A permanent direct electric current

1 GENERAL CHARACTERISTICS OF THE MAGNETIC FIELD 5.1.1 A permanent (direct) electric current 

An electric current can run in a substance in which free charges (current carriers) are present. They can be either electrons (in metals, for example) or ions (in liqnids or solid electrolytes). Such substances are referred to as the conductors of an electric current. However, if a conductor is brought into an external electrostatic field there will be only an instant displacement of charges according to electrostatic laws this will lead to the creation of an internal electrostatic field in a conductor directed opposite to the external electrostatic field and numerically equal to it the current instantly stops. Therefore, inside a conductor the electric field is always zero. This means that additional conditions to support the current flow are necessary. We shall consider these a Uttle later, but first we shall introduce some ideas about the electric current. 

The ordered motion of electric charges is referred to as an electric current (a current of conductivity). The scalar value I determined by a total charge dQ having run through a cross-section of the conductor in a unit time interval dt, i.e.. 

Taking into account the possible movement of both positive and negative charges (for instance in an electrolyte) the total current density is 

Another characteristic of a current is the mobility of the current carrier b, which is the average speed acquired by the current carrier in a field of unit electric field strength. If charges have average speed u in a field E then, by definition, their mobility is h = (,v /E. The mobility b can be expressed through the specific electroconductivity p and carrier concentration n. As the current density is j = nq vy and j = aE, therefore, cfE = nq( vy. Having divided both parts of equality by E we shall obtain a = nqb or 

---