Matter is made by atoms. The atom has a nucleus and one or more electrons. In the nucleus there are protons and neutrons. Protons have positive electrical charge while electrons have negative electrical charge. Neutrons are neutral, they don’t have any electrical charge.
Image: Components of an lithium atom: nucleus, protons, neutrons and electrons
Depending on the chemical element, every atom contains at least one or more electrons. Because usually the number of electrons are equal with the number of protons, atoms are electrically neutral.
If there is a physical contact between two bodies, electrons can move from one body to another. The body which will lose electrons will become positively charged, the body which gains electrons will become negatively charged.
For example, if we rub a glass rod with a piece of silk cloth, electrons from the glass rod will move onto the silk cloth. The glass rod will become positively charged because will lose some electrons and the silk cloth will become negatively charged because it will gain electrons.
If we bring together two bodies with the same type of charge (positive or negative) they will repel each other. On the other hand if we bring together two bodies with opposite charge, one positively charged and the second negatively charged, they will attract each other.
Image: Positive and negative charge
In terms of magnitude the charge of an electron is equal with the charge of a proton, but they are opposite in sign, the electron has negative charge and the proton has positive charge.
The total electrical charge of any matter, body is a multiple of a basic quantity of charge named elementary charge. In physics electrical charge is represented with the letter q. The elementary charge has the symbol qe and has the magnitude of:
\[ \begin{equation} \begin{split}\bbox[#FFFF9D]{|q_e| = 1.60 \cdot 10^{-19} C}
\end{split} \end{equation} \]
qe is the charge of a single electron or proton.
The SI unit of electrical charge is the Coulomb (C). 1 Coulomb is defined as the amount of charge needed for each of two point-like objects, separated at a distance of 1 m, such that an attraction/repulsion force of 9.0 · 109 N occurs between them.
The number of protons needed to have a total charge of 1.0 C is:
\[1.0 C \cdot \frac{1 \text{ proton}}{1.6 \cdot 10^{-19} C} = 6.25 \cdot 10^{18} \text{ protons}\]The same number of electrons is needed to have a combined charge of -1.0 C.
There are some elements that allows electrical charge to move through them relatively easy. These elements are named conductors. Some electrons in metals are not bound to the atom and can move freely. These are named free electrons. Since metals (e.g. iron, copper, aluminium, etc.) allow free movement of electrons they are conductors.
On the other hand, insulators do not allow free movement of electrons which means that they can not carry electrical charge. Example of insulators: glass, plastic, paper, etc.
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