Why do we use formula of magnetic fields for low speed charges? I have read a answer on the site about how moving charges create magnetic fields, which included special relativity. But in low speeds occasions the effects of relativity should be incrediblely small, why we still have to consider it? Since we don't consider the effect of relativity for moving mass point moving with low speed, why consider it for magnetic fields?
I am just a sophomore student and my mother tongue is not English, i would be extremely thankful if you can use simple words.  :D
 A: 
But in low speeds occasions the effects of relativity should be incrediblely small, why we still have to consider it?

The effects of relativity are very small but the effects of electromagnetism are huge! It takes only a very small excess of charge to have an enormous electrostatic effect. So although the relativistic effects are small they are not negligible for electromagnetism. They are essentially scaled up by the strength of the electromagnetic force which is roughly $10^{36}$ times stronger than gravity. 
A: 1.Special Relativity doesn't fully explain the relation between electric fields and magnetic fields. It explains some occasions.
2.Relativistic effects are neglible but the electrostatic force is just very strong with little effects we can see consequences.
3.No mass and charge are not even close to similar. Charge is something fundamental (at low energies at least), mass can be converted to energy and vice-versa.
A: 
These laws can explain the magnetic fields very well, but the nature for magnetics isn't relativity? If not, what is it?

This question - which you asked in the comments - is the key to understanding magnetism. The fact that electrons (as well as protons and neutrons) have a magnetic moment was expressed in 1907 and thus long after Maxwell's equations. At the time when the magnetic dipole moment of electrons was discovered, Bohr's atomic model of the electron orbiting the nucleus was still in use, and scientists explained the magnetic moment by a rotating electron.
Today, the magnetic dipole moment of subatomic particles is defined as an intrinsic property. So if we accept the magnetic dipole as a property that exists under all circumstances, it is clear why the magnetic field has to be taken into account even for low speed electrons.
