Which of the following pairs is NOT correctly matched ?
Quantity – S. I. Unit

Question

UnlockIAS · Accepted Answer

Correct — C, Activity of radio-active substance – Curie. The SI unit of radioactivity is the BECQUEREL, symbol Bq, defined as one nuclear disintegration per second and named after Henri Becquerel, who discovered radioactivity in 1896. The curie is the older unit it replaced: it was pegged to the activity of about one gram of radium-226 and equals 3.7 x 10^10 becquerel, which is why activities quoted in curies are enormous numbers when converted. The curie survives in medicine and industry out of habit, but it is not an SI unit, and that is exactly the confusion this pair is built on. The other three pairs hold: pressure is measured in pascal and heat, being a form of energy, in joule, both SI derived units with special names; the dioptre used for the power of a lens is simply the reciprocal metre under another name, 1 D = 1 m^-1, so it is dimensionally the SI expression for lens power.

Why the other options are wrong — (a) Correctly matched, so it is not the answer. The power of a lens is the reciprocal of its focal length in metres, so its unit is the reciprocal metre, and 'dioptre' is that unit under a familiar name — a lens of focal length 0.5 m has a power of +2 D. A converging lens carries a positive power and a diverging lens a negative one, which is why spectacle prescriptions are written with signs.  (b) Correctly matched, so it is not the answer. The pascal is the SI derived unit of pressure, one newton per square metre, or kg m^-1 s^-2 in base units. Everyday pressures are usually quoted in bar, atmosphere or millimetres of mercury — none of which is SI — but the pascal is the SI unit, and it is what the question asks for.  (d) Correctly matched, so it is not the answer. Heat is energy in transit, so it takes the SI unit of energy, the joule, which is one newton-metre or kg m^2 s^-2. The calorie is the older unit still used for food energy — one calorie is about 4.18 joule — but like the curie it is a legacy unit, not the SI one.

Concept: The International System of Units builds every measurement out of seven base units and expresses everything else as derived units, some of which are given special names — pascal for pressure, joule for energy, watt for power, becquerel for radioactivity, gray for absorbed dose, sievert for equivalent dose. Alongside these sits a large family of legacy units that remain in daily use but are not part of the SI: the calorie, the bar, the atmosphere, the angstrom, the rad, the rem and the curie. Each has a fixed conversion to its SI counterpart. The three radiation quantities are the ones most often mixed up: activity is how fast a source decays (becquerel), absorbed dose is how much energy a body actually takes in (gray), and equivalent dose weights that energy by the type of radiation to give a measure of biological harm (sievert).

State PCS 2022 Q74 — Science & Technology PYQ with Answer

State PCS 2022 Q74 — Science & Technology PYQ with Answer