Is it possible to build a spherical magnet? If we could manage to do it, where would the north and south poles be, and what could such a magnet be used for?
鈥� The simple answer to this question is yes. Just grind the corners off a normal bar magnet until it is spherical. The north and south pole field lines would emerge from opposite points on the sphere.
The more intriguing question is whether a monopole can be created 鈥� the magnetic equivalent of a particle with either positive or negative electric charge. I鈥檓 not aware of a physical law that prevents their existence but, despite a long search, no real example has been found.
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A collection of particles all of the same pole could have many applications. Because they would magnetically repel each other, if the particles were small enough that the repulsion was stronger than their weight, they would behave in a fluid-like manner. Such fluidised particles have many applications in chemical processing because of their heat and mass transfer properties.
Normally, this state is created by passing a fluid up through a bed of particles to keep them in suspension 鈥� air is blown up through coal particles to burn them in some power plants, for example. Fluidising particles without a large upwards flow would reduce the energy required and also reduce losses due to attrition because the collisions between particles would be less violent.
This might enable applications that were previously impossible. In particular, fine particles (say less than 30 micrometres) are notoriously hard to fluidise because surface forces tend to make them stick together. However, such particles are very useful because of their large surface area to mass ratio. So if you find a way to make monopoles, I strongly suggest you patent it.
Simon Iveson, Chemical Engineering Faculty, The University of Newcastle, Callaghan, New South Wales, Australia
鈥� Your correspondent asks if it is possible to build a spherical magnet. The answer is yes 鈥� we all live on one: Earth.
Where would the poles be? The North Pole is where polar bears live and the South Pole is where penguins live.
What鈥檚 it used for? I suppose the magnetic field鈥檚 most important role is protecting the planet from the solar wind, which would otherwise strip away our atmosphere meaning there would be no 快猫短视频, and nowhere to print this answer.
Perry Bebbington, Kimberley, Nottinghamshire, UK
鈥� The first spherical magnets were made in the 16th century or earlier by grinding naturally mined magnets, called lodestones, into spheres. These were called 鈥渢errellas鈥� (little Earths) by , the physician to Queen Elizabeth I.
Gilbert spent much of his life investigating these natural magnets and plotted the first diagram of the magnetic field of a dipole, although he had no concept of field lines. He presciently believed that Earth itself was a giant lodestone with a weathered surface. His book De Magnete was published in 1600.
Rod Wilson, Liverpool, UK
Thanks to all who wrote in to tell us that small, magnetic marbles covered in brightly coloured plastic can actually be bought as toys 鈥� Ed