# B-Field of Permanent Magnets

## Main Question or Discussion Point

I have what I thought should be a relatively simple problem, but a quick trip around the Google makes me wonder. It might be easier to just ask some real humans for once...

I'm working with Hall Effect sensors and neodymium (NIB) magnets. I basically know the strength of my magnets, and the sensitivity range of my Hall Effect Sensors, but I wanted to know if there was a way to calculate how close the magnet had to be to the Hall Effect Sensor to make it come on.

In other words, I know that my magnets are rated at about 2,000 Gauss (I thought Gauss was a unit depending on distance, though, so if I'm right about that, I don't know what distance these are measured at), and I know that my Hall Effect Sensors should activate in the presence of a 5 Gauss B-field. I've noticed that I have to get my magnets really close to the sensors to get them to come on.

I was wondering how I could calculate (no need for a lot of precision, just a good estimate within a cm or two) how close I need to bring the magnet to the Hall Effect sensor to get it to come on.

I've seen a similar question involving a bar magnet, but I'm not sure if it applied. For clarification, the Hall Effect sensors we have are polarized, so I'm bringing the magnet closer to the sensor with the proper side facing it.

Thank you to anyone who takes the time to read this. Even the smallest advice will be appreciated.

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Neodymium magnets are quite strong (if properly magnetized). The field intensity decreases with distance from the magnet, but 5 Gauss is a small field. Therefore, your Hall sensor should perform well at several centimeters from the magnet. This makes me think there's something wrong with the sensor.

Sorry for being unclear earlier.

The Hall Effect sensor is picking it up at a few cm away, but we're going to try choosing new Hall Effect Sensors (for a different reason), and as I look up their sensitivities, I need to know how close our magnets will need to be for those different sensitivities.

Thanks so much for the reply!

The field of a long bar magnet is approximately given by a formula that resembles Coulomb's law (although there are no magnetic charges). Try googling it (I can't remember the exact form)

Don't mean to de-rail your thread, but when dealing with magnets aren't you dealing with an H-field and not a B-field?

I always though B referred to magnetic induction.

Both H and B are present in every magnetic problem. However, Hall sensors give an output proportional to B.
As for the names, I've always considered they're confusing. Thus I always refer to B or H explicitly. The word induction has been overused. First in electrostatics, later with Faraday's law and, in this context, with B.
No wonder students get confused.

According to the manufacturers' data sheets on all of the Hall Effect Sensors I'm looking at, it's the B field. I myself never remember which one is which, just the equations relating the two. My EMag Engineering Prof had a good way of telling them apart, but I've forgotten it now.

I'll check out the concept you mentioned Gordianus. Is the length of my magnet going to factor in? I know looking at the magnetic field lines, it seems that if it's a shorter magnet, both sides of the magnet are more likely to effect my sensor if the magnet is shorter, as opposed to longer.

I mentioned a long bar magnet only because the field B can be computed with the aid of an approximate formula. However, both long and short magnets can be used with Hall sensors. The closest "pole" always dominates.

I think I have it covered now.

I found this link, which has the B-field equation partially derived, based on Coulomb's Law, like you mentioned, Gordianus.

I'm posting it here so if anyone searches the same question, they'll get this information as well, and not have to post about it.

http://instruct.tri-c.edu/fgram/web/mdipole.htm [Broken]

Thanks to everyone who replied!

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