Abstract :
In engineering, accurate measurement is a way of life - that is if you want to remain employed. But there are many times when estimations based on rules of thumb are extremely efficient and can be quite accurate. Most adults have had experience with common blue-tip kitchen matches somewhere in their childhood. Some of these experiences got us into trouble, but thatʹs another story. So we as engineers have an experience-based feel for the amount of heat released when we strike up one of these (now hard to find) matches. But do you have a feel for a British thermal unit (BTU)? Probably not. Oh, you can do the math, but as a quick estimate, how many BTUs of thermal energy are released when you burn a single blue-tip match? Or how many joules of energy are released when you jam that first match back into the box? You have a feel for the heat through past experience, but can you relate that to the measure? Another easy thumb applies to the preferred standard international (SI) energy unit, the joule (J). One J is equal to 1 W delivered over 1 s, about the amount of energy required to raise Newtonʹs apple back up to a height of 1 m. One thousand Js is equal to 1 BTU, or put another way, 1 kj is equal to one match, and 1 MJ is the energy released from that match you jammed into the matchbox during your youth. In theory, that box of matches could boil your 1/2 1 pot dry, but in real world appliances, it will take the energy equivalent of one box to make an eight-cup pot of coffee within a reasonable amount of time. Be careful when transitioning from theory to practice. Summarizing these thumbs: 1 J = 1 W for 1 s is the energy required to raise an apple 1 m; 1,000 J = 1 BTU = one wooden match; 1 MJ = one box of wooden matches or one, eight-cup pot of coffee (in the real world).
