Charging an object through friction is called triboelectric charging. A glass rod rubbed with silk carries a positive charge; a rubber rod rubbed with fur carries a negative charge.
Matter is composed of molecules, and molecules are composed of atoms. Normally, because atoms are neutral (the positive charge of the atomic nucleus is equal to the total negative charge of the electrons outside the nucleus), an object is also electrically neutral. Different substances have different binding forces on their atomic nuclei to their electrons. When two objects rub against each other, the object with the weaker binding force will transfer some electrons to the other object. The object that loses electrons becomes positively charged due to the lack of electrons, while the object that gains electrons becomes negatively charged with an equal amount of excess electrons.
