Why does everything electrical we see give its voltage?

[kaotak]

What is voltage? So many words, so many explanations are just hollow echoes off the walls of "authoritative sources"

What is voltage? Why does everything electrical we see give its voltage? Does it cause current to flow? If so, how? Because of electric forces? How do we harnest the energy from our power outlets? Where are the positive and negative terminals on our power outlets? Don't all voltage drops need positive and negative terminals?

What does it really DOOOOOOOOO.

Does it give us current? If so, how?

Where does the current come from? Where do we get the energy?

Electromagnetism makes no sense.

 

[interested_learner]

Another name for voltage is potential. In other words, potential energy. It is like being on top of a hill and current is like rolling balls off the top of the hill.

Ok not quite, but does someone have a better analogy?

 

[kaotak]

Another name for voltage is potential. In other words, potential energy. It is like being on top of a hill and current is like rolling balls off the top of the hill.

Ok not quite, but does someone have a better analogy?

Where do the balls (current) come from?

 

[russ_watters]

You've used magnets before, right? You've felt the force...?

 

[vanesch]

Where are the positive and negative terminals on our power outlets? [ ... ]
Electromagnetism makes no sense.

You've discovered a super hoax actually, but keep it quiet because there's a whole big industry living off its spread. You don't need two outlets on a power outlet, that's just to increase its price. One single outlet is quite sufficient, as long as you do a rain dance when you switch on your appliance of choice.

BTW, yes, I'm making (gently) some fun of your statements

 

[Kison]

Voltage is another word for potential difference.

Imagine a D-sized battery. You have the positive and negative side.

Voltage describes the potential difference between the positive and negative side. This relates to the electric field & Coulomb's Law. Basically, the positive side of the battery has an unbalanced proportion of protons & electrons, and thus, when you connect a wire from the positive end to the negative, it is theorized that protons move from the positive side to the negative side. This is called electric current. However, it has been realized to be incorrect, although calculations may still be useful today.

In reality, however, electrons move from the negative side to the positive side. This idea is known as the electric flow.

So the more the voltage/potential difference, the more the positive side of the battery is unbalanced between protons/electrons in comparison to the negative side. So when you connect them with a wire, more charges will flow through the circuit every second.

 

[TuviaDaCat]

You've discovered a super hoax actually, but keep it quiet because there's a whole big industry living off its spread. You don't need two outlets on a power outlet, that's just to increase its price. One single outlet is quite sufficient, as long as you do a rain dance when you switch on your appliance of choice.

BTW, yes, I'm making (gently) some fun of your statements

its all theft i tell you! every electron i get from the power station i give one back to them, so what the hell am i paying for?!

 

[Xezlec]

I like Kison's answer. But let me throw in another one. Think of the electrons in a metal as a sort of "gas" of electrons. They are free to move around in the metal, and they repel one another. If you pack more electrons into the same volume of metal, then they have a higher "pressure" because they are all repelling one another. Conversely, if you suck some electrons out, you have a "vacuum" of electrons, because the positively charged nuclei are "trying to attract more electrons". Sort of.

Now what if you build a "pump" that pumps electrons from one piece of metal to another? Well then you have a pressure that builds in one piece of metal and a vacuum that builds in the other. The vacuum piece is said to have a higher (more positive) "voltage". The voltage between two terminals is simply the difference in electron pressure between one and the other.

And if you connect some conductive path (wire) between the two, the electrons will zoom from the higher pressure to the lower pressure, through that path. The bigger the pressure difference (voltage), the faster they will whiz through the path. The rate that the electrons zip through that path is called the "current", and is measured in Amperes. So current through a path is proportional to voltage. Want more?

 

[Coto]

In other words one prong of the plug is +'ve and the other -'ve... and the third round one (american outlet) is the ground.

 

[cesiumfrog]

In other words one prong of the plug is +'ve and the other -'ve... and the third round one (american outlet) is the ground.

That's only true for about a hundreth of a second.

 

[Coto]

ok ok.. AC.. but for simplicities sake. Plus isn't it then 60Hz AC?

 

[Xezlec]

ok ok.. AC.. but for simplicities sake. Plus isn't it then 60Hz AC?

In some places it's 50 Hz. Each cycle is one polarity for exactly half the time. That's 1/100 second.

 

[cesiumfrog]

Coto, my statement applies regardless of whether your power grid supplies 50Hz or 60Hz (I presume the US uses the latter, and wonder if it's related to their anti-metric stance). My wording was that good. People these days depend too much on calculators - they pick them up too early (before they've rearranged and properly simplified the algebra) and then hang on to them for too long (trusting the 9 digit precision regardless of lower certainty in initial data). </rant>

 

[Coto]

Actually what I was implying was AC power versus DC power (in my first statement). The second statement was only questioning the 1/100th of a second, where Xezlec seems to have answered it, but it appears you disagree with his answer.. so I'm confused 0_o, are you disagreeing with my question? with my first statement? or did you mean Xezlec?

Plus, I think you're over generalizing how people plug and chug numbers too often in their calculator.. depends who the person is and what they've studied. Personally I haven't had to deal with a calculator since 2nd year.. we have no choice but to simplify algebraically.. that or we get the answer wrong.

 

[Xezlec]

Lively little conversation.

cesiumfrog: 1/120 sec is more than 10% different from 1/100 sec. When people say "about", I sometimes assume they mean within +/- 10% or so. I think it was "about" as reasonable of me to assume you meant specifically 50 Hz as it would have been for me to assume you intended your statement to cover 60 Hz. ;-)

As for calculators, I think whatever is faster is better (I'm an engineer). People who use their calculators earlier in the process almost always get done quicker than people who feel the need to get everything nice and perfect and reduced first. And I don't even reduce algebraically anymore. Waste of time -- my calculator's algebra system is better at reducing than I am, and makes far fewer mistakes!

On tests in school, I used to always get the problems right conceptually, and make errors in the algebra on almost every problem. I recognized this tendency, so I would go over and over and over my work to make sure I didn't screw up, being as careful as possible. But I still made algebra mistakes half the time, just because of all the eye-boggling formulas dancing in front of me (and no, I'm not dyslexic, just clumsy). My TI-82 turned me into an excellent and functional engineer. And I got an even better calc later.

So there. :-)

 

[mr200backstrok]

Xezlec-

I have been trying to understand volts for the last year. Reading through multitudes of definitions (including one thread for about 45 minutes just now) did not do for me what your description did in about 45 seconds.

one question: do the electrons actually travel faster? or is it just that more of them flow? if the latter is true, how fast do electrons flow?

 

[Xezlec]

Xezlec-

I have been trying to understand volts for the last year. Reading through multitudes of definitions (including one thread for about 45 minutes just now) did not do for me what your description did in about 45 seconds.

That's one of the nicest things anyone has ever said to me.

In school I was constantly annoyed by professors and books that explained things using terminology no one could possibly understand unless they were already an expert. I guess I try really hard not to do that myself.

one question: do the electrons actually travel faster? or is it just that more of them flow?

DISCLAIMER: Remember that electrons are really obeying quantum principles, so they aren't really just particles, yadda yadda. But assuming you just want to discuss the subject using the "classical approximation" (pretending that electrons are little charged balls that bounce around, rather than probabilistic wave functions), I'll try to give a reasonable answer. No purchase necessary. Void where prohibited.

That's an interesting question. I would say, sort of both.

Electrons are constantly bouncing into atoms and into each other, so they're always flying around in random directions, even when no current is flowing at all. When no current is flowing, the average velocity over all the electrons is 0. This just means that for every electron heading south, there's probably another one heading north. They all have their own little agendas but it all adds up to nothing. Exactly like Congress.

When a current is flowing, it just means that the electrons are heading in one direction more often than other directions. So they have an "average velocity" in one direction. There's a little more to it than that, but you can probably picture what I'm saying. This average velocity is often called "electron drift velocity" by people with college degrees in stuff.

So, yes, a larger current means that when they go in that direction, the individual electrons are more likely to be physically moving faster. And yes, it also means more of them are likely to be heading in that direction in the first place.

if the latter is true, how fast do electrons flow?

My dad (a biologist, so forgive him) once told me he thought electrons all moved at the same speed, nearly the speed of light. This is totally wrong. The actual speed of individual electrons depends on all kinds of things, mainly temperature and the type of material they are flowing in. They move pretty quick, but most of their speed is wasted flying around in circles and loops and bouncing off of things.

Drift velocities actually turn out to be much slower than you might expect! http://www.amasci.com/miscon/speed.html" gives a value of 3 inches per hour in a typical 100 Watt light bulb situation! Of course, when you consider how many trillions of electrons are flowing at that rate, you realize it's still a significant amount of current.

This might leave you wondering how the light turns on so fast when you flip the switch. Actually, the electromagnetic wave that transmits the "urge to move" to the electrons throughout the wire does move at nearly the speed of light, even though the electrons themselves move at a ludicrously slow average pace. The details of this can get complicated, and my reply is already too long. I'll just stop here for now.

Check out http://en.wikipedia.org/wiki/Drift_velocity" or search for "drift velocity", "electron mobility", "charge carrier density", and things like that online.

 

[Pathway]

A simple way to understand voltage is just to look at its units.

1 Volt = 1 Joule / 1 Coulomb.

This means that if there is 1 Volt of potential, that means each Coulumb of charge has 1 Joule of potential energy. So you can think of voltage as telling you each charged particle has a certain potential energy. It's like your voltage tells you the particles are on the top of a hill--you can roll each particle down the hill to convert that potential energy each charged particle has into other forms of energy.

 

[triggernum5]

The way modern outlets work in Canada (and I believe the USA) goes like this.. 220V potential comes into a breaker in the form of 1 +110V pole, and 1 -110V pole.. With GND at 0V.. Large appliances that run on 220V get connected to both both poles, thus the potential difference is 220V.. Smaller appliances that run on 110V however are connected to 1 pole, and GND.. On a 110V circuit only 1 prong is 'hot'.. If the home is wired properly then the skinny prong is hot, and the fatter one should be as neutral as the third prong..
Obviously that's dumbed down and looks like DC.. The poles as I badly referred to them are 60 Hz AC voltage sources identical in every way except the waves are 180° (0.5 wavelengths) out of phase with each other..

 

[russ_watters]

There is no positive or negative in AC, since the two 110V "hot" wires are simply 180 degrees out of phase with each other.

 

[Voltage]

I'm saying nothing.

 

[triggernum5]

If you pick a moment in time to define the potential of the AC sources relative to ground then for the sake of explanation WRT ground there is positive/negative.. Just important to realize that the 2 'poles' sinusoidally oscilate peak (+110V) to valley (-110) every ~0.01s or so on a 220V breaker.. Only reason I replied was to clarify the way north american houses are wired since the thread was implying 2 'hot' prongs when I first read it.. I spent enough time arguing about how AC should be explained to novices back in electronics class..

 

[LHarriger]

This might leave you wondering how the light turns on so fast when you flip the switch. Actually, the electromagnetic wave that transmits the "urge to move" to the electrons throughout the wire does move at nearly the speed of light, even though the electrons themselves move at a ludicrously slow average pace. The details of this can get complicated, and my reply is already too long. I'll just stop here for now.

Here's one quick way to explain how the EM field moves so quickly through the wire. First, suppose the field is not the same everywhere inside the wire. Since the field does the "pushing" this means there will be different current flow rates at different points in the wire. As a result, charge will begin to build up at spots in the wire. This built up charge will create its own field. Now draw a diagram of a segment of wire with a built up region of positive charge. This positive charge will create a field pointing away from it. Hence, incoming current is "slowed" while outgoing current is "sped up." We now have a self correcting mechanism for both propogating and smoothing out the field in a wire. Woohooo

 

[heafnerj]

Another name for voltage is potential. In other words, potential energy. It is like being on top of a hill and current is like rolling balls off the top of the hill.

Ok not quite, but does someone have a better analogy?

UGH! *Potential* and *potential energy* are most definitely not the same thing! They are indeed related, but they are not the same thing. Potential difference is the *difference in electric potential* between two points. Fundamentally, potential difference is calculated by taking the line integral of the electric field that exists in the region of space where our two points of interest lie.

As for the term *voltage*, it is imprecise and should be eliminated from the vocabulary. I do not permit my students to use it as it conveys nothing concrete.

 

[barton]

Great question! I've been trying to figure out the same thing for myself. All of circuit analysis seems to be built on this idea of "voltage", but very little explanation is offered to what it truly is.

In electrostatics, voltage is defined as the amount of joules of energy you would get when a small test charge from point A (at a high potential), moves to point B (at a lower potential). The small test charge gains X amount of Joules of kinetic energy, depending on the difference in potential and the amount of test charge. The difference in potential is measured in Joules/Coloumb

In electric circuits, voltage is defined similarly, as the amount of joules of energy you would get when a certain amount of charge moves from one wire (at X volts) to your ground.

Both seem to be a very useful energy bookkeeping systems.

What really bothers me though, is that there seems to be no real explanation of what creates voltage in a circuit. In electrostatics, you have the electric field creating the voltage difference between your two points. Within the wires of an electric circuit, what is there?

Be weary of people who use Ohm's law as the explanation. Ohm's law gives you the experimentally derived correlation between voltage and current in a conductor, not the cause of voltage.

 

[Xezlec]

Great question! I've been trying to figure out the same thing for myself. All of circuit analysis seems to be built on this idea of "voltage", but very little explanation is offered to what it truly is.

I hope your question was adequately answered in the other posts in this thread, in particular the fluid pressure analogy, which is an accurate and useful analogy. If not, feel free to refine your question.

What really bothers me though, is that there seems to be no real explanation of what creates voltage in a circuit. In electrostatics, you have the electric field creating the voltage difference between your two points. Within the wires of an electric circuit, what is there?

I wouldn't say that an electric field "creates" a voltage. Rather, voltage and electric field are just two different quantities that help us describe the physical situation. In electrostatics, an electric field and a voltage are just different ways of looking at the same thing. Field is the derivative of voltage and voltage is the integral of field.

Be weary of people who use Ohm's law as the explanation. Ohm's law gives you the experimentally derived correlation between voltage and current in a conductor, not the cause of voltage.

If you insist on thinking in terms of "cause and effect" (a mistake if you really want to study physics, in my experience), then you can think of voltage (or electric field, or whatever terms you want to speak in) pushing electrons through a wire. You can think of a voltage source as a black box that "somehow" maintains a constant voltage between its terminals, and a current source as a black box that "somehow" sets the voltage between its terminals to whatever value is needed to produce the desired current.

Finally, you can think of a resistor as something which physically obstructs the flow of electrons so that it takes more pressure (voltage) to push them through at a desired rate. Therefore, a current source trying to maintain a current I will have to raise the voltage to V=IR in order to get that amount of current to flow through the resistor.

Were you asking how the sources work? There are a lot of different kinds of circuits, and a lot of different ways to build voltage and current sources. You can read about the mechanisms of individual sources if you want to know how they work. Were you asking how Ohm's Law works? That one is more complicated, but can also be explained if you want. Though that one should be easily understandable using the fluid analogy discussed earlier.

 

[barton]

I understand the fluid pressure analogy from the earlier posts, but what I'm looking for is a true answer, not an analogy. The reason why is because I just finished college with a degree in Electrical Engineering, however, my understand of circuit analysis seems to be built on a shaky understanding of what actually goes on inside the wires of a circuit. This really limits both my ability and confidence in analyzing high frequency circuits,where normal electronic assumptions break down (such as that a wire is always at the same potential everywhere).

Other EE majors don't seem to care the way I do, they just want to be able to know how to use spoon fed formulas so they can make $$ from their employers.


So do you really think that there is some sort of "electron pressure" that builds up in a wire, which is where potential difference measurement comes from?

 

[Xezlec]

I understand the fluid pressure analogy from the earlier posts, but what I'm looking for is a true answer, not an analogy.

So do you really think that there is some sort of "electron pressure" that builds up in a wire, which is where potential difference measurement comes from?

Yes, there is an electron pressure. The electrons repel each other. The negative terminal of a battery is very slightly negatively charged and the positive terminal is very slightly positively charged. If you give the electrons a path to flow between those two, they will be repelled from the negative terminal and attracted to the positive terminal.

The really true explanation of how things work requires quantum physics and Fermi levels, and the details were too much for my brain when I took that class. Nonetheless, this isn't relevant considering your next remark:

The reason why is because I just finished college with a degree in Electrical Engineering, however, my understand of circuit analysis seems to be built on a shaky understanding of what actually goes on inside the wires of a circuit. This really limits both my ability and confidence in analyzing high frequency circuits,where normal electronic assumptions break down (such as that a wire is always at the same potential everywhere).

I'm shocked that you got a (Bachelor's?) degree and didn't learn the underlying physics behind electronic circuits. Where I went, it was a requirement. We had to take quite a few physics classes for that reason. Also, analyzing high-frequency circuits does not require anything beyond the normal assumptions. The assumption that a wire is the same potential everywhere was only made for the first semester of the first year when I went to school. After that it was very quickly pointed out that all conductors have some small but nonzero resistance, capacitance, and inductance. We were also required to learn how to analyze circuits where those facts matter. Do you mind if I ask where you went to school?

Anyway, high-frequency electronics are a whole field of study. I don't think you can expect to just "pick it up" from a forum like this. I'd recommend a good book on engineering electromagnetics, and maybe some better books on electric circuits before that.

That said, the gist of it is that once a wire becomes "long" (relative to the wavelength of the electromagnetic waves you're sending through it), you have to start thinking of it as a "transmission line". That's a very specific technical phrase that is worth googling. A transmission line is a wire where the resistance, capacitance, and inductance of the wire itself are a Big Deal, so you have to look at it like this:

          R      L           R      L
... ----/\/\/---%%%---+----/\/\/---%%%---+--- ...
                      |                  |
                      = C                = C
                      |                  |
                      V                  V

...where each R-L-C represents one little segment of wire. If you make these segments infinitely small and use infinitely many of them (which you should be able to do using ordinary calculus), you have a model that is accurate for both low and high frequencies.

The R/L/C model for analysis of circuits correctly accounts for all of Maxwell's equations as they apply to these situations. And Maxwell's equations are all that there is to know until you are forced to get down into quantum stuff, which only happens in unusual circumstances (such as those in semiconductor devices).

 

[cabraham]

Great question! I've been trying to figure out the same thing for myself. All of circuit analysis seems to be built on this idea of "voltage", but very little explanation is offered to what it truly is.

In electrostatics, voltage is defined as the amount of joules of energy you would get when a small test charge from point A (at a high potential), moves to point B (at a lower potential). The small test charge gains X amount of Joules of kinetic energy, depending on the difference in potential and the amount of test charge. The difference in potential is measured in Joules/Coloumb

In electric circuits, voltage is defined similarly, as the amount of joules of energy you would get when a certain amount of charge moves from one wire (at X volts) to your ground.

Both seem to be a very useful energy bookkeeping systems.

What really bothers me though, is that there seems to be no real explanation of what creates voltage in a circuit. In electrostatics, you have the electric field creating the voltage difference between your two points. Within the wires of an electric circuit, what is there?

Be weary of people who use Ohm's law as the explanation. Ohm's law gives you the experimentally derived correlation between voltage and current in a conductor, not the cause of voltage.

Circuit theory is built on "charge", not current or voltage. Charge is the most fundamental quantity, as I and V are both defined in terms of charge.

Well, what "causes" voltage? That is like asking "what causes energy, or where does energy come from?" Voltage in and of itself is not a particle, or matter. It is a mathematical ratio defined for the purpose of quantifying electric/magnetic behavior. The best answer to the question as to what voltage is is the following.

The line integral along a specific path from a to b, of the electric field wrt distance is voltage. In plain terms, the work done per unit charge moving said charge from a to b along a specified path is the voltage from a to b.

It is a ratio, a mathematical construct used for keeping track of circuit behavior. Like wise with current. The base quantity for all electric phenomena is **charge**. A volt is by definition 1 joule / coulomb. An amp is 1 coulomb / second. This is referenced to charge and coulombs.

It takes some studying to get a good grip on it. For anyone who wants a deeper understanding of circuit theory, the only advice I can give is to seriously study field theory. The study of e/m fields takes the student to a deeper level of understanding, then of course, a wall is hit. What gives charges their inherent Coulomb force, what is the reason for magnetic force, etc. are questions we cannot at the present moment answer.

Claude

 

[barton]

Rutgers University, which surprisingly did not have Electromagnetic Fields and Waves as a requirement for graduation, nor any classes on transmission line theory.

What evidence do you have for your "electron pressure" theory? Can you reference any papers or books?

 

[Dadface]

Voltage is a measure of how much energy is given to each coulomb of charge You pay the power companies for the energy you use not how many electrons that flow.

 

[barton]

How do you give energy to a coulomb of charge?

 

[Dadface]

By elecrical attraction-repulsion,an electron will be attracted to the positive side and repelled from the negative side.

 

[barton]

Xezlec, I found proof for your theory. See: Preyer, "Surface charges and fields of simple circuits", 1999 <http://galaxy.cofc.edu/pubs/AJP01002.pdf> , specifically section IV.B.

 

[Phrak]

Isn't there a voltage in space even when there aren't any wires right there?