68 Chapter 4 • Heat


A rise of 1 °C is the same as a rise of 1 K. A comparison of the two scales shown in fig. 4.1, illustrates this.


Degrees Celsius scale


120 °C 100 °C 80 °C 60 °C 40 °C 20 °C 0 °C


–20 °C –40 °C –60 °C –80 °C


–100 °C –120 °C –140 °C –160 °C –180 °C –200 °C –220 °C –240 °C –260 °C –273 °C


Kelvin scale


393 K


373 K (Boiling point of water) 353 K 333 K 313 K 293 K


273 K (Freezing point of water) 253 K 233 K 213 K 193 K 173 K 153 K 133 K 113 K 93 K 73 K 53 K 33 K 13 K


0 K (Absolute zero)


Fig. 4.1 Celsius and Kelvin scales Fig. 4.1 Celsius and Kelvin scales


4.1.4 Scale conversion


It was seen that: 0 K = –273 °C (absolute zero)


273 K = 0 °C 373 K = 100 °C (freezing point of water) (boiling point of water at sea level)


From this, a general conversion formula can be deduced: t = T – 273 °C ........................................................................................................ (4.1)


or T = t + 273 kelvin ................................................................................................... (4.2) where t = temperature in °C T = thermodynamic temperature in K 273 = a constant


7 °C 6 °C 5 °C 4 °C 3 °C 2 °C 1 °C 0 °C


–1 °C –2 °C –3 °C –4 °C –5 °C –6 °C –7 °C –8 °C


280 K 279 K 278 K 277 K 276 K 275 K 274 K 273 K 272 K 271 K 270 K 269 K 268 K 67 K


266 K 265 K


Portion of scale enlarged


Degrees Celsius


Kelvin scale scale


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