# Magnetic field strength dependent on distance from source

by babemagnetics
Tags: dependent, distance, field, magnetic, source, strength
 Mentor P: 11,614 See here for the magnetic field along the axis of a finite solenoid: http://www.phys.uri.edu/~gerhard/PHY204/tsl215.pdf This assumes the solenoid has no iron or other magnetic material inside it.
 P: 2 Thank you all for the help on this little project! I have to apologize about the delay for my thanks. School became overwhelming so I had to put this on hold. I reviewed the info you all provided, along with a great text a professor lent me. While not one of those individual sources described exactly my situation, I was able to synthesize it all into a decent model to describe my data. Thanks again!
 P: 6 Hi everyone, I am trying to use the 1/r^3 formula to calculate the decrease in magnetic field strength over distance, but I must be doing something very, very wrong following this formula because the numbers I get don't make sense to me. I have clearly made a mistake applying the formula and I am wondering if someone could tell me where I have gone wrong. I am trying to calculate how a field of 0.005 Tesla will decrease at a distance of 0.0005 meters away. I have done the following calculation 0.005T/0.0005^3 meters, which gives me the number 0.005/0.0005^3 = 40,000,000 Obviously I have gone horribly wrong, and I have a feeling I am making a very basic mistake somewhere with either the calculations and/or the understanding of the formula. If any kind-hearted charitable soul out there wants to tell me how this poor fool has gone wrong, I would be very grateful!
PF Gold
P: 937
 Quote by downtownjapan I am trying to calculate how a field of 0.005 Tesla will decrease at a distance of 0.0005 meters away.
Away from where? You need to sort out from where your distances are measured… We need to know the set-up.
Mentor
P: 11,589
 Quote by downtownjapan Hi everyone, I am trying to use the 1/r^3 formula to calculate the decrease in magnetic field strength over distance, but I must be doing something very, very wrong following this formula because the numbers I get don't make sense to me. I have clearly made a mistake applying the formula and I am wondering if someone could tell me where I have gone wrong. I am trying to calculate how a field of 0.005 Tesla will decrease at a distance of 0.0005 meters away. I have done the following calculation 0.005T/0.0005^3 meters, which gives me the number 0.005/0.0005^3 = 40,000,000 Obviously I have gone horribly wrong, and I have a feeling I am making a very basic mistake somewhere with either the calculations and/or the understanding of the formula. If any kind-hearted charitable soul out there wants to tell me how this poor fool has gone wrong, I would be very grateful!
A simple way to see why your answer is wrong: work with units. Your calculation gives 40000000 T/m3 - this has the wrong units for a magnetic field strength. And the numeric value depends on the length units you use.

Those 40000000T would be the magnetic field at this distance if you had a point-like source that has a field of 0.005 T in a distance of 1m. Such a source is completely unrealistic.