Measuring Power, HP myths and Energy Dissipation



There is such a lot of reading material on power and energy that I was surprised to receive a mail from a reader as to why he finds it all rather unglamorous – too many factors are taken into consideration and as far as he was concerned their should be a universal standard i.e. kilowatts or horsepower,  not both – also intriguingly enough why we cannot just turn calories into joules into horsepower or kilowatt.

Energy Equations - Parts Ring

Although I appreciate that this was written tongue in cheek there is sometimes merit to what is mentioned: why all these hundreds of standards when in fact power after all is derived from energy divided by time.   Imagine someone telling you that the breakfast cereal you had just consumed is equal to one horsepower or 746 watts. Ummm, now there’s an idea. So your car delivers 100HP and your breakfast 1/4HP.  Your fat content is equal to 4HP and your dinner 1/8HP. Or your swimming pool pump is 24 000 calories, or how about your air-conditioner equal to 68dB.  All these figures and really no way to make a comparison. In fact there is a comparison which the reader did in fact mention: Power. Power is equal to Energy/Time.  The common factor here is TIME (and of course ENERGY).  What does “how long is a piece of string?” mean or “how many units in a weight?” or more to the point “How much could a wood chopper chop if a wood chopper could chop wood?”.  Because one watt is equal to one Joule per second or P=E/t and that’s about it.

I think like all technical people we are inclined to be very finicky about how technical words or terms are used in a question? When I was still studying electronics, subject being radar theory our lecturer was absolutely pedantic about how we expressed ourselves. Unfortunately this trait was carried over and I have become just as pedantic as my mentor no matter how wrong I am, either in my usage of the English language or usage of technical terms. So let’s get to the point, my own favourite: “what are you benchmarking it against?”  In other words, “my amplifier dissipates 100W” doesn’t sound right. In fact it’s downright obscene to say your amplifier dissipates 100W.  Into what? At what frequency?

Getting down to a grass root level any budding engineer will know that power dissipation or absorbtion is energy transference, which cannot be created or destroyed, plain and simple. However simple is not as simple simply is, is it? Energy has a time constraint.  You see we work in averages and to get an average in an energy calculation we need to know the beginning time and the ending time or if it’s an instantaneous value it must be as near to zero seconds as darn’it.  But near to zero as possible is also a period of time although we may give it a period of 0 seconds.  “BUT” I hear you say… in mechanics we use another way to describe power – the force acted on an on object in motion.  To be more clear, if we are looking at an electric motor we may have something akin to a spinning shaft which has “torque” and the speed the shaft rotates in radians per second. (angular velocity). Now things are becoming interesting because firstly torque is the force required to move the shaft around the axis and therefore also the force that is required to move the axis at x-radians/second.  Power in this case is Torque * Angular Velocity (measured in radians per second). Golly gosh, so now we have power which is Energy/Time and also Torque*Angular Velocity. In fact the two equate to the same thing although the one may be electrical energy and the other mechanical energy. There is a common factor again: Time. Angular velocity is a measurement of x-radians per second (the time constant).

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