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# 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 Q flowing past a point in a time interval t :

 I = . (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.

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

 I = = nAq = nAqvd. (2)

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

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