CDS (I) 2020 General Knowledge · Question 24 of 120

Correct — B, x = 273 + y. The Kelvin and Celsius scales are graded in steps of the same size — a rise of one kelvin and a rise of one degree Celsius are the same amount of warming — but their zeros sit in different places. Absolute zero, 0 K, is −273.15 °C, so the Kelvin reading is always the Celsius reading shifted up by 273.15, rounded in school work to 273. Test it on a familiar pair: water freezes at 0 °C, which is 273 K, and boils at 100 °C, which is 373 K. Both fit x = 273 + y and none of the other three options survive either check.

• (a)x = 273 − y — Subtracting would make the Kelvin temperature fall as the Celsius temperature rises. At 100 °C it gives 173 K, colder than freezing point, which is impossible.
• (c)x = 173 + y — The right operation with the wrong offset. It would put absolute zero at −173 °C and the freezing point of water at 173 K.
• (d)x = 173 − y — Wrong on both counts — wrong offset and wrong sign. It fails the freezing-point check at once.

The kelvin is the SI base unit of temperature and the Kelvin scale is absolute, starting at the lowest temperature there is. Because the kelvin and the degree Celsius are the same size, converting between them is a shift and never a stretch: add 273.15 to go from Celsius to kelvin, subtract it to come back. The Fahrenheit scale is different in kind, since its degree is a different size, so converting to it needs both a multiplication and a shift.

Two of the four options carry the wrong offset and two carry the wrong sign, so a single substitution kills three of them. The habit worth building is to test any conversion formula on a value you already know rather than trying to recall which way the sign goes — the freezing point of water is the cheapest such test, since 0 °C must give 273 K. Watch the difference in wording between a temperature and a temperature change, because a rise of 30 °C is a rise of 30 K with no shift at all; the 273 only enters when converting a reading. Note also that the paper writes 273 rather than 273.15, which is the usual school rounding.

• Absolute zero is 0 K, equal to −273.15 °C.
• A rise of one kelvin and a rise of one degree Celsius are the same amount of temperature change.
• Water freezes at 0 °C or 273 K and boils at 100 °C or 373 K under standard pressure.
• Lord Kelvin proposed the scale in the nineteenth century; the kelvin joined the SI in 1954.
• The 2019 revision of the SI defines the kelvin by fixing the Boltzmann constant at 1.380649 × 10⁻²³ joules per kelvin.

• Reversing the sign and subtracting the Celsius reading from 273.
• Adding 273 to a temperature difference; a change of 30 °C is simply a change of 30 K.
• Writing the unit as degrees kelvin — the kelvin takes no degree sign.

As the conversion relation itself, as a numerical conversion, or as the value of absolute zero in Celsius.

• practice — not a real PYQ

  The temperature of a body rises from 300 K to 330 K. The rise expressed in degrees Celsius is

  • (a)30 °C
  • (b)57 °C
  • (c)273 °C
  • (d)303 °C

  Answer(a) 30 °C — the two scales share a step size, so a difference is numerically the same in both.

• practice — not a real PYQ

  The normal boiling point of water on the Kelvin scale is

  • (a)100 K
  • (b)273 K
  • (c)373 K
  • (d)473 K

  Answer(c) 373 K — 100 °C plus 273.