notes%202 - Class notes#2 Voltage references The amount of...

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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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Sometimes 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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notes%202 - Class notes#2 Voltage references The amount of...

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