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Voltage and Current in a Practical Circuit
Because it takes energy to force electrons to flow against the opposition of
a resistance, there will be voltage manifested (or "dropped") between
any points in a circuit with resistance between them. It is important to note
that although the amount of current (the quantity of electrons moving past a
given point every second) is uniform in a simple circuit, the amount of voltage
(potential energy per unit charge) between different sets of points in a single
circuit may vary considerably:
Take this circuit as an example. If we label four points in this circuit with
the numbers 1, 2, 3, and 4, we will find that the amount of current conducted
through the wire between points 1 and 2 is exactly the same as the amount of
current conducted through the lamp (between points 2 and 3). This same quantity
of current passes through the wire between points 3 and 4, and through the
battery (between points 1 and 4).
However, we will find the voltage appearing between any two of these points
to be directly proportional to the resistance within the conductive path between
those two points, given that the amount of current along any part of the
circuit's path is the same (which, for this simple circuit, it is). In a normal
lamp circuit, the resistance of a lamp will be much greater than the resistance
of the connecting wires, so we should expect to see a substantial amount of
voltage between points 2 and 3, with very little between points 1 and 2, or
between 3 and 4. The voltage between points 1 and 4, of course, will be the full
amount of "force" offered by the battery, which will be only slightly
greater than the voltage across the lamp (between points 2 and 3).
This, again, is analogous to the water reservoir system:
Between points 2 and 3, where the falling water is releasing energy at the
water-wheel, there is a difference of pressure between the two points,
reflecting the opposition to the flow of water through the water-wheel. From
point 1 to point 2, or from point 3 to point 4, where water is flowing freely
through reservoirs with little opposition, there is little or no difference of
pressure (no potential energy). However, the rate of water flow in this
continuous system is the same everywhere (assuming the water levels in both pond
and reservoir are unchanging): through the pump, through the water-wheel, and
through all the pipes. So it is with simple electric circuits: the rate of
electron flow is the same at every point in the circuit, although voltages may
differ between different sets of points.
Lessons In Electric Circuits copyright (C) 2000-2002 Tony R. Kuphaldt,
under the terms and conditions of the Design
Science License.
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