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Fridge magnets and the LHC?!



Extended list of materials and tips:

  1. Two fridge magnets (we got ours here)
  2. A magnetic viewing film (we found ours here)

Delving a little deeper into the physics:

Fridge magnets only “stick” to the fridge on one side, not on the other. [1,3,4] This is because the magnetic field they create is very different from the one of simple dipole magnets, such as horseshoe or bar magnets. One can visualize this with a magnetic viewing film. Such films contain suspensions of metal particles which react to the magnetic field. Areas where the film becomes brighter indicate a strong gradient in the magnetic field – regions of space where the field is changing abruptly. In our case, instead of having only one North pole and one South pole, these fridge magnets consist of strips with alternating polarity. We call them multipolar magnets. This setup, called a Halbach array, generates a magnetic field on one side which is two times as strong as it would be if not arranged in this particular way, and on the other almost zero. [2] Multipolar magnets can be used to perform many different advanced tasks. This idea is used in the CERN accelerators: while traditional dipole magnets bend the particle beam so the protons travel in a loop around the circular collider, quadrupole magnets focus the beam to counteract the fact that the electrically charged protons naturally tend to repel each other. If there were no quadrupole magnets the beam would diverge very quickly. Magnets with an even greater number of poles (sextupole, octupole and decapole magnets) are used to perform finer corrections on the beam. [6,7,8]

Links for further information:

  • [1] Jean-Michel Courty & Edouard Kierlik, Les magnets, aimants à un seul pôle ?, Pour la Science n°391 (2010). Investigating the properties of fridge magnets (in French).
  • [2] K&J Magnetics, Inc., Halbach Arrays, kjmagnetics.com. Field schematics and explanations about Halbach arrays. 
  • [3] The McMoran Lab, Mysteries of fridge magnets, YouTube  (2020). Intro to fridge magnets.
  • [4] Steve Mould, Fridge magnet trick (2016). Intro to fridge magnets.
  • [5] Smarter Every Day, Mind-blowing magic magnets, YouTube (2016). “Smart” magnets, ie. magnets with engineered pole design used to perform specific tasks. 
  • [6] Xabier Cid Vidal & Ramon Cid, Magnetic dipoles & magnetic multipoles, lhc-closer.es. Magnets section of an outreach website about the LHC.
  • [7] Thomas Hortala, Why LHC magnets are blue – and other colorful accelerator questions answered, home.cern (2021)
  • [8] Gijs de Rijk, Introduction to Magnets (2019). Lecture slides from a presentation about CERN magnets.