In other types of materials such as wood or ceramic, the atoms' electrons have very little freedom to move around. The different freedom of electrons makes different capability of material to transfer electrons.

Materials with high electron mobility (many free electrons) are called conductors, while materials with low electron mobility (few or no free electrons) are called insulators. Conductivity of all materials is not the same. For instance, aluminum is better than dirty water. 

Physical dimension also impacts conductivity. This is analogous to water flow in a pipe: a fat pipe offers easier passage than a skinny pipe, and a short pipe is easier for water to move through than a long pipe, all other dimensions being equal. Besides, some materials experience changes in their electrical properties under different conditions, such as when exposed to different temperatures.

Remember that electrons can flow only when they have the opportunity to move in the space between the atoms of a material. This means that there can be electric current only where there exists a continuous path of conductive material providing a conduit for electrons to travel through.

When a length of copper wire is connected to positive and negative source, such as a dry cell battery, the following process occurs. An electron (-) is forced out of orbit and attracted to the positive (+) end of the battery. The atom is now positive (+) charged because it now has a deficiency of electrons (-). The atom in turn attracts an electron from a neighbor. The neighbor in turn receives an electron from the next atom, and so on until the last copper atom receives an electron from the negative end of the battery.

The result of this chain reaction is that the electrons move through the conductor from the negative end to the positive end of the battery. The flow of electrons will continue as long as the positive charges and negative charges from the battery are maintained at each end of the conductor.

It is interesting to note that no "wear" occurs within wires due to this electric current, unlike water carrying pipes which are eventually corroded and worn by prolonged flows. Electrons do encounter some degree of friction as they move, however, and this friction can generate heat in a conductor.