Physics · Thermal Properties Of Matter · NEET
The Kelvin scale starts at absolute zero, the lowest possible temperature, which is -273.15 degrees Celsius. So 0 K is the same as -273.15 C. Both scales grow at the same speed (one Kelvin equals one degree Celsius in size), so you only need to shift the starting point. Adding 273.15 moves you from the Celsius zero (melting ice) up to where the Kelvin count begins. For NEET numericals, 273 is usually accepted unless the question needs 273.15.
Two things differ between Celsius and Fahrenheit: the size of one degree and the zero point. Between freezing and boiling of water, Celsius has 100 divisions (0 to 100) but Fahrenheit has 180 divisions (32 to 212). So one Celsius degree equals 180/100 = 9/5 Fahrenheit degrees; that gives the 9/5 factor. The +32 is added because Fahrenheit freezing point is 32, not 0. First stretch the scale by 9/5, then shift by 32.
Yes. The Celsius and Kelvin scales have the same degree size; only their zero points differ. So a temperature difference or a temperature rise of 20 C is exactly a rise of 20 K. This is why formulas like Q = m c delta T give the same numerical answer whether delta T is in Celsius or Kelvin. But Celsius and Fahrenheit differences are NOT equal: a change of 1 C equals a change of 1.8 F.
Set C = F in the formula F = (9/5)C + 32. Then C = (9/5)C + 32, so C - (9/5)C = 32, which gives -(4/5)C = 32, so C = -40. So -40 C equals -40 F. This single point is a favourite NEET and quiz question; remember minus forty as the meeting point.
By modern SI rules, Kelvin is an absolute (thermodynamic) scale, so we write 300 K, not 300 degree K. The degree symbol is used for Celsius (25 C is read as twenty-five degree Celsius) and Fahrenheit. Kelvin uses the plain word kelvin and the symbol K only. Writing 'degree K' is marked wrong.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Celsius (C), Fahrenheit (F) and Kelvin (K). Celsius and Kelvin are used in science; Kelvin is the SI unit of temperature. Fahrenheit is common in daily life in some countries.
F = (9/5)C + 32, C = (5/9)(F - 32), K = C + 273.15, and C = K - 273.15. To go from Fahrenheit to Kelvin, first find C, then add 273.15.
Freezing point: 0 C, 32 F, 273.15 K. Boiling point: 100 C, 212 F, 373.15 K. Between these, Celsius has 100 parts, Fahrenheit has 180 parts, Kelvin has 100 parts.
F = (9/5)(37) + 32 = 66.6 + 32 = 98.6 F. That is the normal human body temperature. In Kelvin it is 37 + 273.15 = 310.15 K.
Use Kelvin for gas laws, Stefan-Boltzmann law and any formula with absolute temperature. For a temperature change (delta T) you may keep Celsius, since the change is the same as in Kelvin.
