為什麼世上大部分物質都具有電磁力?

It is simply because matters are made up of atoms (and molecules). Atoms compose of charged particles, like electrons and protons.

Electromagnetic force thus arises because of the charges carried by these particles (electrostatic force), and their motions (orbital or spinning motions which give rise to magnetic forces).

2010-03-20 23:21:19 補充：
In fact, most of the contact forces we observe in the macroscopic world, like friction, impact force...etc., are electromagnetic in nature.