How many VW Bugs = 1 coulomb?
May 1, 2010 4:01 AM Subscribe
PhysicsTeacherFilter: Where can I find a good reference for various units and what they practically mean. (As in, 2mA feels like a slight shock, 30mA feels like an extremely painful shock, 75mA will kill you, ~10A in a toaster, 30kA in lightning, etc)
I'm TAing some physics classes, and it would be nice to come armed with this sort of information for any units one might encounter. Any good references on that front? If my students find that their small electric motor is using 20 megawatts or 20 microwatts of power, I want them to know that both of those answers are very likely wrong.
I'm TAing some physics classes, and it would be nice to come armed with this sort of information for any units one might encounter. Any good references on that front? If my students find that their small electric motor is using 20 megawatts or 20 microwatts of power, I want them to know that both of those answers are very likely wrong.
Wikipedia has a whole series of orders of magnitude: power, mass, voltage, charge, energy, speed, and so on (they're all listed at the infobox at the bottom of each page.)
posted by Rhomboid at 4:58 AM on May 1, 2010 [1 favorite]
posted by Rhomboid at 4:58 AM on May 1, 2010 [1 favorite]
Someone's going to mention this, so it might as well be me.
posted by baf at 5:51 AM on May 1, 2010 [3 favorites]
posted by baf at 5:51 AM on May 1, 2010 [3 favorites]
(As in, 2mA feels like a slight shock, 30mA feels like an extremely painful shock, 75mA will kill you, ~10A in a toaster, 30kA in lightning, etc)
Volts hurt, amps kill. You can take a car battery, capable of delivering hundreds of amps, and touch the posts, and feel nothing. Touch a 9v battery to your tongue and you can feel it. So the idea of "practical knowledge of amps by feel" is a bad analogy at best, deadly at worst.
A better analogy is water- pressure versus flow. The ocean has lots of flow capability (amps), but low pressure (volts) unless you dig a really deep hole. If you open a fire hydrant, you get a lot of water. If you place a cap over it and drill a tiny hole in it, you get very little water. Same pressure, less flow. Same volts, less amps drawn.
I would recommend doing practical demomstrations. Get motors of known power, some ammeters and some different batteries that can provide different amperages. Construct an aparatus that shows what happens when the motors try to lift different weights with different sources. Including hooking a 12V motor to a 9v battery and seeing what the amperage draw difference is.
(Or, less expensively, just coils of wire and how hot they get when shorted to batteries. You'll kill a lot of batteries, but I think your students will benefit.)
I don't think there is a practical way to "know" physics without actually seeing it in action. Nothing beats simple experiments to illustrate the abstract concepts.
(Funny story- in high school, they were unprepared for a state law change that required a third year of science. So they had to use a teacher that wasn't quite ready for primetime. He foolishly tried to "wing" a problem solving exercise where he made up the size and mass of a ball that was thrown over the edge of a cliff. Unfortunately for him, he did it in a way that made the math more complicated than the students knew. So, when we did the math as we were taught, we ended up with null set as the answer. Apparently, the ball disappeared soon after it was thrown...)
posted by gjc at 6:33 AM on May 1, 2010 [1 favorite]
Volts hurt, amps kill. You can take a car battery, capable of delivering hundreds of amps, and touch the posts, and feel nothing. Touch a 9v battery to your tongue and you can feel it. So the idea of "practical knowledge of amps by feel" is a bad analogy at best, deadly at worst.
A better analogy is water- pressure versus flow. The ocean has lots of flow capability (amps), but low pressure (volts) unless you dig a really deep hole. If you open a fire hydrant, you get a lot of water. If you place a cap over it and drill a tiny hole in it, you get very little water. Same pressure, less flow. Same volts, less amps drawn.
I would recommend doing practical demomstrations. Get motors of known power, some ammeters and some different batteries that can provide different amperages. Construct an aparatus that shows what happens when the motors try to lift different weights with different sources. Including hooking a 12V motor to a 9v battery and seeing what the amperage draw difference is.
(Or, less expensively, just coils of wire and how hot they get when shorted to batteries. You'll kill a lot of batteries, but I think your students will benefit.)
I don't think there is a practical way to "know" physics without actually seeing it in action. Nothing beats simple experiments to illustrate the abstract concepts.
(Funny story- in high school, they were unprepared for a state law change that required a third year of science. So they had to use a teacher that wasn't quite ready for primetime. He foolishly tried to "wing" a problem solving exercise where he made up the size and mass of a ball that was thrown over the edge of a cliff. Unfortunately for him, he did it in a way that made the math more complicated than the students knew. So, when we did the math as we were taught, we ended up with null set as the answer. Apparently, the ball disappeared soon after it was thrown...)
posted by gjc at 6:33 AM on May 1, 2010 [1 favorite]
For electricity:
http://www.osha.gov/SLTC/etools/construction/electrical_incidents/eleccurrent.html
posted by Maxwell_Smart at 8:32 AM on May 1, 2010
http://www.osha.gov/SLTC/etools/construction/electrical_incidents/eleccurrent.html
posted by Maxwell_Smart at 8:32 AM on May 1, 2010
Ok, let's try to make a link this time:
http://www.osha.gov/SLTC/etools/construction/electrical_incidents/eleccurrent.html
posted by Maxwell_Smart at 8:33 AM on May 1, 2010
http://www.osha.gov/SLTC/etools/construction/electrical_incidents/eleccurrent.html
posted by Maxwell_Smart at 8:33 AM on May 1, 2010
Infochimps has a concise cheat-sheet of Scale Landmarks of the Universe.
posted by James Scott-Brown at 10:17 AM on May 2, 2010
posted by James Scott-Brown at 10:17 AM on May 2, 2010
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posted by infini at 4:05 AM on May 1, 2010