Answer to Question #17900 in Electricity and Magnetism for aawaz

Question

ok..heres one problem....i am inside a train,,the train is moving..and there is one electron inside a train ..and i am looking at that electron..in my perspective the electron is not moving and (as my high school teacher told) only moving charged particle creates magnetic field...so i will not see any magnetic field..but suppose my freind is looking that electron sitting outside the train..then he will see a magnetic field is created...then here is a problem...what is happenning in reality..is there a magnetic field or not,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,

Answer 1

There are two origins of magnetism: movement of charged particles and quantum properties of some particles. The magnetic field of moving charged particles like electrons is a relativistic effect (mathematically it is explained by Einstein's theory). If particle moves relatively to observer, observe detect magnetic field. In your example your friend will see it and you not. Such magnetic fields create conductors, electromagnets etc. Magnetism as inner nature of matter is linked with so-called property of elementary particles as spin. It can be imagined as vector of some lenght. Briefly says, if spin of a given partilce is nonzero than there is a magnetic field around it. Protons and electrons have spin length 1/2 (often scientists say "spin" without "length"). Some other particles or groups can have 1, 3/2, 2 etc. Direction of this magnetic field is defined by spin direction. When many electrons in a bulk medium have the same direction of their spins, such field can be easily detected: such phenomenon happens in permanent magnets.