Naked Science Forum
General Science => Question of the Week => Topic started by: thedoc on 07/06/2012 15:47:14
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Can you create a magnet so powerful it crushes the object it attracts?
Asked by Mark Andrew, Sheffield
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We answered this question on the show...
We posed this question to Joe Brown who works at Oxford Instruments designing superconducting magnets for use in research labs and the clinic....
Some of the work I do involves calculating the forces of attraction between the magnets and any magnetic materials in the environment. This means that the question, can we make a magnet so strong that it will crush the object it attracts is of particular interest to me. Magnetic materials in the general sense are those known as ferromagnets. The most common example of which is iron fou[img float=right]/forum/copies/RTEmagicC_Refrigerator_magnet_1.jpg.jpg[/img]nd in the steel that makes up many objects in our everyday lives.
When positioning our superconducting magnets in laboratories which often contain large quantities of steel in the building framework, laboratory furniture, and experimental equipment, we have to perform calculation of forces between the magnet and this local environment.
This calculation shows that for any magnet which we can conceive of today, the simple answer to the question is no. The magnetic forces are insufficient to crush any object it attracts. This is not to say that the forces involved are small and it is quite possible to have situations where a loose metal object such as a steel waste bin can be accidentally attracted and firmly stuck to the outside of a magnet system. In such circumstance, the bin will come off worse in the encounter, ending up severely bent. To help prevent such accidents, large magnets such as those used in MRI scanners are magnetically shielded using special configurations of the windings making up a magnet, reducing the magnetic field around it. Special consideration is also made to the position and surroundings of the magnet and you'll not find objects like steel waste bins within an MRI suite.
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Depending on what you mean by 'crush'- there was an embarrassing case I heard about where a superconducting magnet intended for a particle accelerator mechanically failed when they established the magnetic field. Magnetic pressures are quite high, about 50 tonnes per square metre (5 atmospheres), and crushing things is perfectly possible, if rare. But it's too weak to crush solid materials, it would only destroy things if they're fairly thin and holding larger, thicker magnetic objects.
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I have no doubt that a strong magnet could crush a small sheet metal container.
However, the magnetic field strength falls off with the cube of the distance. So, for example, in a wrecking yard that uses a magnet to move cars, the magnet doesn't crush the car by the strength of magnetic pull because when you would attach to the roof of the car, the frame would be several feet away, and experience a much weaker magnetic field.
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To be strictly accurate, magnetism is a 'near field'. With near fields the drop off in field with distance depends on the shape of the object generating it, although as the distance increases it does usually go as inverse cube (for dipoles), but can die away more quickly with quadrupole fields and such like.
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... you'll not find objects like steel waste bins within an MRI suite.
Maybe not in Oxford ...
Spontaneous Discharge of a Firearm in an MR Imaging Environment
http://www.ajronline.org/content/178/5/1092.full