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# how far can an electromagnetic field reach? Answered

For my project I need to know how far electromagnetic fields can reach. I am guessing its related to power and materials, but perhaps there is some kind of rough formula for calculating potential reach? By reach, I mean, how far can the magnetic field reach and attract (move) something, like metal fillings. Also, is there anyway to know how quickly a magnetic field can move something. In other words what speed will metal fillings move at when the electromagnet is switched on? Thanks all, Joel (the snow athlete)

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For monopoles, 1/r2. For dipoles, 1/r3, but with a theta dependence.

If you don't know what that means, then do some research. Wikipedia is a reasonable basic resource for physics information. If you or your teacher doesn't like Wikipedia, then a basic freshman physics textbook will give you more information than you need.

thanks for the answers. I looked those laws up on Wikipedia, but its way up there above my head. For my project all I really need is some way to calculate roughly how many millimeters an electromagnet running on a 12 volt battery can reach to pick up a pile of metal fillings. Is there some more basic way i can work this out without lots of equations i dont understand?

It's one of those things where the theory doesn't help you. Experiment and find out. If you want to use math' - ask how to calculate the field strength of electromagnets - it's not much use for answering the question, but it should help to build the magnet. L

:-) I get your point. I think your project may have too many variables to calculate from first principles: how many windings? What current? What's the permeability of the iron alloy you're using? and so on.... A better method for you might be just empirical. Make your magnet, and hook it up to a smaller (1.5V) battery. Slowly bring a little pile of filings in from "infinity" (a couple of feet away), and measure where they start to be attracted. Do the same thing with two 1.5V batteries. The second case will reach farther than the first. Now, the magnet strength is just proportional to the current in the wire (Lenz's Law), and the current in the wire is proportional to the voltage (Ohm's law). So with those two results, you can estimate the behaviour at 12V. Technically, you just need the first point, but seeing that the proportionality works is a good sanity check.

Wups, missed the part about how quickly. EM propagates at the speed of light (by definition), so for anything you can possibly look at it's effectively instantaneous (one nanosecond per foot). If you're talking about "dragging metal filings with a magnet," then you're dominated by friction both among the filings and between them and whatever surface you put them on.