All is about electricity. It is something that we cannot see with our own eyes, but is easy to perceive through its effects.
Which is the typical media for conducting electricity?
A cable of course! For example copper is a conductive material, a good one.
A very basic definition of electric current can be "moving electric charges", so we need electric charges able to move. The subatomic particle responsible for electricity is the electron.
All the materials have electrons, because materials are made of atoms, and electrons are part of the atoms. This does not mean that they are all conductive materials. The condition is that they must have free electrons.
Most metals have free electrons, particularly gold, silver, copper and aluminum have free electrons that can be easily moved, that's why they are so good conductors. This means that if we apply an electric field (for example by connecting a battery), the electrons will actually travel, and we will have an electric current into the cable.
The electron is too small to be considered for practical purposes. The unit for electric charge is the Coulomb, which represents millions of millions of millions electrons, and still is a small electrical charge.
We can now give a quantitative definition of electric current intensity:
Where Q is electric charge (in Coulombs), and t is time (in seconds). Thus, this simply means that current (or intensity) represents how many Coulombs by second are circulating for a particular point.
The unit of measurement for current intensity is the Ampere (or Amp). For thousandths of an Ampere, the "m" (milli) prefix is used, for example: 0.20 A = 200mA and 0.035 A = 35mA. For millionths of Amps, the "μ" (micro) prefix is used, for example: 0.000058 A = 58μA and 0.120 mA = 120μA.
In the other end of conductors, we can find the insulating materials, like cable's PVC protection, porcelain, dry wood, dry air. They are just the opposite, their electrons are so firmly attached to the atoms that they can be exposed to certain voltage levels and stay still, avoiding the flow of electric current.
But remember that nothing is absolute, they have a limit which is called breakdown voltage, and depends on each material.
And in an intermediate point between conductive and insulating materials, we have the resistive materials. For example graphite, iron, and certain synthetic materials, which offer some resistance to the current flow.
On the next chapter we will learn how resistive materials are used to give life to the most basic electronic component, which is the resistor.
We have presented conductive, insulating and resistive materials as different things. Again, these classifications are not absolute. For example, we said that copper is a good conductor, what means that it has very very low resistance, but not nule. So tthere is no a definite line between one type and other, but a gradient from "good conductor" to "isolator" passing by "resistive material".