90. Conditions required to generate electric current

In order for an electric current to exist, there must be free charged particles. If positive and negative charges are linked into neutral systems (atoms or molecules), their ordered movement will obviously not lead to electric current.

For now we will not touch upon the question of where free charges in electrolytes and gases come from (metals were already discussed in section 72). Now let us pay attention to the fact that the presence of free charges is still not enough to generate electric current. For creation and maintenance of directed movement of charged particles the force acting on them in a certain direction is necessary.

The reason for the directed motion of charged particles, i.e. the electric current in a conductor, is usually (but not always) an electric field. This field acts on charges with force \(\,\overrightarrow{F}=q\overrightarrow{E}\,\) and causes them to move. Consequently, there is an electric field inside the conductor with current. However, the presence of an electric field is due to the presence of a potential difference at the ends of the conductor.

If the potential difference at the ends of the conductor does not change over time, the current in the conductor does not change either. This kind of current is called direct electric current.

Along the DC conductor, the potential decreases from a maximum value at one end of the conductor to a minimum value at the other. This drop in potential can be detected by simple experiment.

As a conductor we take a not very dry stick and hang it horizontally. (Such a stick, although bad, still conducts current.) Let the voltage source be a car battery. To register the potential of the different parts of the conductor relative to the ground, you can use leaves of metal foil attached to the stick. We will connect one battery pole to the ground and the other to one end of our conductor. The circuit will not be closed. We will find that all the leaves will deviate at the same angle (fig. 1). So, the potential of all points of the conductor relative to the ground is the same. This is the way it should be when the charges on the conductor are balanced. If now the other end of the stick is grounded, the picture will change. (Since the earth is the conductor, the circuit becomes closed when the conductor is grounded). At the earthed end, the leaves will not deviate at all - the potential of this end of the conductor is almost equal to the potential of the earth (the drop of potential in the metal wire is small). The maximum angle of deviation will be at the end of the conductor connected to the battery (fig. 2). A decrease in the deviation angle of the leaves as they move away from the accumulator indicates a potential drop along the conductor.