Chapter 19
Electric Current
If we place a potential differenceVacross opposite ends of a conductor (a copper wire, for example), then
there will be an electric fieldEDV=xcreated inside the conductor. The free electrons will respond
to this electric field, moving opposite the direction of the electric field. This motion of electrons is called an
electric current, and is analogous to the flow of water in a stream.
Current is measured as the amount of current passing a fixed point in the conductor per unit time. A
current of 1 coulomb of charge per second is defined to be 1 ampere (A), after the French physicist Andr ́e-
Marie Amp`ere: 1 AD1 C/s.
By convention, the direction of electric current is taken to beoppositethe direction of the flow of electrons.
Another way to think of this is to imagine electric current to be due to the flow of positive charges through
the conductor (even though it’s actually the negative electrons that are moving). (This somewhat confusing
situation is related to Benjamin Franklin’s unfortunate choice of which type of charge to call “positive” and
which to call “negative”.) Conventional current moves in the direction from high potential to low potential.
If a conductor is connected to the terminals of a battery, then conventional current flows from theCterminal,
through the conductor, back to theterminal.
Electric current does not flow smoothly through through a conductor. Electrons inside the conductor
are moving around at random, bumping into other electrons in their vicinity. Superimposed on this random
motion is a gradual drift of the electrons opposite the direction of the electric field. This speed of the electrons
through the conductor is called thedrift velocity. If the density of free electrons (electrons per unit volume)
isn, then the total charge per unit volume isne(whereeis the elementary charge). In timet, the volume of
electrons that move through the wire isAvdt, whereAis the cross-sectional area andvdis the drift velocity.
This means the total charge moving through the wire in timetis.ne/.Avdt/, and so the current is found by
dividing this byt:
IDneAvd: (19.1)Here’s an important point to keep in mind: one speaks of the potential difference (or voltage)between two
pointsin an electrical circuit; but one speaks of the electric currentat one pointin the circuit. For example,
you refer to the voltageacrossa resistor, but the currentthrougha resistor.