Working with units

[Han]

(04-05-2017 06:21 PM)tdh79 Wrote:  Also, why won't it add 2 temperatures when they're both including the unit?

This comes up quite often. Here's an example of why adding temperatures does not make too much sense.

1°C + 1°C = 2°C

However, if we convert to an absolute scale and do:

1°C + 1°C = 274.15K + 274.15K = 548.3K = 275.15°C

Clearly 2°C is not the same as 275.15°C.

Almost all physical units have the common minimal value of 0. (For example, 0 degrees is the same as 0 radians, which is also the same as 0 gradians. And 0 meters is the same as 0 inches, or 0 feet, etc.) However, 0°C is not the same as 0°F, and neither are equal to 0K. This is why adding Kelvin temperatures is not a problem (it is an absolute scale with no temperature less than 0) but it will not work for °C or °F. The same can be said for scalar multiplication of a unit. Multiplication by scalars only make sense if there is some fixed notion of "0" -- there is no fixed "0" for the various temperature units.

That said, if one of the two temperatures is a change in temperature, then the sum makes sense. But even then, the use of mixed units still requires a lot of care on the part of the user (which can be avoided by converting to Kelvin).