Units and Constants

Although many different units are employed in energy work, whenever possible we shall adopt the Systeme International (SI). This means joules and watts. If we are talking about large energies, we'll speak of MJ, GJ, TJ, PJ, and EJ—that is, 106, 109, 1012, 1015, and 1018 joules, respectively, (See Table 1.1).

One might wish for greater consistency in the choice of names and symbols of the different prefixes adopted by the SI. The symbols for submultiplier prefixes are all in lowercase letters, and it would make sense if the multipliers were all in uppercase letters, which they are not. All symbols are single letters, except the one for "deca" which has two letters ("da"). Perhaps that explains why deciliters are popular and decaliters are extremely rare. Unlike the rest of the multipliers, "deca," "hecta," and "kilo" start with lowercase letters. The names of the prefixes are derived mostly from Greek or Latin with some severe corruptions, but there are also Danish words and one "Spanish" word—"pico"—which is not listed in most Spanish dictionaries. Some prefixes allude to the power of 1000 of the multiplier—"exa" (meaning six), for instance, refers to 10006: others allude to the multiplier itself—"kilo" (meaning one thousand) indicates the multiplier directly.

We cannot entirely resist tradition. Most of the time we will express pressures in pascals, but we will occasionally use atmospheres because most of the existing data are based on the latter. Sometimes electronvolts are more convenient than joules. Also, expressing energy in barrels of oil or kWh may better convey the idea of cost. On the whole, however, we shall avoid quads, BTUs, calories, and other non-SI units. The reason for this choice is threefold: SI units are easier to use, they have been adopted by most countries, and they are frequently better defined.

Consider, for instance, the calorie, a unit preferred by chemists. Does one mean the international steam table calorie (4.18674 J)? or the mean calorie (4.19002 J)? or the thermochemical calorie (4.18400 J)? or the calorie measured at 15 C (4.18580 J)? or at 20 C (4.18190 J)?

Americans like to use the BTU, but, again, there are numerous BTUs: steam table, mean, thermochemical, at 39 F, at 60 F. The ratio of the BTU to the calorie of the same species is about 251.956, with some variations in the sixth significant figure. Remember that 1 BTU is roughly equal to 1 kJ, whereas 1quad equals roughly 1EJ. The conversion factors between

Table 1.1 SI Prefixes and Symbols

Multiplier

Symbol

Prefix

Etymology

1024

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