MPSC 2023 Prelims Paper-I · Question 36 of 100

Correct — option (3), 'Super conductors', printed in the paper as two words. The stem is a definition with the term taken out, and every element of it points to one phenomenon. Resistivity is the property of a material that measures how strongly it opposes the flow of current, defined independently of the size and shape of any particular specimen, so a statement about resistivity is a statement about a substance rather than about a wire. The stem says this quantity becomes zero — not small, not negligible, but zero — and that it does so below a critical temperature, that is, abruptly at a definite temperature characteristic of the material rather than gradually as the specimen is cooled. Both features together define superconductivity. Below its critical or transition temperature a superconductor carries current with no resistance at all, which has the striking consequence that a current once started in a superconducting ring continues to circulate indefinitely with no source of emf to maintain it, since there is nothing to dissipate its energy. The phenomenon was discovered by Heike Kamerlingh Onnes in 1911, after he had succeeded in liquefying helium and could reach temperatures within a few degrees of absolute zero; on cooling mercury he found its resistance vanish at about 4.2 kelvin, and he was awarded the Nobel Prize in Physics in 1913 for his work at low temperatures. A superconductor turned out to be more than a perfect conductor: below the critical temperature it also expels magnetic flux from its interior, the Meissner effect, making it a perfect diamagnet, which is what allows a magnet to be held levitating above a superconducting surface. The practical importance lies in magnets, since a coil with no resistance can carry very large currents without heating, and superconducting magnets are what make MRI scanners and large particle accelerators possible. No other option describes a material whose resistivity reaches zero, so option (3) is the answer.

• (1)Conductors — Conductors have low resistivity, which is what makes them useful for carrying current, but low is not zero and the difference is absolute rather than one of degree. In a metallic conductor the resistivity falls as the temperature is lowered, because the lattice vibrations that scatter the moving electrons subside, but it does not fall indefinitely: it levels off at a small residual value determined by the impurities and structural defects in the metal, which do not disappear however cold the specimen becomes. There is no critical temperature at which anything abrupt happens. This distinction is worth holding firmly, because the stem's phrasing tempts a candidate to reason that a very good conductor cooled very far must eventually reach zero. It does not, and a metal that does reach zero has entered the superconducting state, which is a different phase with properties an ordinary conductor never shows.
• (2)Insulators — Insulators are at the opposite end of the scale from what the stem describes, having resistivities many orders of magnitude greater than those of metals, which is precisely why they are used to prevent the flow of current. Their behaviour with temperature also runs the wrong way for this question. In insulators and semiconductors resistivity decreases as the temperature rises, because heating frees more charge carriers to conduct, and it therefore increases as the material is cooled. Cooling an insulator makes it a better insulator, so no amount of it will bring the resistivity to zero. This is a useful contrast to keep alongside the behaviour of metals, whose resistivity rises with temperature, and it is asked about in its own right often enough to be worth learning as a pair.
• (4)None of these — An escape option of this kind can only be correct if all three of the preceding choices can be positively ruled out, and here the third of them is not merely defensible but is the standard textbook definition of the term. The stem's two elements — resistivity falling to exactly zero, and doing so below a critical temperature — are the two elements used to define a superconductor in every account of the subject, so the sentence is not a description in search of a name. Choosing 'none of these' generally means either that the term was not recognised or that its printing as two words caused hesitation. Neither is a sound reason: the commission's spacing and spelling are unreliable throughout this paper, and an option should be judged by what it names rather than by how it is typeset.

Two quantities are easily confused here and the question depends on the difference. Resistance belongs to a particular object and depends on how long and how thick it is; resistivity belongs to the material itself, being the resistance of a specimen of unit length and unit cross-section, so it can be quoted for copper or for glass without reference to any specimen. Materials sort themselves by resistivity into conductors, semiconductors and insulators, and they also differ in how that resistivity responds to temperature. In metals the resistivity rises with temperature, because hotter lattice ions vibrate more and scatter the conduction electrons more effectively; cooling therefore reduces resistivity, but only down to a residual value fixed by impurities and defects. In semiconductors and insulators the relationship is reversed, since heating liberates additional charge carriers and cooling removes them. Superconductivity does not fit anywhere on this scale, because it is not an extension of good conduction but a distinct state of matter that certain materials enter below a critical temperature. In that state the resistivity is exactly zero, currents persist indefinitely, and magnetic flux is expelled from the interior. For seventy-five years superconductors were known only at temperatures within a few degrees of absolute zero, requiring liquid helium; the discovery of the ceramic high-temperature superconductors in 1986, which won Bednorz and Muller the Nobel Prize the following year, raised critical temperatures far enough that liquid nitrogen became sufficient for some materials and made practical applications very much cheaper.

The general science section of an MPSC paper mixes short definitional items with simple numerical ones, and the definitional items are usually written as a fill-in-the-blank or as a one-line statement of a property, exactly as here. They are among the most efficient marks in the paper for a prepared candidate, because they can be answered in a few seconds and require no working, but they are unforgiving in a particular way: the four options are typically the members of one classification, so no option can be dismissed as absurd and there is no partial credit for knowing the general area. The preparation that suits them is a set of one-line definitions for the terms in the physics and chemistry syllabus, each fixed by the property that distinguishes it from its neighbours. For this cluster the distinguishing properties are the size of the resistivity, the direction in which it changes with temperature, and whether there is a critical temperature at all. It is also worth being unbothered by the paper's typography. This edition prints 'Super conductors' as two words, and elsewhere misspells names and breaks concord; an option should be read for the thing it names.

• Resistivity is a property of a material rather than of a specimen, equal to the resistance of a piece of unit length and unit cross-sectional area, so it can be quoted for a substance without reference to the size or shape of any particular sample.
• In a metallic conductor resistivity increases with temperature and decreases on cooling, but it settles at a small non-zero residual value set by impurities and lattice defects, and there is no temperature at which it vanishes.
• In semiconductors and insulators the relationship is reversed: resistivity falls as temperature rises, because heating liberates additional charge carriers, so cooling an insulator makes it a better insulator.
• A superconductor is a material whose resistivity falls abruptly to exactly zero below a characteristic critical or transition temperature, so that a current once established in a superconducting loop persists indefinitely without any source of emf.
• Superconductivity was discovered by Heike Kamerlingh Onnes in 1911, when mercury cooled with liquid helium lost all resistance at about 4.2 kelvin; a superconductor also expels magnetic flux from its interior, the Meissner effect, and superconducting magnets are used in MRI scanners and particle accelerators.

• Assuming that a very good conductor cooled far enough will eventually reach zero resistivity, when a metal's resistivity levels off at a residual value and only a superconducting transition takes it to zero
• Reversing the direction in which resistivity changes with temperature, which runs one way in metals and the opposite way in semiconductors and insulators
• Confusing resistance, which belongs to a particular object, with resistivity, which is a property of the material
• Choosing an escape option such as 'none of these' without positively eliminating each of the substantive choices first

General science in MPSC papers is examined through short definitional and numerical items rather than through extended reasoning, and physics contributes questions on electricity, optics, heat, motion and modern physics in roughly equal measure. Definitions of material properties — resistivity, conductivity, specific heat, latent heat, refractive index — recur, usually as a fill-in-the-blank or as a single descriptive sentence with the term removed. The commission builds the option set from one family so that the candidate must know the defining property rather than the general subject, and it often includes an escape option such as 'none of these' to punish hesitation. Superconductivity in particular appears regularly, since its definition is unambiguous and its discovery, its critical temperature and the Meissner effect provide several distinct questions from one topic.

No directly related past PYQ was found.

• practice — not a real PYQ

  Superconductivity was first observed in 1911 by Heike Kamerlingh Onnes, who found that the resistance of which of the following metals vanished at about 4.2 kelvin ?

  • (a)Mercury
  • (b)Copper
  • (c)Aluminium
  • (d)Silver

  Answer(a) Mercury — having succeeded in liquefying helium, Onnes was able to cool mercury to within a few degrees of absolute zero and found its resistance disappear abruptly at about 4.2 kelvin, for which he received the Nobel Prize in Physics in 1913. Copper and silver are excellent ordinary conductors whose resistivity falls on cooling but levels off at a residual value, and aluminium, though it does superconduct, was not the metal of the original observation.

• practice — not a real PYQ

  The expulsion of magnetic flux from the interior of a material when it passes into the superconducting state is known as which of the following ?

  • (a)The Hall effect
  • (b)The Meissner effect
  • (c)The Seebeck effect
  • (d)The Peltier effect

  Answer(b) The Meissner effect — below the critical temperature a superconductor drives magnetic flux out of its interior and behaves as a perfect diamagnet, which is what allows a magnet to float above a superconducting surface. The Hall effect is the transverse voltage that appears across a current-carrying conductor placed in a magnetic field, the Seebeck effect is the emf generated when two junctions of dissimilar metals are at different temperatures, and the Peltier effect is the heating or cooling produced at such a junction when a current passes through it.