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My teachers (also the internet) tells me that Kelvin can never be negative. But when I converted $-275^\circ$C to Kelvin, it becomes $-1.85\,\rm K$. So, why everyone says that Kelvin can never be negative?

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You've converted correctly, but the problem is that -275°C is colder than absolute zero, -273.15°C. Temperatures colder than this are not possible.

The conversion formula is just a maths formula, if you plug in a number it will spit out another number. The formula doesn't check that what you've entered is possible.

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  • They tell you correctly that a Kelvin value can't be negative, $T>0 \,\mathrm K$.

  • Also, they should have told you that a degrees Celsius value can't be below $T>-273.15 \,^\circ\mathrm C$.

Sure, you can make up a number below this value, such as $-275\,^\circ \mathrm C$. Just like you can make up a number such as $-10\,\mathrm K$. These numbers just don't represent reality. A lower temperature limit is a feature of the world we live in. The above two values for the lower limit are identical - called absolute zero - just given in different unit systems.

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