next up previous index
Next: Resistance Up: Current and Resistance (Ch. Previous: Current and Resistance (Ch.

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 :

I = $\displaystyle{\frac{\Delta Q}{\Delta t}}$. (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 vd 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
\begin{figure}
\begin{center}
\leavevmode
\epsfxsize=3 in
\epsfbox{/export/home/fyde/randy/figs/fig17-1.eps}\end{center}\end{figure}

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

 
I = $\displaystyle{\frac{\Delta Q}{\Delta t}}$ = nAq$\displaystyle{\frac{\Delta x}{\Delta t}}$ = nAqvd. (2)

As will be seen later in an example, the drift velocity vd is surprisingly small for typical currents.
next up previous index
Next: Resistance Up: Current and Resistance (Ch. Previous: Current and Resistance (Ch.

www-admin@theory.uwinnipeg.ca
10/9/1997