MOTION MOUNTAIN

liquid electricity, invisible fields and maximum speed 37
magnet
diamagnetic
material
magnet
paramagnetic
material
F I G U R E 15 The two basic
types of magnetic material
behaviour (tested in an
inhomogeneous field):
diamagnetism and
paramagnetism.
Ref. 19
gives the direction of the magnetic field, and the density of the lines gives the magnitude
of the field.
Many systems in nature are magnets, as shown in Figure 13.The existence of twomagneticpolesis
valid for all magnets in nature: molecules, atoms and elementary particles
are either dipoles or non-magnetic.
⊳ Therearenomagneticmonopoles.
Magneticfieldlinescouldstartorendatamagneticmonopole–if oneexisted. Despite
thepromise of eternal fame, no magnetic monopolehas ever been found. The searches
are summarized in Table 7.
Magnets have a second important property, shown in Figure 15: magnets, through
their magnetic field, transform non-magneticmaterialsintomagneticones.Thereisthus
a magnetic polarization,similar to the electric polarization. The amount of polarization
dependsonthematerial; some values are given in Table 9.
— Certain materials, the so-called diamagnetic materials, are repelled by magnets,
thoughusually onlybyweakforces.
— Others,theso-calledparamagneticmaterials,areattracted tomagnets.
— Some important materials, the ferromagnetic materials, such as steel, retain the induced
magnetic polarization: they become permanently magnetized. This happens
when the atoms in the material get aligned by an external magnet. Ferromagnetic
materialsareusedtoproducepermanentmagnets–thusartificial lodestone.
Magneticmaterialsareessentialfortheindustrialproductionofelectriccurrentandare
partofmostdevices that use electricity.
How do animals feel magnetic fields?
“Anyfoolcan askmore questionsthan seven
sagescan answer.
Antiquity”
It is known that honey bees, sharks, pigeons, the sandhill crane, various other birds, salmon,trout,seaturtles,dolphinsandcertainbacteriacanfeelmagneticfields.
Onespeaks
oftheabilityformagnetoreception.Alltheselifeformsusethisabilityfornavigation.The
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net