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Just a general electronics question: What is negative voltage, like -5 Volt?
From my basic knowledge, power is generated by electrons wandering from the minus to the plus side of the power source (assuming DC power here). Is negative voltage when electrons wander from + to -?
Why do some devices even need it, what is so special about it?
Someone may have better words to explain this than me, but the big thing you have to remember is voltage is a potential difference. In most cases the "difference" part is a difference between some potential and ground potential. When someone says -5v, they're saying that you are below ground.
You also need to keep in mind that voltage is relative. So like I mentioned before, most people reference to "ground"; but what is ground? You can say ground is earth ground, but what about the case when you have a battery powered device that has no contact to ground. In this situation we have to treat some arbitrary point as "ground". Usually the negative terminal on the battery is what we consider from this reference.
Now consider the case that you have 2 batteries in series. If both were 5 volts, then you would say you would have 10 volts total.
But the assumption that you get 0/+10 is based off of "ground" as being the negative terminal on the battery that isn't touching the other battery and then 10V as being the location of the positive terminal that isn't touching the other battery. In this situation we can make the decision that we want to make the connection between the 2 batteries be our "ground" reference. This would then result in +5v on one end and -5v on the other end.
Here is what I was trying to explain:
+10v +++ +5v
| |
| | < Battery
| |
+5v --- 0v
+++
| |
| | < Another Battery
| |
0v --- -5v
Imagine you're measuring the height of a car. You could take a tape measure and measure the distance from the ground to the roof of the car. "The roof of this car is 4 feet above the ground."
You could also stand on the roof of the car and dangle the same tape measure down to the ground. "The ground is 4 feet below the roof of this car."
Voltage works the same way. The negative sign is just a convention, in the same way that the car has the same height, regardless of which way you measure it. Flip your multimeter leads, and the negative sign will disappear.
Voltage is a difference in potential. If I connect terminal A of a device to potential of 30 volts and terminal B to a potential 20 volts. The potential from A to B is 10 volts, but the potential from B to A is - 10 volts.
Think of it a tall building To go from floor 30 to floor 20 you go down 10 floors.
oh why not ..
To say you have a voltage of +5V at point A means that point A is 5 volts more positive than your chosen ground.
To say you have a voltage of -5V at point B means that 'ground' is 5 volts more positive than point B.
The sign just tells you the polarity of the voltage, with respect to the ground node.
The first part of this question has already been answered very well.
As for the second part, you could take the lowest supply terminal from the power supply and call that 0V, then every other voltage would be positive. However, that would be very inconvenient for many circuits. For example, common power supplies for an op-amp circuit are +12V and -12V. You could relabel the -12V output as "ground", then the old ground would be +12V and the old +12V would be +24V. Also, all your signals would be referenced to +12V and you would have to take that into account any time you measure things. Also, a lot of the power is used between the highest and middle output (in effect the charge in the top output originally came from the middle output and wants to go back there), same with the lowest output. All in all it is easier to label the middle output 0V (ground) and work with positive and negative voltages.
All this is ignoring earth grounding issues. In real life often the ground output on the supply is literally grounded, and thinking of the whole Earth as being at +12V would just be weird.
Some OP-Amps, for example the veteran 741, require their supply as two voltages, one positive and the other negative with respect to the ground or zero level of the signal input and output.In this context it is natural to speak of the supplies being +15v and -15v (these are the values commonly quoted for the 741)
An alternating voltage, such as the a.c mains supply, swings positive and negative with respect to the neutral line, which is very close to the earth potential, so "neutral" is regarded as at zero voltage.
Voltage between two physical points \$A\$ and \$B\$ is defined as
\$ V_{AB} = - \displaystyle \int\limits_{A}^{B} \vec{E}.d\vec{\ell}, \$
where \$\vec{E}\$ is the electric field on the path of the integration.
\$V_{AB}\$ becomes negative or positive (or just zero) according to this integration. For example, if you swap the points \$A\$ and \$B\$, the sign changes.
If for example your power supply shows ground, 5v, -5v then your ground terminal is positive to the -5v, you get 5v from the ground that way. If you use the 5v and -5v terminals, then you will be using the -5v as the ground and you will get 10v from the 5v terminal If say there was also a 3v terminal, you would get 8v from the 3v terminal using the -5v as ground. Simple question, simple answer people. I probably know less about this than everyone else who commented here.
Well, just to chuck in my two cents, let's say you have an ungrounded device. With +10 volts, you're expecting the current to come of the battery, through the widget, and then... where? It's only 10 volts, so arcing through the air to ground isn't really possible, but it could go through the case into the hand of the user, or the charge could simply stay on the far end of the widget. So now you have +10 volts on one side and +8 volts or something on the other, relative to ground. The wiget only sees a 2 volt difference though.
With a a +5V and a -5V the current is both pushed into the widget and pulled out of the widget.
I'll try to explain it as I understand it...there is no +ve and -ve, its only a denotation of two directions. Voltage means Potential "difference". i.e, it is not a quantity, but a comparative value, in a direction. If you like a direction, then its +ve, if you don't like it then its -ve. An apple falls from the tree to the ground, both the ground and the branch of the tree exert a certain pull on that apple, but the ground exerts "X" amount more than the branch, the ground wins. Then Jupiter decides to take a cosmic stroll and climbs 2 feet above the tree, now the apple flies up. Actually there is no up and down, there never was. If the sight of an apple flying up distresses you, you say down is where earth is so towards the earth is the +ve "direction", and up is where Jupiter is so flying to Jupiter is -ve "direction".
In the same way there is no +ve and -ve in electrical engineering, its just a denotation of two directions (scalar not vector in the case of electronics), one we prefer, one we don't so much but still use it to our benefit.
Power is not generated by electrons wandering around. In fact, an electron might wander all over the place with no voltage at all applied. Energy cannot be created or destroyed. It comes from someplace, and it goes someplace. For example, when you turn the crank of a generator, you are not really generating power; you are merely transferring the power that plants absorbed from the sun before you ate and digested them. You should market such a generator as "powered by thermonuclear fusion".
