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Natural Science Forum / Physics / General Physics / July 2008Galvanometer Deflection
There's a problem in my physics books relating to galvanometers that I have a query about. Basically it runs like this: A fixed magnetic field strength, B, and current, 50 mA, cause a full- scale reading in a galvanometer. If the field strength is decreased to 80% of the original value, what current is now required to cause a full scale reading? Surely the current must increase to compensate for the reduced magnetic field strength? I get the answer 62.5 mA, but the book says 40 mA. Any help appreciated. Cheers, Anon > There's a problem in my physics books relating to galvanometers that I > have a query about. Basically it runs like this: [quoted text clipped - 13 lines] > > Anon 20% change in B, book says 20% change in amps. seems OK thought there could be a squared term, guess not. >> There's a problem in my physics books relating to galvanometers that I >> have a query about. Basically it runs like this: [quoted text clipped - 18 lines] > > thought there could be a squared term, guess not. I think he questions why the current required decreases rather than increases. I thought a galvanometer had a coil, a permanant magnet and a spring. When a current flows the coils repels the magnet against the spring and moving the pointer. If the strength of the magnet is reduced wouldn't it take more current to overpower the same spring? > I think he questions why the current required decreases rather than > increases. [quoted text clipped - 3 lines] > the pointer. If the strength of the magnet is reduced wouldn't it take > more current to overpower the same spring? In fact isn't the torque T = BAIN? So weaker B means less torque. More current needed to compensate? > > I think he questions why the current required decreases rather than >> increases. [quoted text clipped - 7 lines] > > So weaker B means less torque. More current needed to compensate? Yes, current should be more, not less | There's a problem in my physics books relating to galvanometers that I | have a query about. Basically it runs like this: | | A fixed magnetic field strength, B, and current, 50 mA, cause a full- | scale reading in a galvanometer. Yeah... and the read/write arm in your hard drive. Take an old one apart and have a look (don't expect it to work afterwards, though.) http://techluver.com/wp-content/uploads/2007/10/hard-drive.jpg We've come a long way since Galvani but the principle hasn't changed. | If the field strength is decreased to | 80% of the original value, what current is now required to cause a | full scale reading? Take the magnet out altogether and then try it. | Surely the current must increase to compensate for the reduced | magnetic field strength? I get the answer 62.5 mA, but the book says | 40 mA. Well, its a physics book, right? You don't expect it to be correct, do you? | Any help appreciated. The best help is to get some hands-on experience. Don't you have any toy trains or old VCRs with motors in them? There is plenty of junk around you can experiment with and it doesn't matter if you wreck it, it was junk anyway. You are not going to get much help out of a book some schoolteacher wrote. > There's a problem in my physics books relating to galvanometers that I > have a query about. Basically it runs like this: [quoted text clipped - 13 lines] > > Anon You are correct. The book is wrong. Deflection of a galvanomenter is proportonal to the product of the magnetic field (B) and the current flowing though the coil. Harry C. I expect ALL physics books to have 100% correct answers. And then I woke up. Thanks. Nice to know I'm not going (magnetic coil) loopy. > I expect ALL physics books to have 100% correct answers. And then I > woke up. > > Thanks. Nice to know I'm not going (magnetic coil) loopy. Trust me that you are not going 'loopy'! Some of these error to answeres in the problem set are intentional, designed to create discussion in the class session. In more advance physics, errors in the published solutions for the problem sets are often more due to accidental errors.In advanced physics courses like Theoretical Mechanics, it was not uncommon to find 10% or more of the published probem set solution to be incorrect. Please don't blame the author of your textbook for this. The textbook author generally creates the problem sets, but does not compute a solution. He relies on his senior students or TA's to solve and publish the solutions, and it's been my personal observation that the author of the text rarely checks them. Take heart, when you advance in physics, the large lectures vanish, and the majority of discussion relates to the solution of assigned problem sets at a classroom level. Trust me, any mistake you made in a solution will be highlighter in magic marker, and displayed to the entire class via on overhead projector. Your mistake will be ridiculed by your professor or instructor,and the class will laugh nervously realizing they are next. Then too, it is a documented fact that you will never likely again make that same error. In many ways it is comparable to Marine Corps training, although far more expensive. Some readers will immediately grasp this post, while other readers will never get it. Harry C, >> I expect ALL physics books to have 100% correct answers. And then I >> woke up. [quoted text clipped - 32 lines] > > Harry C, :-) Thanks Harry -- I must say I certainly am pleased that you are taking time to post here! -Sam Thanks Harry. I'd never thought about the possibility that the errors are intentional, but it makes sense - keeps you on your toes. But recently I went back over all the physics problems I'd been doing and could convincingly tear through (with reasoning) many solutions in the book that disagreed with my own. Heck, who's to say their math is better than mine! I like the idea of thoroughly going over problems in a class discussion, like you mention. When it's participatory, things stick better. Cheers, Anon |
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