Understanding voltage

Voltage is *potential* energy. The most common analogy I've seen is with that of water pressure. Think of voltage as pressure, and current as flow. Resistance is opposition to, or restriction of current (e.g. with the water analogy, a smaller diameter pipe). That's why the greater the amount of resistance, the greater the voltage you'd measure across the resistance.

-Mike

This easily yields to experiment. Try this:

Grab one coulomb of positive charge in one hand, and one coulomb of negative charge in the other. Now draw your hands apart until they are separated by 1 meter. You'll note that the force trying to draw your hands together is about 9e9 Newtons (it helps to do this test in a universe where you are many times stronger than here, and where you can hold point charges in your hands -- there may be one in your Physics building, check with a prof).

Now as you drew your hands apart, you will have noted that you had to exert force to do so, and this force was exerted over some distance -- i.e. you performed work on the charges. Solving for energy = work * distance, you conclude that you have added energy to the charges.

HTH.

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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
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Have you looked at Wikipedia yet

You might find that entry helpful.

The energy is contained in the electric field, more-or-less by definition: the 'field' is a region in which forces act on charges. But if you seek a physical explanation of why there should be forces at all then you may never find a clear, simple explanation, as with gravitation.

Chris

What I can't understand is how a person can be in third year engineering and not have a strong understanding of what voltage is.

3rd year _mechanical_ engineering. And he's working to fill in the gaps, which is more than a lot of mechanical engineering students might do.

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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

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It's a big gap to fill and a strange place to come to fill it.

Note that everything you have learned about gravity can almost be directly applied to electricity. In fact the governing equations of EM are almost exactly the same as that of gravity if you could "remove" the polarity aspect. (but the consequences can be dramatically different)

Electrons have attraction and repulsion while "matter" has only attraction.

What causet he attraction/repulsion? It is a force. Gravity on one hand and the electrostatic force on the other.

What about potential energy? Same thing holds for charge. If you take two charges and bring them close together they will have some sort of potential energy... they will either attract or repell and that potential will be converted into kinetic energy just as in mechanics.

Now voltage is a measure of that attraction.

How do you know something has potential energy? You have to let act out on it. (there is no other way except through analysis but that came about from observation.

Voltage, or the electric potential(vs the mechanical potential) is a really a difference in potential energy.

So suppose we have +Q C at (-1, 0) and -Q C at (1, 0), they will attract each other and have forces on them. this would be simiar case to M kg and M kg but the magnitudes of hte forces would be different.

This attraction gives rise to a potential and the potential difference is precisely the voltage. (in the right units for charge)

People tend to speak of voltage as if it were a force(such as electromotive force which has the same units as voltage) but it is not a force just as mechanical potential isn't a force... but it can be used to create a force.

You hopefully know that the gravitational force can have an associated potential with it(the mechanical potential). The same is done with the electric force. Since the forces are conservative we know by mathematics that there is a scalar field who's gradient is the force. It's much easier to work with a scalar field and it's called the potential.

In any case thats more theoretical.

What does it mean in practice?

If someone says that they have a capacitor with 10V "across" it what do they mean? It means they can do some work... and if they were smart they could compute just how much work. All you need ot know is that if there is a potential difference between two points, any two points, and you stick a wire at those two points(a conductor) then current will flow. If you have a lightbulb or led in series with that wire then it might light up... or you might be able to turn a wire.

The mere fact that there is a potential difference implies that you can do work and vice versa. (they are identical concepts as force but viewed from a different perspective)

It doesn't tell you have much work you can do and infact you might not be able to do any depending on the circumstances... but at least in theory you can do some work.

It also is related to current... because current flowing means there is a potential difference. (but not vice versa)

Analogy: A book on a table. The table is a resistance to the book "flowing" down to the ground. The book has potential due only to it's position w.r.t to the earth. If you remove the table the book will convert the potential(voltage) into kinetic energy(think of current) and when it hits the ground or something inbetween it would apply a force that can continue to do work on other things.

There is nothing special about voltage... it's just what we call the potential for electricity. If you understand the gravitational potential then you shouldn't have any problem if you just realize that the basic quanitities one is dealing with are analogous. current = mass flow, voltage = mechanical potential, force = force, electric field = gravitational field, etc..

What's more important is that you have some concept of magnitude of voltage... what is 10V? what is 1000V? Also helps to know something about current and what is 1A vs 100A, etc...

By having that kinda knowledge you'll have a better working understanding. It's similar to mass and energy. Everyone knows what 100lbs is about... or maybe even 1000lbs but most people don't know much about energy. Most people have a better concept of power than energy as they know their lightbulb is using maybe 100W. They still don't really have any clue what it means but they do know it is doing something(i.e. work).

And that's all this boils down too... voltage is a measure of work! Work is what is important! mass is useless if it can't do any work! current is useless if it can't do any work!! Current is a measure of charge in motion... which is usefull to determine how much work it can do.

So ultimately in all the things we are trying to do is to simply things to determine how much work something can do... by knowing that we know how much less work we have to do. But of course we can't always measure work directly... we don't have a special machine that we can ask how much work x is doing and it tells us. We have to break the problem down and learn how to measure it which involves measuring bits and pieces.

(I don't mean to sound dramatic about it but the fundamentals of physics is concerned with it)

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Nice job. Now he can go into fourth year. No wonder our bridges are falling down.

Even worse... they somehow get good grades, and after they get their degrees, some of them come to work at our place, and they STILL don't understand it. Then they pay them engineering salaries and let them design satellite gear!

Working at a high voltage power supply company did allow me to review, and lock in a lot of basics. That was a good eight year experience in my life.

Especially the "one-hand-in-the-pocket" rule. :-]

Not quite: Voltage is NOT potential energy nor is it kinetic or any other kind of energy. Voltage is NOT energy.

Your pressure analogy is a good one however. Keep in mind that pressure is not energy either.

Voltage in analogous to pressure or force sometimes called electro-motive force, emf.

Current is analogous to flow rate.

Charge is amount or quantity of electricity.

If you tell me where the potential energy goes when I lift a weight up in a gravitational field, I'll tell you where the energy goes when you separate two charges.

-- Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com

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Do you also "can\'t understand" why he may not have a strong
understanding of what a "parsec" is, or a "millihelen", or a zillion
other terms which aren\'t relevant to his discipline?

Instead of offering him help, as others have done, why do you ridicule
him for seeking knowledge when you should be commending him for the
courage he mustered in exposing his ignorance and asking for help before
the likes of you?

Perhaps because you have no help to give and are intent on pulling your
betters down in order to make your position on the food chain seem
higher than it actually is?

Some roses by any other name would smell as foul.

JF

Because it's mechanical engineering?

John

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At least I wasn't being cranky.

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Please...

You were being much worse than cranky, you were being intentionally
disingenuous.

JF

It might bring some comfort to you to know that the equation to find the amperes that a conductor can safely carry without overheating comes from Mechanical Engineering. It is the Fourier heat transfer equation. Mechanical engineers know a whole lot more about this than electrical engineers. The equation is (TC - TA) =3D I**2 R (RCA) Solving I =3D SRT((TC-TA)/(R*RCA)) I in amperes, TC is maximum conductor insulation temperature in degrees C, TA is ambient temperature in degrees C, R is dc resistance in ohms of conductor, RCA is thermal Resistance in thermal ohm feet. I square R is the heat generated in the conductor when I amperes flows through the conductor with resistance R in ohms. The amperes flow because of a potential difference in voltage that exist between conductors. Variations of this equation were used by Rosch in 1938 and by Msgs Neher and McGrath in 1957 to develop ampacity tables found in the National Electrical Code. This does not tell you what voltage is but it does put some rather elite electrical engineers that like to poke fun at mechanical engineers in their place.

...

When did one have to understand electricity to understand how to build a bridge?

I'd rather the guy know squat about electricity and be a great bridge builder than build shitty ass bridges cause he spent to much time trying to learn about electricity for some school requirements to "broaden his horizons". He could have spent that time more wisely.

It goes into the field! At least thats what field theorists say. They say the field is what has the energy... but of course they only say this because the that is how they interpret the field equations ;/ (has to do with the fact that potential energy depends only on the relative positions)

But to answer your question, when lift up a weight in a gravitational field you are supplying work, i.e. energy, to the weight giving it potential energy... you did that by first giving it kinetic energy to move it. So you have actually increased it's potential energy... hence it's not "where did the potential energy go" but "where did it come from" ;)

Mechanical engineers don't build bridges either. They do build automobiles and robots, though. Basic electricity would seem to be a useful thing for MEs. Basic physics is rather useful, and required, for EEs. MEs don't have to take the EM semester of physics?

Try a civil engineer if you want a bridge built. I'd rather my civil engineer had the full load of physics too. We *are* talking about basic electricity here.

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Keith