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Registered Member #1512
Joined: Fri May 30 2008, 01:16AM
Location:
Posts: 4
Hey everybody,
Working on a science fair project, and I'm trying to measure the magnetic fields of some magnets. I tried building Rick Hoadley's simple Gaussmeter, but it's not sensitive enough for the measurements I need to take. Anyway, I'm trying to figure out if there's a simple way to measure magnetic fields, or at least estimate. Does anyone know if placing two equal size, equal field strength magnets end-to-end cause the magnetic field to double?
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
It depends where you measure the field.
If putting two magnets together causes the length of the path that "field lines" have to traverse to increase, like putting them end to end, then the doubling of mmf you get with two magnets is compensated by the increase of path length, so the H field (mmf/path) doesn't double, so neither does the B field.
It gets worse than that however, as the magnetic material is subjetc to saturation, and the increased field in each magnet may change the permeability a lot.
If you put a small permanent magnet on the 0" mark of a ruler, another on the 12" mark, and measure the field at 6", then you have a geometry where removing one of the magnets will halve the field. The reason being that each magnet doesn't "see" much of the field of the other, so saturation effects don't come into play. The path length for each magnet doesn't change, but the mmf doubles (or goes to zero) when the other magnet is moved in or away.
If you use electromagnets, they aren't subject to permaebility changes with changing field.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
MagLev wrote ...
Does anyone know if placing two equal size, equal field strength magnets end-to-end cause the magnetic field to double?
The two magnets may be close, but do not occupy exactly the same space, so the combined field cannot be exactly doubled at any particular point of measurement.
But if we are to consider the simple and practical, if one magnet could lift 1kg, then two magnets working together would lift 2kg, near as makes not much difference.
The most simple type of magnetometer is, of course, the compass, and I would think you could do your demonstration showing the degree of a deflection of a compass needle caused by your different combinations of magnets. For best measurement results, keep the compass as far away from the magnets as you can whilst still getting a usable deflection.
If you are a chap who enjoys calculation and measurement, you should be able to calculate the field strength of your magnets at a given distance by the degree of compass deviation they cause away from the Earth's ambient field, (about 600 milligauss).
The entire foundations of modern science were made with such simple equipment!
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Do magnetic fields of permanent magnets add linearly?
