I came across this definition for the calorie, which I found... strange:

"The (15°) calorie is the heat required to raise the temperature of one gram of water from 14.5°C to 15.5°C."

Subsequent searches on-line revealed that this was (one of) the standard definitions for the Calorie.

Now two issues came to my mind after I read that definition:

1) Why is the term 'heat' and not 'energy' used?

From what I recall from my Thermodynamics classes at school last year, heat is energy in transit. I'm under the impression 'heat' is more of a 'surface phenomenon' if you will; there's 'heat' at the junction between two surfaces, but no matter how much you 'heat' up a body it can never contain heat (strictly speaking).

2) What's so special about the 14.5°C and the 15.5°C range?

Seriously, why that particular range? Why not 0°C to 1°C, or even 1°C to 2°C? Heck even 14°C to 15°C would go down (mildly) well with me, but 14 .5 °C to 15 .5 °C? What's with the inordinate fondness for the 0.5°C? A 1°C difference is a 1°C difference in temperature, so why specify any range at all? Why not define the calorie as 'The energy required to raise the temperature of one gram of water by 1°C' ?

I didn't have this question before because I, for all practical purposes, only used to see the calorie as being roughly equal to 4.19 Joules.

Would anyone happen to have the answer to those two queries?


I've already seen this particular question, and I believe it is necessary to state that this does NOT answer my question. My question is distinct from it in the following respects:

1) I'm asking for a clarification regarding the use of the term 'heat' over energy.

2) I want to know why a generalization of the definition, replacing the arbitrary temperature range with a '1° difference' wouldn't suffice.

3) I want to know why the range is demarcated by two fractions (14.5° and 15.5°) instead of integers.


A number of the Chemistry texts I've used or examined for adoption have indeed confused heat and thermal energy. They sometime refer to a body possessing heat. But Physics books are pretty unaminous that heat is the transfer of thermal energy and not a state property of matter.

I've never seen, in texts I've used, heat restricted strictly to the interface between the participants in the exchange. It refers to the total amount of thermal energy transferred between two bodies. It can also be used to describe how much energy was lost by one of the bodies in the exchange and how much was gained by the other. One way to reconcile this with your interface statement is that once the surface of a body warms due to contact with a hotter body, the higher temperature at the surface relative to the rest of the body will continue to cause heat to flow within the body.

As to the choice of temperature, it shouldn't near zero since this is where a phase change occurs.

Edit. In response to your edit, #2. Search for a table of "specific heat of water". Notice the specific heat is not constant as temperature varies. I'll hazard a guess on the 14.5-15.5 part. Think 15 degrees. But, since an interval is required, let's begin a half degree before and end an equal amount after. As to why 15 degrees, hmmm. One answer to a recent question on the Physics forum says this is roughly room temperature. Pretty cold for me but maybe true when and where this was defined. (Sorry for no link, I'm not that fascile on iPad.)

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  • $\begingroup$ So basically heat is a macroscopic phenomenon while energy is a microscopic phenomenon. $\endgroup$ – DHMO Oct 9 '16 at 16:23
  • $\begingroup$ Heat relies on the exchange of energy via particle collisions. Internal energy, of which one aspect is thermal, relies on the motion and relative positions of particles with respect to one another within a body. I think it would be fair so say both exhibit facets of microscopic and macroscopic properties. Also, although it's particle motion and position involved in internal energy, there is also energy associated with the bulk motion or position of the body itself. $\endgroup$ – bpedit Oct 9 '16 at 17:00

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