Calculating magnetic field as a function of magnet size

AI Thread Summary
The discussion focuses on determining the size of a magnet needed to achieve a specific magnetic field at a certain distance from its surface. An N48 neodymium magnet has a Bremanence of approximately 1.47T, and the relationship between the magnet's physical size and the magnetic field strength is explored. It is noted that the field strength diminishes as the distance increases, following a d^-3 relationship, which is specific to the axis of the magnet. The formula provided estimates the magnetic field at a distance based on the height of the magnet and its surface field strength. Understanding the influence of magnet shape on field distribution is also highlighted as an important consideration.
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I want to determine how large a magnet I need in order to get a given field a certain distance away from the surface.

An N48 neodymium magnet has a Bremanence of ~1.47T. How does the physical size of the magnet affect the field a given distance away?

https://www.physicsforums.com/showthread.php?t=519563 seems to imply it's proportional to the volume. However, I'm unclear on how to apply the equations. I'm also surprised that no mention is given to the shape -- shouldn't a flat, thin magnet have its magnetic field fall off more slowly than a long, deep one?
 
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That formula you refer to is for the field of a magnetic dipole and ONLY on the axis of the dipole. There, d is the distance from the dipole, it has nothing to do with the volume. In other words, the field of a static dipole decays as the reciprocal of the cube of the distance.

As an estimation to your problem, I am assuming that the (manufacturer?) specified field of 1.47 T is specified at the surface of the end of the magnet. Then, as long as you stay on the axis of the magnet, the field at a distance d away from the surface of the magnet is
B(d) = \frac{(\frac{H}{2})^3}{(d+(\frac{H}{2}))^3} \cdot 1.47 [\rm{T}] \; ,
where H is the height of the magnet.
 
Thanks! How did you derive that?
 
No problem; it's just the only way to enforce the d^-3 relation with the field referenced to the surface. Look up "magnetic dipole" for the derivation of the equation from the other thread.
 
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