximum when the conductor is placed perpendicular to the plane of the paper through A, because the circular field lines then lie in the paper itself and the whole of the field acts in the plane in which the needle turns. Since $B\propto I/r$, the deflection grows further if the current is increased or the compass is brought nearer to the conductor.

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2. Q14
   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.
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   Use a steel core, pass a steady direct current, and make the field strong with many close turns and a large current.

   Two conditions must be met together. First, the rod placed inside the solenoid must be of a hard magnetic material such as steel rather than soft iron, because steel keeps its magnetism after the magnetising field is removed while soft iron loses nearly all of it. Second, the solenoid must carry a steady direct current, supplied by a battery, so that the magnetising field keeps one direction and definite N and S poles are established; with alternating current the field would reverse a hundred times a second and no settled poles would be left. The field must also be strong enough, which is why the solenoid is wound with a large number of close turns and carries a sufficiently large current. The circuit is the solenoid, wound on the steel rod, joined in series with a battery and a plug key, as drawn in the labelled diagram.

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3. Q15
   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?

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   At P the field is into the paper, at Q out of the paper; the field is stronger at Q, the nearer point.

   The current flows upwards, as the arrow on the conductor shows. At P, which lies to the right of the conductor, the magnetic field is directed into the plane of the paper; at Q, which lies to the left of it, the field is directed out of the plane of the paper. Since $B\propto 1/r$ for a straight conductor and $r_2<r_1$, the strength of the magnetic field is larger at Q.

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4. Q16
   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.
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   The deflection increases, because the wire's field at the compass is proportional to the current.

   The deflection increases. The compass is not moved, so its distance from the wire is unchanged, and the field of a straight conductor at a fixed distance is directly proportional to the current, $B\propto I/r$. The needle rests along the resultant of that field and the earth's horizontal field. Raising the current lengthens the wire's contribution while the earth's field stays exactly as it was, so the resultant turns further away from the north-south direction and the needle deflects more. In the usual arrangement, in which the wire's field at the needle is at right angles to the earth's field, $\tan \theta =B/B_H$, so the tangent of the deflection grows in proportion to the current and the deflection itself approaches, but never reaches, a right angle.

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5. Q17
   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

   1. (i)alpha particles, ​
   2. (ii)​neutrons? Justify your answer.
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   Yes for alpha particles, because moving charge forms a current; no for neutrons, which carry no charge.

   (i) Yes. An alpha particle carries a charge of $+2e$, so a thin beam of moving alpha particles is a stream of moving charge, which is an electric current even though no wire is involved. A magnetic field is therefore produced around the beam, its lines forming circles that encircle the beam; since the particles are positive the conventional current is along the beam, and the right-hand thumb rule with the thumb pointing along the beam gives the sense of those circles. (ii) No. A neutron carries no charge at all, so however fast and however intense the beam is, no charge passes any point of it and it constitutes no current. With no current there is no magnetic field of this kind around a neutron beam.

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6. Q18
   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?
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   The thumb shows the direction of the current; the thumb rule gives the field produced by a current, Fleming's rule the force on it.

   In the right-hand thumb rule the direction of the thumb indicates the direction of the **current** in the straight conductor; the curled fingers then give the direction in which the magnetic field lines encircle it. The rule differs from Fleming's left-hand rule in four ways. It uses the right hand, while Fleming's rule uses the left. It involves only two directions, the current and the field, while Fleming's rule involves three mutually perpendicular directions, the field on the forefinger, the current on the centre finger and the force on the thumb. The thumb stands for the current in the first rule but for the force in the second. Most importantly, the right-hand thumb rule gives the magnetic field that a current **produces** around itself, whereas Fleming's left-hand rule gives the force that a current-carrying conductor **experiences** when it is placed in an external magnetic field.

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7. Q19
   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.
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   The lines diverge because the field grows weaker away from the centre, and widely spaced lines show a weaker field.

   The spacing of magnetic field lines is what shows the strength of the field: closely packed lines mean a strong field and widely separated lines a weak one. At the centre of the loop every part of the wire is the same distance away and each part contributes a field pointing the same way along the axis, so the contributions add and the field is at its largest; that is why the lines there are almost straight, parallel and close together. As Meena moves away from the centre every part of the loop is further off, its contribution is smaller and the field grows weaker, so the lines have to be drawn further apart. The same thing can be said using the lines alone: field lines are closed curves, so the same bundle of lines continues outwards, and a fixed bundle spread over a larger area means fewer lines crossing unit area. Her observation of diverging lines is therefore simply the picture of a magnetic field that is getting weaker with distance from the centre of the loop.

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8. Q20
   What ​does ​the ​divergence of magnetic field lines near ​the ​ends ​of a current carrying straight solenoid indicate?
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   Divergence of the field lines near the ends shows that the magnetic field is weaker there.

   It indicates that the strength of the magnetic field is decreasing near the ends of the solenoid. The spacing of field lines is what records the strength of a field: inside the solenoid the lines are straight, parallel and equally spaced, so the field there is uniform and strong, while near the ends the same lines spread apart, so fewer of them cross unit area and the field is weaker. The lines fan out because they are closed curves that must leave the coil and return through the much larger space outside it; for a long solenoid the field on the axis at the end face has already fallen to half its value in the middle.

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9. Q21
   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?
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   Fan, washing machine, refrigerator and mixer use motors; a motor converts electrical to mechanical energy, a generator the reverse.

   Four appliances that use an electric motor as an important component are an electric fan, a washing machine, a refrigerator, whose compressor is driven by a motor, and a mixer or grinder; a water pump, an electric drill or a hair dryer would do equally well. Motors differ from generators in four connected respects. The energy conversion runs the opposite way: a motor converts electrical energy into mechanical energy, a generator converts mechanical energy into electrical energy. The principle is different: a motor works on the force experienced by a current-carrying coil placed in a magnetic field, a generator on the current induced in a coil rotated in a magnetic field, that is, on electromagnetic induction. What must be supplied is different: a motor has to be fed with a current and gives out rotation, while a generator has to be turned and gives out a current. And the rule that fixes the direction is different: Fleming's left-hand rule for the motor, Fleming's right-hand rule for the generator.

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10. Q22
    What ​is the role ​of ​the two conducting ​stationary ​brushes in ​a simple electric motor?
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    The brushes give sliding contact between the battery and the rotating coil and, with the split ring, reverse the current every half turn.

    The two brushes are held stationary and press against the two halves of the split-ring commutator, which turns with the coil. They have two roles. First, they provide a continuous sliding electrical contact between the fixed battery and the rotating coil, so that current can be carried into and out of a coil that is turning without the connecting wires twisting and breaking. Second, together with the split ring they reverse the direction of the current in the coil after every half rotation, because each half-ring comes into contact with the other brush at that point. Since each arm of the coil also changes sides between the poles every half turn, reversing the current at the same instant keeps the force on each arm acting in the same rotational sense, so the coil goes on rotating continuously in one direction instead of stopping after half a turn.

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11. Q23
    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?
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    DC flows in one direction while AC reverses periodically; AC in India changes direction 100 times in one second.

    A direct current always flows in one and the same direction, so its direction never changes with time, whereas an alternating current reverses its direction periodically, so that both its direction and its magnitude change with time. The alternating current supplied in India has a frequency of $50$ Hz, that is, $50$ complete cycles each second, and the current changes direction twice during every cycle, once as it falls through zero from one direction and once as it falls back through zero from the other. It therefore changes direction $2\times 50=100$ times in one second, which is once every $1/100$ s, while a complete cycle takes $1/50$ s.

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12. Q24
    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?
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    A fuse melts and breaks the circuit when the current becomes too large; a larger-rated fuse would not melt at a dangerous current.

    A fuse is a short piece of wire of high resistance and low melting point, joined in series with the appliance on the live wire so that it carries exactly the same current as the appliance. Its role is to protect the appliance and the wiring: if the current rises above the safe value, because of a short circuit or an overload, the heat produced in the fuse, $H=I^{2}Rt$, is enough to melt it, the circuit is broken and the current stops before the appliance is damaged or a fire starts. A fuse with a defined rating must not be replaced by one with a larger rating because the rating is the current at which the fuse melts, and it is chosen just above the appliance's own working current. A larger fuse would not melt at the current that is already dangerous for that appliance, so it would allow a much larger current to keep flowing; the appliance and the wiring would overheat and could be destroyed or set on fire. Raising the rating does not make the protection stronger, it removes it.

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