Explain negative voltage - Page 1
Forum section: electronics / beginners

author: MrBond
author_rank: Contributor
timestamp: January 12, 2023, 11:49:34 pm
content: Can anyone please explain what negative voltage is? I keep hearing about -15v rail for example and i can feel myself going crazy trying to understand what that means. My understanding of electricity is that if there's nothing it's 0,if there's something its positive, negative being just a neant for the electrons to flow into.

author: james_s
author_rank: Super Contributor
timestamp: January 12, 2023, 11:53:00 pm
content: Voltage is relative, it's the difference in potential between any two nodes. Typically we pick one node and call it "ground" and use that as the reference point to which all other voltages in a circuit are measured. If you take for example two 12V batteries and connect them in series you could call the connection between them ground. Now if you measure from that point to the other terminal on each battery you will measure +12V on one and -12V on the other. -12V is 12 volts lower potential than ground, it's not negative in the sense of a negative bank account balance, it's just negative relative to the point from which you are measuring.

author: Kim Christensen
author_rank: Super Contributor
timestamp: January 12, 2023, 11:57:34 pm
content: Negative voltage just means it's negative relative to a reference point (usually ground).ie: If you took a 9v battery and connected it's positive terminal to ground, while leaving the negative terminal open, and then measured between it's negative terminal and ground you would measure -9V

author: ejeffrey
author_rank: Super Contributor
timestamp: January 13, 2023, 12:10:09 am
content: The electric field is a vector (arrow) pointing in the direction of the force a charged particle would experience.It's annoying to deal with electric fields because they have a direction and magnitude. Voltage is a number (not a vector/arrow) chosen such that the electric field points in the direction from "high voltage" to "low voltage". The overall offset doesn't matter, just the difference, because the electric field arrows that it represents are the same in either case.

author: MarkS
author_rank: Unknown
timestamp: January 13, 2023, 12:31:07 am
content: Removed really bad joke...

author: Psi
author_rank: Super Contributor
timestamp: January 13, 2023, 12:42:38 am
content: The biggest error people make is assuming "this wire has 5 volts on it" It does not.It might have 5V on it with respect to another wire in the circuit. But it might also have 10,000V on it with respect to a different place in the circuit. or -500V, all at the same time!A voltage becomes negative or positive when you choose to make a specific voltage 'the ground'. A voltage measurement is only a measurement between two points, so you define polarity by your measurement. You might decide "this point is ground" and that means you assume that point is zero volts and measure everything as an amount of volts compared to that point. But since you decided what to use as ground it's all relative to your decision.You can have two 1.5V AA batteries in series and choose to view that as (+3V) or (-3V) or (two voltages +1.5V and -1.5V) depending on what point you consider to be ground.This is why 'isolated' power supplies are useful, you get to decide how to use the output voltage.If the power supply is not isolated then someone has already decided ground for you.

author: ledtester
author_rank: Super Contributor
timestamp: January 13, 2023, 02:25:29 am
content: Here is an analogy which might be helpful: Voltage is like height.You decide on a certain reference point (called zero volts or ground) and positive voltage is a height above that reference point and negative voltage is a vertical distance below that reference point.For instance, you may decide that sea level is your zero volts. Then Mount Everest will be at a positive voltage and the Mariana Trench will be at a negative voltage.On the other hand, you could decide to call the Mariana Trench your zero volts. Then sea level will be at positive voltage and Mount Everest will be at an even larger positive voltage.In electronics all that matters is the difference in voltage. Note that the altitude difference between Mount Everest and the Mariana Trench is the same regardless of what altitude you decide on for your zero volts.In a circuit design the zero volts reference point is usually chosen to help make understanding the circuit easier.

author: golden_labels
author_rank: Super Contributor
timestamp: January 13, 2023, 03:41:01 am
content: Ignoring specific scales for a moment, I feel it’s important to straighten out one other misconception. This isn’t like density, where at some point there is nothing and this is the end of the scale. Voltage is not single-ended. With two(1)particles representing electric charge — electron and proton — the electric field is representing collective effect of all electrons and protons in the system. There is never “nothing”. The influences may sum up to zero, but that’s not the same as there being no influences. If you have a train and a crowd pushes and pulls it, perhaps it doesn’t move (forces sum to zero), but you can’t say that there is no forces acting on the train.Since that kinda contradicts ledtester’s description,(2)let’s make another example. Instead height, use time at the scale of human life. You wrote this question today james_s answered 4 minutes later. That is positive, +4 minutes. But if we take james_s’s response as our reference point, you asked 4 minutes before that. That is negative, -4 minutes. Time in this case is just a long line and we arbitrarily chose our reference point, measuring everything else relative to it. Voltage works the same.(1)We already know more than two, but two is sufficient to explain the point.(2)The description is very good, just in the light of the previous paragraph it may seem a bit inconvenient to accept it.

author: IanB
author_rank: Super Contributor
timestamp: January 13, 2023, 04:01:08 am
content: Quote from: golden_labels on January 13, 2023, 03:41:01 amI think this is entirely confusing and just muddies the issue.Voltage is a difference between two values, like an arithmetic subtraction, A minus B. You can evaluate A minus B, or B minus A. Both have the same magnitude, but the sign is reversed.You see this really simply if you just swap the red and black leads on a multimeter when measuring the voltage of a battery.

author: golden_labels
author_rank: Super Contributor
timestamp: January 13, 2023, 06:49:53 am
content: Quote from: IanB on January 13, 2023, 04:01:08 amLet me repeat the first sentence (emphasis added):Quote from: golden_labels on January 13, 2023, 03:41:01 am

author: bidrohini
author_rank: Regular Contributor
timestamp: January 13, 2023, 07:29:16 am
content: You can also read these stuffs:http://www.learningaboutelectronics.com/Articles/What-is-negative-voltagehttps://electrouniversity.com/what-does-negative-voltage-mean-on-a-multimeter/

author: CaptDon
author_rank: Super Contributor
timestamp: January 13, 2023, 02:19:45 pm
content: Wasn't this exact same question posted about 6 months or so back?

author: Terry Bites
author_rank: Super Contributor
timestamp: January 13, 2023, 03:17:32 pm
content: Surely you're joking Mr Bond?Please don't insert my batteries.

author: MrBond
author_rank: Contributor
timestamp: January 13, 2023, 08:58:05 pm
content: I would like to thank all of you for explaining this to me. It's been remarcably hard to find anything about it on both google and youtube, and the actual answer was surprinsingly easy.