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.
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Figure
17.1: Cross-section of a wire carrying moving charges |
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|
In a time
t
a charge
Q
has moved a distance
x
. Since
Q
= (nA
x)q
, we have for the current
|
|
I
=
|
(2) |
As will be seen later in an example, the drift velocity vd is surprisingly small for typical currents.