Two additional example of Jim's interpretation of energy and power are: 1. The *energy* stored in the battery of an electric car. That total amount of energy stored (joules) at the start of the day, determines how far the car can be driven during that day. The speed at which the car travels is determined by the power (watts) delivered by the electric motor at any given moment. ….. 2. A similar consideration of energy and power applies for a gasoline fueled automobile. The range is determined by the stored energy, the size of the fuel tank (and the quality of the gasoline, 10% ethylene?); joules. The speed is determined by the power (watts) delivered by the engine at any particular moment.
Eugene Mechtly. On Dec 17, 2014, at 3:44 PM, James <j...@metricmethods.com<mailto:j...@metricmethods.com>> wrote: Stan, I appreciate your point that citing some number of joules used in a year is a rate of energy usage, which falls in the category of power or energy rate. But I think that one needs to be flexible and sensitive to rhetoric and context. If an author compares the energy consumed in various forms in a year's time, then most certainly the author is comparing amounts of energy and that should be measured in joules. As long as all categories cover the same time period the comparison is fair. Some of those forms of usage might be rather sporadic, such as the consumption of stove wood. I use stove wood in the cold months here. So I endeavor to enter that cold period with a certain quantity of wood, which I roughly measure by volume. It contains a certain amount of useful energy -- depending on that volume, the wood's moisture content, and the types of trees that it came from. As I am accumulating that supply of stove wood, I don't give a single thought to its rate of usage and therefore not to its output energy rate. I want to have some minimum amount of heat available, regardless of its rate of usage. So I have no heartburn with folks who would like to cite values for the amount of energy produced by each source of that energy in a set period of time and using the joule as the unit of measurement. I'm certainly smart enough to divide by the number of seconds in that period of time to view this usage in terms of rate of energy production, measured in watts. Of course, one could make that comparison in watts, as you prefer, and then multiplying by the number of seconds in that period to obtain the total amounts of energy produced in that period by each source. Presenting such data in watts invites easy comparison to other "benchmark" figures people recall, such as the power rating of their nearest power plant. Again, let the rhetoric and the context provide the guidance. The beauty of the SI is that it lets readers quickly make such calculations. It's our collection of U.S. Customary units that makes such calculations difficult. If we were stuck with those units, then it would be worth arguing about which presentation is preferable before going to the bother of publishing a comparison. But since we have the SI available, citing either annual energy production or the rate of energy production is sufficient and understandable. This dispute reminds me of arguments about the direction in which those angels dancing on the head of a pin are circling. Clockwise or counterclockwise? Hmm ... as viewed from above or below? The same can be said for claiming (incorrectly) that the SI prefers the joule over the watt second. Neither is preferred. Specially named units are for convenience and are not mandated. If you disagree with me on this, Stan, cite the clause in the SI brochure that contradicts me. Let's not discourage any budding metric fans here by bogging down into arguments of relative purity, especially those that are not substantiated by the standards. Jim On 2014-12-17 09:10, Stanislav Jakuba wrote: In due respect, Gene, there is no need for the alternative W.s as there is no need for the N.m (for work) or the kg.m2/s2. Associating any quantity with only one form of its unit represents a major advantage of SI. Let's stick with it. The joule is the only form for the unit of energy to be used in practice. To those who commented on this email, and I thank you all, you should know that the Editor "bought" the reasoning and plans the article for the January issue. Stan Jakuba PS: While on this topic, the above principle applies also to the quantity "energy consumption" that is universally expressed, wrongly, as Btu/day, MWh/year, etc. In the spirit of SI, the ONLY unit for power is the W. And because energy consumption is a flow, the unit of power (W) is correct here despite the almost universal, worldwide opposition. Imagine that "... per day, per year, per hour, per minute, per decade ..." all gone with the watt. Suddenly all statistics are comparable directly. On Mon, Dec 15, 2014 at 11:58 AM, mechtly, eugene a <mech...@illinois.edu<mailto:mech...@illinois.edu>> wrote: Stan, The “joule” of energy equals exactly one “watt.second”; the product of the watt and the second, where all reasonably literate persons have heard of the watt and the second, and might even know that the watt of power is *defined* as one joule per second for the time rate of energy processing or transfer. Try using the watt.second (Ws or W.s) in your writings, until your readers are more familiar with the joule. Gene Mechtly. On Dec 15, 2014, at 8:17 AM, Stanislav Jakuba <jakub...@gmail.com<mailto:jakub...@gmail.com>> wrote: USMA members will, I hope, be pleased reading the answer to an editor who desired that American units be presented alongside the SI values in my manuscript. Dear .... I attempted the parenthesis you suggested but found the resulting complexity of the text disruptive and harder to read. And there are other problems associated with doubling each unit. Allow me a few words of explanation. The article is about comparing two sets of numbers. For that, there need not be any unit at all. The majority of readers will skip the units anyway, and the few curious engineers and physics teachers will know how to convert to whatever units they like. As to the several common values I doubt that there are readers that wouldn't know that water freezes at 0 deg. Celsius or what a km is. As to the substitution for the one still unfamiliar unit – the joule or MJ – here Americans use several different units for energy such as calorie, Btu, kilowatt hour, lb-ft. As a result. I am at a loss which one to select. Different professions use them all – that would be four parentheses. Instead, I spelled out the symbol MJ and use it consistently thereafter. Thus no problem with comparing numbers. In any case non-technical readers will not care, and experts who might be checking the math will convert the value to whichever unit they like. I am pleased to say that, up to now, no publisher asked me to add conversions. Dozens of articles, no complains. May I say that one might underestimate one’s readers? As a side issue, you may be interested in why I insist on the units of a system that has only one unit for any measurement, be it energy, power, or length. That’s because, with the multiplicity of the U.S. energy and power units, it is common to present false or misleading numbers and get away with it for it is too difficult and bothersome for readers to look up all the conversion factors to check. I might also point out that since it is the Federal Law and Exec. Order that state that "SI metric is the preferred measurement system in the U.S." my writing in SI only should help citizens learning it. Once they see how easy comparisons are with SI units, they might actually prefer that system particularly when noticing the cheating in the daily press with American units such as the one illustrated in the other enclosed treatise. Yours, Stan Jakuba
