Class notes #2
Voltage references
The amount of effort (energy or work) required to lift a heavy box is equal to
mgh
, where
h
is the distance we must lift the box,
m
is the mass of the box, and
g
is a constant
representing the gravitational attraction of the earth. The amount of work required to lift
the box from the first floor of a building to the second floor is equal to the amount of
work required to lift the same box from the 7
th
floor to the 8
th
floor. It doesn’t matter
(within our assumption that we stay ‘near’ the earth’s surface) where the lift starts, we are
only concerned about the
change
in height. In fact, it makes no sense to describe height at
all without some kind of reference. When we casually describe a height without
providing a reference, it is usually possible to infer the reference from the context. For
instance, if someone tells you a skyscraper (the Empire State building) is 381 meters
high, you can assume that it stands 381 meters above Fifth Avenue. If someone tells you
a mountain (Everest) is 8841 meters high, you can assume that height is measured
relative to sea level.
Similarly, it only makes sense to talk about voltages relative to some reference level. We
can indicate this using the notation
V
AB
, which means the voltage of node
A
with respect
to node
B
.
We can consider an apartment window that is 12 meters above a street. We may note this
as:
where
h
WS
is the height of the window relative to the street. Equivalently, we can say that
the street is 12 meters
below
the window:
where
h
SW
is the height of the street relative to the window. Since the street is below the
window we assign this measurement a negative value. Note that
A similar situation exists for voltages:
When we wish to mark a voltage on a drawing, the
V
AB
notation in not convenient, and
we will indicate the reference direction using ‘+’ and ‘–‘ signs, where the ‘–‘ sign
indicates the reference node:
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View Full DocumentSometimes we find it convenient to have an electrical version of ‘sea level’. The
electrical equivalent of sea level is ‘ground’, which is the potential of the planet earth.
Since moist soil is usually fairly conductive, we can often establish a ground reference
simply by pounding a metal rod or pipe into the earth. Typically, buildings have at least
one metallic rod driven into the earth to establish a ground reference, and this reference is
wired to the ground pin of every outlet in the building. Often this ground wire is
connected to the metal chassis of our electronic equipment and appliances.
We can extend this idea of a ground reference to situations where there is no actual
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 Fall '07
 Westerfield
 Electrons, Electric charge

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