____________ are materials whose resistivities become zero below a critical temperature.

Question

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (a) 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.  (b) 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.  (d) 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.

Concept: 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.

State PCS 2023 Q36 — Science & Technology PYQ with Answer

State PCS 2023 Q36 — Science & Technology PYQ with Answer