Let's suppose we have temperatures
30°C and 35°C. Converting them to Kelvin we have 303.15K and 308.15K.
In the second case, the temperature difference is 5K. While in first case, temperature difference is 5°C, which is convertible to 278.15K.
This clearly is absurd. I have some ideas why it goes wrong, but could someone please provide a clear explanation? Cannot we use conversions on difference?
This seems to be a general problem with units which have additive relationship (Kelvin and Celsius) rather than a multiplicative one(metre -centimetres).
Best Answer
Celsius and Kelvin are two scales that differs only for an additive factor, but the single increment corresponds to the same temperature difference. In other words, an object become "hotter" in the same way if you rise its temperature by 1K or 1°C.
You can use conversion formula in differences, just make sure you use it for both terms and keep in mind that in these cases you are referring to a temperature difference.
To make things more clear, consider the conversion to the Fahrenheit scale: 35°C - 30°C = 95°F - 86°F = 9°F which is telling you that the same temperature drop is represented by either 5°C or 9°F.
If that helps you, think about the potential gravitational energy on the Earth surface. That quantity is defined by choosing an arbitrary zero point, and this is exactly the case of Kelvin and Celsius scales. (Farenheit scale, instead, include a scale value).