My copy of "Resnick and Halliday" states the following:

"Before scientists realized that heat is transferred energy, heat was measured in terms of its ability to raise the temperature of water. Thus, the calorie(cal) was defined as the amount of heat that would raise the temperature of 1g water from 14.5°C to 15.5°C."

This definition seems to account for the fact that heat really is energy in transit so why was this definition changed? Exactly what is so inherently wrong with defining heat in this manner? I'm afraid that I may have misunderstood the subtle distinction between heat and energy, if there is one.

Please share your knowledge and help me. Much thanks in advance :) Regards.

  • $\begingroup$ While this is not an answer and still very incomplete, you might get some enlightenment by looking at inexact differentials $\endgroup$ – mikuszefski Sep 7 '16 at 7:04
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    $\begingroup$ @JohnForkosh: Yes, I realize this. I have already studied this chapter several times before but missed some subtleties and so, I am catching up now; I'm well aware of the bigger picture :) $\endgroup$ – user106570 Sep 7 '16 at 7:40
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    $\begingroup$ BTW, I am learning all these concepts on my own; I have no teachers/friends to discuss even small doubts with and so, I am bombarding this website with questions. $\endgroup$ – user106570 Sep 7 '16 at 8:48

You are right, heat really is energy, and the calorie is a unit of energy. However, the definition you gave is not up to current standards of defining units. As far as I know, there are two reasons: First is that the definition is not unambiguous, and the other is that nowadays SI units are the way to go when it comes to science and official measurements, and in SI, the unit of energy, Joule, is defined in a more straightforward way based on other "base units".

First, the specific heat capacity of water changes by temperature and pressure, so the amount of heat (=energy) needed for a temperature change of 1 Celsius varies. The Wikipedia article lists quite a few different definitions for the calorie, where the temperature endpoints are 3.5 °C...4.5 °C, 14.5°C...15.5 °C, 19.5 °C...20.5 °C, and one where the definition is one hundredth of the energy needed to warm form 0 °C to 100 °C. All of these give slightly different amounts of energy. Also the definition you quoted doesn't specify the pressure at all, so even that doesn't give a single fixed value of the calorie.

The second reason is that the international system of units is nowadays used for units used in science. The idea is that there are a few (as few as possible) base units that are defined in terms of things occurring in the nature, and the rest are defined in terms of the base units. The unit of energy (=heat) is Joule whose definition is simply $\text{kg} \text{ m}^2/ \text{s}^2$, where kilogram, metre, and second are defined in a certain way.

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