Electric Current
The movement of charges is always associated with the movement of electrons, protons, ionized atoms or ionized molecules as these entities are carriers of electric charges. There is no such thing as an electric charge "by itself", just the charge and nothing else.
The most common electric current is the one due to the motion of electrons. Such a current is delivered to our houses by means of copper wires and is "produced" by batteries used in many types of equipment.
For now we are not interested in the "type" of electric current - the motion of charges creates a current, that's all what we need to know right now. The motion of these charges is caused by a potential difference which creates electric field E and this field exerts force on the charges.
Charges move with a constant average speed (they do not accelerate) because the force exerted by the electric field is compensated by different interactions between charges and their surrounding.
The direction of the current is defined as the direction in which positive charges would be moving,
regardless of the fact that most common examples of current are due to the motion of electrons, that is negative charges.
The mechanism of the flow of current in conducting materials like metals is as follows.
In metals there are tremendous numbers of so called "free electrons". These are electrons which left atoms and move randomly in the space between ions (ionized atoms) which create the crystal lattice, something like a cage in which electrons can freely move, but cannot leave. This motion is random in nature, and the speed of electrons is very high, of the order of 106 m/s. This is equal 3.6 millions kilometers per hour.
The speed of this chaotic motion of electrons depends on the temperature of metal. The higher the temperature, the higher this speed. As the motion is chaotic there is no net flow of charge in any direction. If we consider a copper wire, the average number of electrons moving to the left from an imaginary surface of the cross section of this wire is equal to the average number moving to the right. Such a situation exists without any electric potential difference along this wire.
Now we connect the ends of the wire to the poles of a battery. A potential difference will be created along the wire. This potential difference forces electrons to drift in the direction from the - to the + poles of the battery. We call this motion a drift as it is superimposed on the chaotic motion mentioned above. The average speed of this drift, which is responsible for the flow of the current, is very slow, of the order of 10-5 m/s (equal to 0.036 km/h). This sounds strange if we recall everyday experience with using an electric current. We press the switch and practically at the same moment the equipment starts working. To explain this let's consider a small hose filled with honey. The flow of honey through the hose, if we position it at a small angle to the horizontal level, is very slow. But, if we push the honey at one end of the hose, the far end moves almost instantly. Analogically, electrons in wire move very slowly, but the energy is transferred very fast.
The main parameter which characterizes electric current, as a physical phenomena, is the amount of charge ΔQ flowing per unit of time Δt. It is called electric current intensity or simply electric current and abbreviated I. The mathematical definition of electric current is
The SI unit for electric current is called Ampere and is defined as