In the famous Thermodynamics book of Moran and SHapiro,

1 cal = 4.1868 J

Searched google:

1 cal = 4.1868 J (1860 times) 1 cal = 4.184 J (9780 times)

Which one is more accurate??

I was so surprised that there are discrepency in such a very classical and standard unit factor.

Thanks.

The issue comes from what is the definition of a "calorie".

1 cal = 4.184 J is for a "thermochemical" calorie, the definition used in the U.S.

1 cal = 4.1868 J is the calories as defined by ISO.

There are other units that have multiple definitions (pounds troy vs. pounds avoirdupois, e.g.).

Source: Engineering Unit Conversions, by M.R. Lindenburg.

If you're that concerned, perhaps stick with the ISO definition (4.1868 J).

Or just do everything in SI units to start with....

Interesting to note, I must say.

Thanks for the info.