Magnets on the work bench
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From: Rich Grise on Google groups on 11 Feb 2010 15:49 On Feb 10, 6:08 pm, "Michael A. Terrell" <mike.terr...(a)earthlink.net> wrote: > Rich Grise on Google groups wrote: > > > I don't know if it's still possible to buy a nonmagnetic screwdriver, > > since beryllium has > > been declared toxic by our lord protectors.[1] > > They were Beryllium Copper. > > <http://en.wikipedia.org/wiki/Beryllium_copper> Hey, thanks for this! Now, is there any chart or anything that shows the relative "strengths" of magnetic fields? Like, Jan Panteltje says, "these are not very strong, specified as about 1.2 Tesla IIRC..." and Chris says, "Since when was 1.2 T 'not very strong'?!" So, how strong is "strong?" Is there some kind of way to relate Teslas to Gauss to whatever other measures of flux density or whatever that can be related to physical stuff, like "A 1 <unit> magnet can support one pound of plain ol' iron" or something that could be related to something I could hold in my hands? What other measures of magnetic force are there? Teslas, Gausses, Ampere-turns, Other? (and how to they relate to physical reality?) Thanks, Rich
From: Spehro Pefhany on 11 Feb 2010 16:04 On Thu, 11 Feb 2010 12:49:53 -0800, Fred Abse <excretatauris(a)invalid.invalid> wrote: >On Thu, 11 Feb 2010 09:39:11 -0500, Spehro Pefhany wrote: > >> On Thu, 11 Feb 2010 01:31:27 -0800, Fred Abse >> <excretatauris(a)invalid.invalid> wrote: >> >>>On Wed, 10 Feb 2010 20:20:37 -0800, Greegor wrote: >>> >>>> Aren't most stainless machine screws made >>>> from alloys of stainless with ferrous steel in them? >>> >>>Name a non-ferrous steel ;-) >> >> Perhaps he means "ferritic". ;-) >> >> The 400-series (ferritic) stainlesses are quite ferromagnetic- you can >> pick up big chunks with an electromagnet. > >Yup. That's what they make nuts, bolts and screws out of. Actually, they're usually 316 or 18-8 if they're stainless.
From: Tim Williams on 11 Feb 2010 17:54 "Rich Grise on Google groups" <richardgrise(a)yahoo.com> wrote in message news:ce079cb4-8170-4d1c-8e82-a5e9e5e3ad02(a)k5g2000pra.googlegroups.com... > So, how strong is "strong?" Well, ferromagnetics saturate in the 1-1.8T range (note: ferrimagnetics (i.e. ferrite) saturate lower, 0.2-0.5T). > Is there some kind of way to relate Teslas to Gauss 1T = 10,000 G > to whatever other measures of flux density or whatever T = Wb/m^2, literally the flux density. Flux (Phi) in Wb = B*A (flux density times area). Wb == V*s, which is interesting because it means you cannot have flux without having put some voltage into it at some point. And that's exactly what happens, you apply volts to an inductor to increase current, V = L*dI/dt. > that can be related to physical stuff, like "A 1 <unit> magnet can > support one pound of plain ol' iron" Maxwell stress sigma_m = B^2/(2*mu_0). In SI units, T^2 / (H / m) = V^2 s^2 m A / (m^4 V s) = V A s / m^3 = J / m^3, an energy density. But: J / m^3 = N / m^2 = Pa, pressure. If you integrate the stress over all space, you'll get the total energy stored in that magnetic field. Or if you integrate it over a cross section, you get the force on that section. If you imagine the flux lines coming out of the N pole of a magnet, looping around and returning to S, these lines carry a mechanical tension equal to this pressure, while expanding perpendicular to the lines with identical pressure. Thus, if you have a uniform 1T magnetic field from top to bottom through a 1 meter cube, there is a force of F = A*sigma_m = 1m^2 * 1T^2/(4*pi x 10^-7 H/m) = 795kN pulling the top and bottom together, and forcing the sides apart (assuming the surfaces are interacting magnetically). That's a lot of force, which is why big magnets are made with even bigger steel. A 1cm cubic neodymium magnet with 1T on each end experiences a net force of 0, but if you stick two together, you get 1T from each magnet at the ends and 2T where they add in the middle (assuming they are actually a lot longer than 1cm, and assuming the material is linear, which certainly isn't; the internal field may not get much above 1.5T or so). The apparent attractive force is then around (10^-4 m^2) / (4*pi x 10^-7) * (1.5T^2 - 1T^2) = 99N, or about 22 lbs (lbf, I should say). Instead of sticking magnets together, you'll get a similar answer for a sufficiently wide and thick hunk of steel, because the magnetic field from one pole is sucked into the iron, loops around and runs back through the air around the magnet, with very little fringing through the back side of the plate. The B field at the surface of the iron is then around 1T, and nearly 0 at the back side. Thin steel or small pieces will act more like extensions of the magnet, > What other measures of magnetic force are there? Teslas, Gausses, > Ampere-turns, Other? (and how to they relate to physical reality?) Amp turns is magnetomotive force. Whereas a battery or generator supplies electromotive force to an electronic circuit, At supplies MMF to a magnetic circuit. Flux replaces current, and reluctance replaces resistance. A mechanical analogy would actually be more appropriate, because reluctance obviously doesn't consume power as resistance does. Magnetization is amps per meter (or At/m to be practical and specific), and is related to flux density as B = mu_0 * H. Because mu_0 is teensy, it takes a *lot* of magnetization to make 1T in air. Very permeable materials (mu_r > 100) are a lot easier to magnetize; this is usually used to 'cheat' because, instead of brute-force magnetizing air, you magnetize a big stinking core and leave a little air gap to play around in. The required magnetization is much more reasonable, maybe 100 to 1000At. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: life imitates life on 11 Feb 2010 20:50 On Thu, 11 Feb 2010 09:50:45 -0600, "Tim Williams" <tmoranwms(a)charter.net> wrote: >"Spehro Pefhany" <speffSNIP(a)interlogDOTyou.knowwhat> wrote in message >news:jl58n5ldh993a68e3hb7jr83usr874a5be(a)4ax.com... >> If you feel like experimenting, it would be interesting to see if a >> BFM (big fat magnet) would kill an energized SMPS wall wart. > >Hmm, depending on orientation, it might enhance efficiency. > >Tim Standing field. Zero effect on a transformer, as well as the rest of the supply circuit, UNLESS you are moving it OVER the transformer placing the transformer inside the field loop. Still won't introduce much into it, since the transformer is a closed loop as well. Typical net effect: Zero. All orientations.
From: life imitates life on 11 Feb 2010 20:53
On Thu, 11 Feb 2010 16:35:10 GMT, Jan Panteltje <pNaonStpealmtje(a)yahoo.com> wrote: >On a sunny day (Thu, 11 Feb 2010 09:50:45 -0600) it happened "Tim Williams" ><tmoranwms(a)charter.net> wrote in <hl190l$8ft$1(a)news.eternal-september.org>: > >>"Spehro Pefhany" <speffSNIP(a)interlogDOTyou.knowwhat> wrote in message >>news:jl58n5ldh993a68e3hb7jr83usr874a5be(a)4ax.com... >>> If you feel like experimenting, it would be interesting to see if a >>> BFM (big fat magnet) would kill an energized SMPS wall wart. >> >>Hmm, depending on orientation, it might enhance efficiency. >> >>Tim > >Core saturation would _enhance_ efficiency? A standing external field will not "saturate" the core. Saturation would be a DC standing field produced from within the transformer's closed magnetic loop. In other words, from one of the windings. |