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# Physics/electricity

Question
QUESTION: Hi!
I am a bit confused I guess. Sometimes we talk about electrical potention, eg -56mV. How can electricity be minus? If I had -10 dollars I would have no money to talk about (unless I owed ten dollars or something). Anyway, how can electricity be minus? Am I just confused about what electricity is all about?

ANSWER: When you think about opposite charges (positive and negative), perhaps it's not as mysterious-seeming.  The actual assignment of positive and negative is a human convention.  But that's charges which are the sources of electric fields and therefor voltages.  With voltage itself, all voltage is relative to some other point in space.  Only changes in voltage matter, there's no such thing as an absolute voltage.  It's more like height.  If fly at 1000 meters in a plane, I say that I'm at 1000 meters.  But that's relative to the ground.  If the ground is already at 1000 meters above sea level, then I'm 2000 meters above sea level.  Similarly, if I go down into Death Valley (86 meters below sea level), I'm at -86 meters.  Voltage is like that, you pick a point and measure higher or lower electrical potential relative to that point.

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QUESTION: And how do you define "electricity"?

"Electricity" is an imprecise term that people use to describe anything electrical in nature, and even romantic attraction between two people.  More precise terms are ususally structured as "electrical charge," "electric current," "electric field," or "electric potential."  In the case you mentioned, voltage, the last term.  You can have negative values on all of these things.  Example: negative numbers associated with electric current indicate the the current is flowing in the opposite direction from what you are measuring.

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