# Is it correct to find ideal engine's efficiency using celsius scale?

I am solving a problem from my book which asks to calculate the maximum efficiency of an engine whose hot body is at temperature 300°C and cold body is at 25°C. Using 1- Tc/Th, I got efficiency equals to 0.916 but my book says that it is 0.479. Then I tried calculating this after converting the temperature into kelvins and got a correct answer. Why didn't the author used temperature in celsius?

In thermodynamics $$e=1-\frac{T_c}{T_h}$$ is the definition of absolute temperature.

Hence by definition you have to use the absolute temperature scale to obtain the efficiency.

• Definition of Abs Temperature? How? – user104909 Nov 20 '16 at 12:23
• $T_c/T_h=1-e$. So for example, if the efficiency is $0.4$, $T_c/T_h=0.6$. – velut luna Nov 20 '16 at 12:25
• OK, it happens this way but I don't understand how this expression defines absolute temperature? And what do you mean by definition of absolute temperature? – user104909 Nov 20 '16 at 12:29
• It defines absolution temperature up to a scale, which is fixed by setting the triple point of water to be $273.16$ exactly. – velut luna Nov 20 '16 at 12:31
• So tell me your definition of absolute temperature. – velut luna Nov 20 '16 at 12:34