VOLTAGE
Now that we know how to measure current, the next question is, What causes it?
Why does an electron drift through the circuit? One answer is to say that there’s an
electric field inside the wire, and since negative charges move in the direction
opposite to the electric field lines, electrons would drift opposite the electric field.
Another (equivalent) answer to the question is that there’s a difference in electric
potential—otherwise known as a potential difference or, most simply, a voltage—
between the ends of the wire. Negative charges naturally move from regions of
higher potential to lower potential. Voltage creates a current.
Voltage
Continuing with the
metaphor of a river, note
that a river flows from
higher ground to lower
ground; it will not flow if
it is flat. Voltage, then, is
the degree of elevation
(anywhere from a molehill
to a mountain) that gives
the river a height to
flow down.It is not uncommon to see the voltage that creates a current referred to as
electromotive force (emf), since it is the cause that sets the charges into motion in a
preferred direction. The voltage of a circuit is generally supplied by a battery.