I like watching science videos on YouTube and I have a very basic understanding of electronics. But I never seem to get the “negative voltage” thing.
Why is it used and how does it differ from normal voltage.
In: 5
It doesn’t really differ. Voltage is a potential energy between two spots. A positive voltage between two locations means that electrons flowing from one to the other will give you energy. A negative voltage means that electrons flowing the other direction will give you energy.
Voltage is always measured as a difference and is like pressure. It’s not something that measures an amount of stuff like mass or volume. A voltmeter’s displayed voltage measurement is the red probe minus the black probe. If you do that and it says +10 volts, swapping the two probes will make it read -10 volts.
Consider electricity being similar to plumbing. A closed circuit one, to be precise.
Imagine we have a pump and a curvy pipe sticking to both ends of the pump.
The pump pushes a liquid to the pipe through the output hole and expects it to get back to the pump from the other side – the input.
The pump generates a pressure at the output – that’s the “positive” pressure. And on the input you get under-pressure as the pump is sucking the liquid in. That’s the “negative” pressure.
The same thing happens in electricity. Just imagine the voltage being the pressure a pump generates.
Hope this is clear 😅
Voltage is a measure of the difference between electric potential(this is not potential energy) between two points.
Positive vs negative is a matter of comparison to a reference point, on the earth our reference point is simply the earth(hence the term ground) which is defined as 0.
In practice positive or negative simply denotes the direction in which current flows.
Electrons move one way down a wire, positive voltage. Flip the wires around so the electrons move the opposite direction down the wire, negative voltage.
Kinda like you can have *negative* potential energy when you compare your current position to a nearby 1000’ mountain top. Just flip the math around and the height becomes positive instead of negative. Potential energy = mass * acceleration of gravity * height. If you are on the mountain top, your height would be 1000’-0’ =1000’. But on the flat next to the mountain the height is 0’-1000’=-1000’ when comparing the two elevations.
This is analogous to the negative voltage you are describing.
A voltage is a difference in electromagnetic potential between two-point.
The important part is that it is a difference. What is zero volts is up to you.
Take two AA batteries that each is at 1.5V and connect them in series. Call the point on the negative pole of the first battery as A. Between the two batteries as B and the end of the last as C like in the diagram below
A -====+ B -====+ C
If you define A as 0 volts then B is at 1.5vt and C is 3v.
If you define B as 0 volt then A is at -1.5V and C at 1.5V
If you define C as 0 volt the A is at -3V and B at -1.5V
Any calculation you make works the same regardless of what you define as 0V it is just a question of what is common and convenient to use.
For the power grid, you have the power plants connected to the physical earth and the same as your location, It is defined as zero volts so there is a common reference
If it is not any different elevation. You can define the floor on the ground level as 0 meters then level up the floor might be at +2meter but the cellar flor is at -2 meter.
Your zero point can be changed to the cellar floor than the ground floor is at 2 meter and the next at 4 meter.
The elevation is always relative to something just like voltage. Elevation can be considered as a difference in gravitational potential. If you ever have calculated potential energy as a function of elevation you have used elevation just as we use voltage
If we’re talking on the physical level, it’s the same as positive voltage. It’s like the difference between moving 3 meters left and moving 3 meters right, they both results in 3 meters from starting point, just different location.
About why is it used… that’s a bit more… complicated. It’s a whole world of electronics black magic forkery.
One of the most popular use of negative voltage is N-channel JFET.
To very briefly explain N-channel JFET, think of a river. And then somewhere down the river there’s a giant water gate. Now the gate is controlled by say, a lever.
By default, the lever is right in the middle, and the gate is half open. Turning the lever up raises the gate, slowly limiting the water flow, while turning the lever down lowers the gate, allowing more water to flow.
Well, that’s N-channel JFET. It has 3 terminals, Source, Drain, and Gate. Electricity flows through a “channel” from Source to Drain, and Gate controls them. Applying positive (up) voltage raises the gate, while negative (down) voltage lowers the gate.
This is of course a very, very simplified explanation.
Why does this phenomenon occur? That’s a whole ‘nother rabbit hole of Semiconductor Forbidden Super-Secret Black Magic Forkery.
It differs from a positive voltage only in what you arbitrarily decide to call zero. Lets say you measure a voltage, and it’s 5 volts. If you swap your leads around, you measure -5 volts. The only difference is which one you’re calling zero.
It’s sort of like negative temperature on the Celsius or Fahrenheit scales, negative temperatures are just temperatures below the point those scales decide are “zero”.
– +
A A
A A
+ –
—–[sheet of metal]—–
imagine you have two batteries touching a sheet of metal. one battery’s negative side is down, the other battery’s positive side is down.
if you take your multimeter and touch the negative probe to the sheet of metal, and the positive probe on the battery that’s got “+” up, you’ll see +1.5v.
if you then move the positive probe and touch the battery that has its “-” sticking up, you’ll see -1.5v.
You’ve now observed negative voltage — but what’s it good for?
Move your negative probe to the battery with the “-” sticking up, and touch the positive probe to the one with the “+” sticking up. Now you’ll see +3.0v.
Where did 3.0v come from?? Ground (the sheet of metal) only showed 1.5v in each direction!
Well, that’s where negative voltages come in handy — given to themselves, they’re only a smaller voltage, but combined with the positive rail, they can be useful as a higher voltage, where needed, if the circuit uses both “rails”. Consider that the “sheet of metal” – the idea of the negative voltage’s “ground” – is totally up to you… it could, for a moment, just be considered an arbitrary wire between two batteries (DC supplies).
In EV charging (where I really, deeply learned about negative voltage in this specific case), +12v/-12v is used in J1772 EVSE signaling to “communicate” (in a crude, analog manner) between the vehicle and station. To ensure the attached “thing” is really a vehicle and not a puddle of water, the EV has a diode in it – so that only the positive side of the signal ever gets resisted (to change the voltage, and “talk back” to the charging station). No diode? Both sides see a voltage decrease. Diode? Only the positive side gets decreased – and it must be a car.
Did Obi-wan have the high ground? Or did Anakin have the low ground?
Both are correct since its all relative. Obi-wan was either on a hill (+V) or Anakin was in a ditch (-V), depending on your perspective.
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