If x is the temperature of a system in Kelvin and y is the temperature of the system in °C, then the correct relation between them is
- A.x = 273 - y
- B.x = 273 + y
- C.x = 173 + y
- D.x = 173 - y
Correct answer
Explanation
The correct answer is B, x = 273 + y. A temperature in kelvin is the Celsius temperature plus 273 (more exactly 273.15).
The Kelvin and Celsius scales have degrees of the same size, so a change of 1 °C equals a change of 1 K; only their zero points differ. The Kelvin scale starts at absolute zero, the lowest possible temperature, which is −273.15 °C. So water freezes at 0 °C = 273 K and boils at 100 °C = 373 K. The kelvin is the SI unit of temperature and is written without the degree sign. On this scale no temperature can be negative.
A is wrong: x = 273 − y would make temperatures in kelvin fall as Celsius temperatures rise, which is impossible.
C is wrong: the offset between the scales is 273, not 173.
D is wrong: both the number and the sign are wrong.
Exam tip: K = °C + 273; absolute zero = 0 K = −273.15 °C; −40 °C = −40 °F; human body ≈ 37 °C ≈ 310 K.
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