The law of conservation of electric charges
Every material in the universe is composed of atoms. Atoms are composed of protons, neutrons, and electrons. Proton is positive electrically charged; electron is negative electrically charged, neutrons are not electrically charged. Inside the atom, there are some electrons, protons, and neutrons. If the number of electrons is equal to the number of protons, the total electrical charge on the atom is zero. Atoms like this are electrically neutral. If the number of electrons is more than the number of protons, then the atom becomes negative electrically charged. If the number of electrons is less than the number of protons, then the atom becomes positively electrically charged.
Each material is composed of atoms, so if the atoms making up an object have electrons as many as the proton, then the object is not charged or neutral. Conversely, if the atoms have more electrons than protons, then the object is negatively charged. Likewise, if the atoms have fewer electrons than protons, then the object is positively charged.
Many objects found in everyday life are electrically neutral. However, these objects can be changed to an electrically charged object. Changing things that are neutral to electricity charged can be done through friction or by induction. In the friction method, objects that to be electrically charged are contacted with other objects, on the contrary in the induction method, objects that to be electrically charged do not in contact but are only closer to other objects that are electrically charged.
A plastic that is initially neutral becomes electrically charged after rubbing with dry hair. Proof that the plastic has been electrically charged are two plastic rods that have previously been rubbed with dry hair, repels each other. Both plastic rods repel each other because they have like charges. As explained in the topic of electric charge, according to the agreement based on the advice of Benyamin Franklin (1706-1790), the electrical charge held by plastic is determined as a negative charge. The plastic is negatively charged so that the overall number of electrons in plastic is more than the number of protons. At neutral plastic, it can be concluded that the excess of electrons possessed by plastic now comes from hair. Electrons in the hair move to plastic during friction between plastic and hair. Hair initially neutral so that when the electrons move to plastic, the hair lack electrons and excess protons. The plastic becomes negatively charged, and the hair becomes positively charged.
There is no charge lost during friction; the charge is not created during friction, what happens is the transfer of charge. Plastic becomes negatively charged, and the hair becomes positively charged.
The law of conservation of electric charge states that in a closed system, electric charges are not created or destroyed but only move from one object to another. The net amount of electric charge produced in any process is zero.
When an object becomes positively charged, at the same time another object becomes negatively charged, where each object has the same net charges. The closed system in the statement above is illustrated as follows. For example, when plastic and hair rub each other, it is considered that the transfer of electric charge only occurs just between plastic and hair, does not involve air.