Positive and Negative voltage - Page 1
Forum section: electronics / beginners

author: BlackHawk2399
author_rank: Newbie
timestamp: August 03, 2012, 11:29:11 pm
content: Hi Guys, I need some help here. I don't really understand the concept of Positive and Negative Voltage. I searched the web but there was no easy rather detailed explanation. Can anyone please explain it to me?

author: IanB
author_rank: Super Contributor
timestamp: August 03, 2012, 11:44:25 pm
content: In most circuit analysis voltage is usefully considered as the difference in potential between two points.Whether the voltage difference is positive or negative depends on which way round you measure the difference.So for example if you have a 1.5 V battery and you measure the difference from the positive end to the negative end you will get +1.5 V.On the other hand, if you measure the difference from the negative end to the positive end you will get -1.5 V.It's just like saying:(a - b) = - (b - a)

author: Rerouter
author_rank: Super Contributor
timestamp: August 03, 2012, 11:53:18 pm
content: also for most circuits you will set one part of it as a reference to measure against, e.g. ground / earth (even if your device is battery powered people still like using the terms) so generally they are talking where the voltage is in respect to the reference point,

author: Psi
author_rank: Super Contributor
timestamp: August 04, 2012, 12:40:36 am
content: In most cars the negative battery terminal is connected to the chassis (ground) so the cars electrics run on +12V.However, a few old cars use the positive battery terminal for chassis ground so the electrics in those cars runs on negative 12V

author: HardBoot
author_rank: Regular Contributor
timestamp: August 04, 2012, 12:41:30 am
content: I don't really think of voltages as positive or negative, just how high in what direction, it's a relativistic thing.With something like an amp design which wants +12 and -12, you can just have an op-amp control which way the +12 and 0 are wired, or go 24v in some cases, depends on if you actually need to push the speaker/motor backwards.

author: IanB
author_rank: Super Contributor
timestamp: August 04, 2012, 12:51:29 am
content: Quote from: HardBoot on August 04, 2012, 12:41:30 amWell precisely, once you set a datum the sign of the measurement tells you in what direction it is measured.For instance, in civil engineering if you set the datum at ground level then the cellar floor might be at an elevation of -2.5 m (down), while an upper floor might be at 5 m (up). It is very common on engineering drawings to see elevations marked this way.

author: amspire
author_rank: Super Contributor
timestamp: August 04, 2012, 02:55:23 am
content: There have been a lot of posts describing volts as a relative value, but it is an absolute value. At any point in time, a point can have a negative absolute voltage (more negative charge then positive), a positive absolute voltage (more positive charge then negative) or zero absolute volts (positive and negative charges are exactly equal).If an object is floating in space so that it is totally electrically isolated from any other circuit, and the object has an excess of electrons, it will have a negative absolute voltage that can be calculated and measured.It just happens that in electronics, we are almost only ever interested in the voltage difference between points or across a component, so we do not worry about determining the absolute voltage very often.Richard.

author: JackOfVA
author_rank: Unknown
timestamp: August 04, 2012, 09:34:08 am
content: Quote from: amspire on August 04, 2012, 02:55:23 amIs it correct that an object with a certain charge has a unique absolute voltage?Charge, yes. The charge on an object is determined by the number of excess electrons in the example.From electrostatics, we know that the relationship amongst charge, voltage and capacitance is Q=CV, or since we are interested in voltage, V=Q/C.But, the capacitance of an isolated object is not definable; it's defined only with respect to some other object(s). Hence the voltage can also only be defined with respect to another object(s).Consequently, I can't see that a single charged object has a unique absolute voltage; rather it has a voltage that can be defined only with respect to another object of a certain size and geometrical relationship to the charged object.Consider, for example, the simple case of two conducting spheres, one with no net charge and one with some excess electrons. The voltage measured between the two spheres depends upon the distance between the two. I agree, of course, that this voltage can be calculated and measured, but since its value depends on the geometry, it's misleading to imply that the value is absolute.

author: amspire
author_rank: Super Contributor
timestamp: August 04, 2012, 02:32:07 pm
content: Quote from: JackOfVA on August 04, 2012, 09:34:08 amThe capacitance is defineable. It is called self-capacitance and it is a function of the surface shape. Given the shape of an object, the self-capacitance can be calculated. For a conducting sphere of radius R, the self capacitance is 4 * pi * R * e0(where e0is the permittivity of a vacuum). The Earth has a self-capacitance just over 700uF.QuoteNo, voltage does not have to be defined with respect to another voltage. The thing is zero absolute voltage is easily definable - it occurs if a body has zero net charge. That sets a refernce point for all other absolute voltages.QuoteThe absolute voltage will decrease as the spheres get closer, but it should still be measurable and calculated. As I said, the absolute voltage is not often relevant to electronics, so we only think about relative voltages.Richard.

author: HardBoot
author_rank: Regular Contributor
timestamp: August 04, 2012, 03:57:44 pm
content: I think of absolute voltage as static charge, something that's generally not an issue in relative low voltage electronics, although if something has an abundance of electrons or lack, things can go pop when that charge gets too strong, or suddenly gets equalized, zap goes the chip.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 05, 2012, 02:40:28 pm
content: Quote from: amspire on August 04, 2012, 02:55:23 amNo, that isn't true. Absolute voltage is technically meaningless. The physical quantity is the electric field (which tells you the force on a charge). The electric field is a vector (has x, y, and z components) which is a bit annoying to handle, but it turns out that we can write the electric field as the gradient (vector derivative) of a scalar field called the electric potential aka voltage. Since voltage an integral of electric field, we can add an arbitrary constant of integration without changing the field (or any measurable quantity) at all.What we usually do when making theoretical calculations is then to take the electric potential "at infinity" to be zero. This is often mathematically convenient, but it is just a choice. It is no more or less valid than any other choice, and it isn't always the most convenient. It certainly isn't of any practical utility since so few of us have long enough DMM probes to measure at infinity.

author: amspire
author_rank: Super Contributor
timestamp: August 05, 2012, 03:01:10 pm
content: Quote from: ejeffrey on August 05, 2012, 02:40:28 pmI think you are getting confused by the formulae, and not considering reality. A charged object in space will have an electric field, and it will be a consistent electric field. The electric field will be nothing at all to do with any arbitrary constant you choose. For a given charge and object shape, there will be only one possible electric field for the object, assuming the object is isolated in space, and is not near any other object or any external electric field.As the field can be determined unambiguously, the voltage can be calculated unambiguously.You never have to seriously take the field to infinity unless you are a theoretician. You only need to do that if you want to calculate a result to infinite accuracy, and that is never needed for any practical accuracy.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 05, 2012, 03:05:12 pm
content: QuoteYou succinctly demonstrated the how poor my long explanation was. This is the statement that is wrong. There is no unambiguous way to go from the electric field to the voltage. You can add a constant value to the voltage everywhere and you don't change the electric field.

author: megajocke
author_rank: Regular Contributor
timestamp: August 05, 2012, 06:53:09 pm
content: I'll second that ejeffrey's statement is correct.For a conservative field like the electric field in electrostatics you can define a potential whose gradient is the field. You can add a constant to the potential without changing its gradient.However, if you have changing magnetic fields you can not define electric potential because the electric field is not conservative in that case. A closed integration path enclosing a changing magnetic field will measure a voltage, even though you start and end in the same point!Or in practical terms: if you set your multimeter to AC volts, put one lead through the winding area of a transformer and short the tips together you will see a voltage!

author: amspire
author_rank: Super Contributor
timestamp: August 06, 2012, 03:26:05 am
content: Quote from: megajocke on August 05, 2012, 06:53:09 pmThe constant has to correspond to a real property of the universe. It is not a number that anyone can arbitrarily make up. For the single object isolated in space, it would mean that the universe would have to have a non-zero charge. Not being a physicist, I cannot be sure that there is not a significant non-zero charge, but if there were it would have a lot of consequences.If the constant is non-zero, it would be the equivalent to enclosing the universe in a conductive sphere charged to a voltage. If the universe does behave this way, it would mean that an uncharged hydrogen molecule will generate a local electric field that should be measurable.The universe has been expanding since the big bang, so this universe charge will be continually decreasing towards zero. If there is a signifigant universe charge/voltage now, then it means that early in the big bang, the voltage of the universe would have been extreme, and this would have been a major factor in the way the universe has expanded.My guess is that this universe voltage constant is probably so low it is insignifigant, but I admit I do not know for sure. I suspect that if you look at the known size of the universe and calculate a voltage based in a possible charge unbalance, the resulting voltage will still be below anything we could detect. So I still stand by my statement that an isolated charged body in space has a measurable absolute voltage.Whatever the voltage of the universe is, it is nonsensical to suggest that the field around a charged isolated object in space is neither measurable or constant. It does not depend on any arbitrary constant. The field is a real physically property of space around the charged object, not a mathematical abstraction that can be altered with imaginary constants.QuoteYou don't use a multimeter to measure the voltage of a charged object, as a multimeter draws current. You would need an electrometer, or any other method to examine the electric field such as the behavior of a charged particle as it passes though the field.Richard.

author: amspire
author_rank: Super Contributor
timestamp: August 06, 2012, 05:00:23 am
content: I have had a go at seeing what the experts say about the net charge of the universe. It would seem that there may be some charge imbalance, but the effect is so small that it is probably unmeasurable.In this paper (http://adsabs.harvard.edu/abs/1978ApJ...220..743B), it seems that bodies in galaxies have a small positive net charge - about 100 coulombs per solar mass. The freely expanding intergalactic medium has a compensating negative charge. I also gather that if the universe had a significant net charge, it would have had a big noticeable effect on the formation of the galaxies.I gather it is a question that is still researched. If anyone has some recent findings, I would be interested.As long as the universe is essentially balanced, then it is safe to take the net voltage at infinity to be zero.Richard.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 06, 2012, 06:29:55 am
content: Quote from: amspire on August 06, 2012, 03:26:05 amNo, it is just a number anyone can make up. It doesn't correspond to any physical quantity. In particular, it doesn't correspond to anything like the net charge of the universe.QuoteThe electric field is not the same thing as voltage. The electric field (measured in volts/meter) is measurable and unambiguously defined. The electrical potential (aka voltage) is only unambiguous up to an arbitrary additive constant which has no physical meaning. All measurable quantities are related to the difference between the electrical potential at two points, and the additive constant drops out.To be clear, this is not speculation on my part. I am telling you the facts of E&M -- there is no such thing as an absolute voltage scale. I don't know how to be any more clear.

author: metalphreak
author_rank: Frequent Contributor
timestamp: August 06, 2012, 08:48:32 am
content: A molecule can have negative or positive charge, but if its in a sea of molecules with the same negative or positive charge, there is no potential difference between them.... thus no voltage."Voltage, otherwise known as electrical potential difference or electric tension (denoted ?V and measured in volts, or joules per coulomb) is the potential difference between two points — or the difference in electric potential energy per unit charge between two points."Two negatively charged (but different quantity of charge) molecules can have a positive voltage (or negative) depending on how you look at it

author: IanB
author_rank: Super Contributor
timestamp: August 06, 2012, 03:24:17 pm
content: Quote from: ejeffrey on August 06, 2012, 06:29:55 amTo explore this further, it is possible to define an absolute temperature scale in thermodynamics by defining what absolute zero means as a temperature. So why is it not possible to do the same thing with voltage? For instance by defining zero volts as the potential of an uncharged body in free space absent of electric or magnetic fields?

author: ejeffrey
author_rank: Super Contributor
timestamp: August 06, 2012, 09:39:15 pm
content: Quote from: IanB on August 06, 2012, 03:24:17 pmWell, that is effectively what you are doing when you define the electric potential to be zero at infinity. You have just picked a reference point and measure everything relative to that. It is often mathematically convenient to do it this way. But it is just a choice, it doesn't effect any physical measurement.Absolute temperature on the other hand has a specific physical meaning -- namely that the random motion of atoms and molecules stops, the system is in the ground state with zero entropy. So we aren't free to choose a different value for absolute zero like we are with the electric potential.By the way, everything I have talked about here is only dealing with electrostatics. As megajocke explained, if magnetic fields are involved it is more complicated and you have to include the magnetic vector potential as well. The end result is the same -- electric potential is only a relative quantity, but the formula is a bit more complicated.

author: megajocke
author_rank: Regular Contributor
timestamp: August 07, 2012, 01:43:00 am
content: Quote from: amspire on August 06, 2012, 03:26:05 amActually, no. In electrostatics, a charge distribution inside a conductive shell will give exactly the same electric field inside regardless of what is outside. The field created outside such a shell will however depend on the amount of enclosed charge, but not on the manner by which it is distributed inside. It might as well have been a solid conducting object with that amount of free charge, as far as the field outside is concerned.This can be useful when doing calculations because it allows separating such a problem into two independent domains.For example, put a negatively charged conducting sphere inside a larger conducting sphere. This is a capacitor; let's say it is charged to 1 kV. If the outside is connected to +20 kV referenced to the surroundings (infinity, if we are alone in the universe), the inner conductor will be at +19 kV compared to the same reference, even though it has an excess of electrons.But for a single object in the universe, sure, the voltage to infinity is directly linked to charge and self capacitance. It is also a good approximation if other objects are sufficiently far away.