Electric Current and Current Carriers

Electric Current

Definition: Current is the rate of flow of electric charge

When the charge is expressed in coulombs and time in seconds, current is in amperes (A).

The following should be noted:

1. The dimensions of the quantities in Equation 1.1.1 are:

Quantities on the RHS cancel out between numerators and denominators, leaving C/s. It is helpful to check the dimensions of all defining relations so as to gain a better appreciation of the quantities involved.

FIGURE 1.1.1
The flow of electric charges in a conducting medium.

2. Unlike charge, which has only magnitude and sign, current is, in general, a vector quantity that has both magnitude and direction. By convention, and for purely historical reasons, the direction of current is considered as that of motion of positive electric charges. In Equation 1.1.1, e is positive, so that i has the same sign as u. If the particles are negatively charged, e is negative, and the direction of i is opposite that of u.
3. In the presence of an electric field, positive and negative charges experience forces in opposite directions. As a result, positive charges move in the direction of the electric field, whereas negative charges move in the opposite direction. However, the direction of the current is the same for both positive and negative charges because, in the case of negative charges, the signs of both e and u in Equation 1.1.1 change simultaneously.

Current Carriers

In metals, current carriers are predominantly conduction electrons, which have sufficient energy to detach from their parent atoms, becoming free electrons within the crystal. These electrons are able to acquire a mean drift velocity in the presence of an applied electric field, as explained more fully in Section 1.7. In some solids, such as semiconductors, current can also be carried by what are effectively positive charges, or holes. In conducting (or electrolytic) solutions, conduction is by means of positively charged ions (cations) or negatively charged ions (anions). In gases, current carriers could be positively charged ions, negatively charged ions, or electrons.

EXERCISE 1.1.2

In a one-dimensional flow of current through a semiconductor, holes move in the

Positive x-direction at a steady rate of 5 . 10^18 holes/min and electrons move in the negative x-direction at a steady rate of 2.5. 10^18 electrons/min. Determine the total current in milliamperes in (a) the positive x-direction and (b) the negative x-direction. (c) What would be current if the holes and electrons are moving in the same direction? Note that the positive charge of a hole has the same magnitude as the charge of an electron.
Answer: (a) 20 mA; (b) -20 mA; (c) 20/3 mA in the direction of movement of holes.