Current

When acted upon by an electric field, a charge experiences a force, and thus moves. One defines the current associated with this flow of charge as the amount of charge $\Delta$Q flowing past a point in a time interval $\Delta$t :

$${\frac{\Delta Q}{\Delta t}}$$ (1)

The units of current are thus C/s, which are given the name Amperes (A). By convention, the flow of current is in the direction of the motion of positive charges.

One can relate the current I in a material to properties of the atomic charges. Suppose in the material there are n charges per unit volume, each carrying a charge q . When acted upon by an electric field these charges begin to move; let us associate an average drift velocity v_d with each individual charge. Consider now a section of the material with cross-sectional area A , as in Fig. 17.1.

  

Figure 17.1: Cross-section of a wire carrying moving charges

In a time $\Delta$t a charge $\Delta$Q has moved a distance $\Delta$x . Since $\Delta$Q = (nA$\Delta$x)q , we have for the current

 

$${\frac{\Delta Q}{\Delta t}} {\frac{\Delta x}{\Delta t}}$$ (2)

As will be seen later in an example, the drift velocity v_d is surprisingly small for typical currents.