QUESTION: In one of your answers you said "all voltage is relative to some other point in space".
I understand that in physics you speak about the concept of ground. I've heard that ground has to do with zero volts. Is this true and what is the definition of ground?
Where I live, a socket has 220volts. If i understand the concept of volts correctly we could never have -220volts coming out of a socket. Is this true?
I also understand that eectricity is electrons moving. 220volts would be a lot of electrons moving but would -220 also be electrons moving?
If I wrote something that is not true then please correct me.
ANSWER: Zero is a chosen point to measure zero at. It's called that because it usually refers to the actual "grounded" electrical connection to the nearby Earth. If I drew a line and called some point zero distance, it's the same idea. Negative and positive, as you refer to them, just indicate the direction of electron flow between two points. Like I wrote before, -220V is just like being below sea level. Totally an arbitrary human choice to make sea level zero and not the center of the earth...but in voltage there is no absolute zero. -220 can be done between two terminals on a battery by just turning the battery around. That +/- just indicates a direction.
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QUESTION: Just to clarify: are you saying that positive and negative refer to the direction the electrical current is moving?
And 1 volt is always the same like 1 meter above sea level refer to the same thing for all people?
I've heard that 500 volt is high voltage and dangerous but that if you rub a balloon on your hair then you also have 500 volts. Is that really true?
If volt is relative then people use different kinds of definition of volt? Is that true?
They do refer to the direction that the current is moving, yes. 1 Volt is a measure of electrical potential difference between two points. One meter above sea level could still refer to two different points and therefore two different potentials if there's any horizontal electric field.
500 Volts is indeed high voltage if you can supply current to it. Static electricity like you're talking about can be harmless at thousands of Volts, though, because the discharge lowers the voltage to zero nearly instantaneously. If you were passing many Amps of current through your body by putting a constant 500 V across it, you would quickly pass many Coulombs of charge through it. That's deadly. But nC (billionths of Coulombs of charge) like in static electricity is generally harmless...once it's flowed through (nanoseconds), it's gone.
Everyone uses the same definition of the Volt. Go take physics and study it, it's the integral over electric field (as you travel along an electric field, voltage drops along the field...builds up going against the field). Read more here: http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elewor.html