Understanding voltage



I don't remember. Maybe I was thinking of the Whitestone Bridge.
Bill
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wrote:

http://www.redrivergorgearches.com /
Volume 1, White's Branch Arch
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On Fri, 03 Oct 2008 00:06:46 -0700, ValleyGirl

My posts are electrical.
John
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On Fri, 03 Oct 2008 08:27:02 -0700, John Larkin

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On Fri, 03 Oct 2008 11:13:02 -0500, John Fields

Some posts are connected in parallel, and some posts are cereal.
John
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On Fri, 03 Oct 2008 10:10:11 -0700, John Larkin

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Cereal posts? That doesn't make a grain of sense.

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snipped-for-privacy@austininstruments.com says...

Back to the wheatstone bridge?
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Keith

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John Fields wrote:

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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.
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Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com
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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" ;)
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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) = I**2 R (RCA) Solving I = 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.
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----------------------------

Voltage is work (or energy) per unit charge required to move a unit charge from a to b in an electrical field. An electrical field is produced by the presence of other charges. Think of mechanical potential energy per unit mass required to move a unit mass from point a to b in a gravitational field. A gravitational field is produced by other masses.
The two are analogous. In both cases it doesn't matter what path you take from a to b.
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Don Kelly snipped-for-privacy@shawcross.ca
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I kinda forgot to answer your questions:
Since you said you understand current, you do realize that I = Q/t? and that if Q goes up and t goes down in proportion then I doesn't change? The same thing is going on with voltage.
Analogy: If you have 10 N weight lifted up 3 feet then it has the same potential energy as a 5 N weight lifted up 6 feet? (ok, not exactly since the gravitational field is a bit weaker but close enough)
Remember that voltage isn't a fundamental quantity but is a function of more than one.
V = J/C is one expression of V but it's also V = W/A as it is A*Ohm. (we get the last two from ohms law)
I think you really need to think about it more. Take 2 C of charge and place them at some distance apart, say it has 10 joules of energy, if you now move them apart you increase the energy to maybe 20 joules? Or equivalently, but with less information, we have went from 5V to 20V.
It is exactly analogous with many other physical quantities that depend on more than one thing. If I have 80 C moving past a point in 2 second then that is 40A but so is 40 C moving past a point in 1 second. I could also have one electron moving past that point in 1/40C of a second and it would also be equivalent to 40A.
Also if I reduce the time I move the 80C then from 2 seconds to 1 second(if I slow them down) then I cut the current. In fact if I "freeze" everything, even though I have 80C of charge sitting there, I have no current at all!
"The difference in voltage measured when moving from point A to point B is equal to the work which would have to be done, per unit charge, against the electric field to move the charge from A to B. "
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elevol.html
Note that it is a "per-unit" quantity that depends only on the distance in the electric field. It doesn't depend on the amount of charge which is why it is being divided out.
Think of it as energy per unit charge. Current gives us the other half of the equation. If we know the energy per unit charge, or the voltage difference, and we know the charge per second(along some path), or the current, then if we multiply them we have
Energy/charge * charge/time = energy/time = power = work/time
i.e., V*I = P
If we had some idea of the time involved we could get the energy too.
If you want a microscopic concept then it is the energy contained per electron in the electric field... (do you understand the electric field?)
The macroscopic concept is one of "force" or some "ability to do work"... note that it is a mixed up concept because it's not fundamentally correct but it is understanding by "consequence". (kinda like understanding anything we have to associate it with things we know)
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wrote:

Just like the electric field is force between charges divided by charge, electric potential (ie, voltage) is potential energy divided by charge.
You aren't having trouble with voltage, though. You are having issues with energy. What is it? Think about that for a bit before continuing...
My answer is that it is the potential to move something. When you hold a hunk of matter above the ground, it has the potential to start accelerating when you let go of it. Thus, it has potential energy, given to it by the attraction of gravity. How much? It depends on how high you hold it.
Same thing for voltage. It is the potential to move charged particles around (ie, create a current).

Just like the attraction of gravity gives a hunk of matter different 'potential energy' at different altitudes above the ground. So, to get the hunk of matter up there, somebody had to give it some energy.
To get 10 volts out of 2 coulombs, you need to put in 20 joules of energy to separate the charges.

Voltage, as used in electronics, is a relative measure. You pick some place in the circuit, and say "that is ground", meaning that is where you measure the rest of the voltages from. Then, take a particle, like an electron, and integrate the force it takes you to move it to some other place in the circuit with respect to distance. Thankfully, it doesn't matter how you go, any path will do. Now, divide by the charge of the particle. That number will equal the voltage at the destination point, relative to the ground of the circuit.
In physics, there are actually two potential fields, the electrostatic potential, and the vector potential. Those fields, the first a scalar field, and the second a vector field, influence how a charge will move. A charge will move along the gradient of the electrostatic field, and a moving charge will turn to align its motion with the vector potential.
Regards, bob monsen
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if you picture current as the rate of flow of water in a pipe, voltage is the water pressure.

in the water analogy: Joules / Cubic meter IOW: pascals

it has it by being displaced by a from where it would like to rest the further it is displaced the higher the voltage.

different pressures, with 10kPa you can spin a turbine nicely, but same flow rate with only 10Pa behind can do very little work.
Bye. Jasen
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That's making it too bloody complicated. Absolutely basic that: Voltage (Or EMF, electro motive force, or potential or whatever you want to call it) is the pressure that can push an electric current through a circuit. The source of the voltage can be various devices, such as a battery, a generator or a storage device such as capacitor. 'Voltages' can be DC (Direct current) or AC (Alternating current). Take a pencil and draw a square to represent 'the source'. Then draw a circuit from and external to the source comprising wires (which have virtually no resistance in most practical applications) and a load (which could be say a single heating resistor of R ohms). Electric current (amps) will flow in the above circuit. The higher the voltage the greater the current that will traverse the circuit. The formula; Ohm's Law is Voltage/Circuit Resistance = Current flow. A practical example being 230 volts, a 20 ohm resistor, and a resulting current flow of 230/20 = 11.5 amps If you want to get into the amount of power (watts, or watts per hour) how many coulombs of energy are being transferred you can make further calculations. But the above is basic. PS. Working in telecommunications for some 40 years we once had a boss who was an 'Industrial Engineer'. We (experienced subordinates) always gave him a hard time saying "Well who can expect an Industrial Engineer to understand electricity with more than one frequency!" So congratulations to the OP on wanting to understand electricity.
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There's a fascinating bit of philosophy hidden in this inquiry. Voltage and charge are two completely different kinds of physical quantities, and the distinction between these is repeated in many different ways over all disciplines (which is why so many analogies are offered when the question comes up).
Voltage is an example of an intensive quantity. Charge is an example of an extensive quantity. If you consider a system (like, let's take a battery/bulb flashlight), the voltage of that battery is an intensive quantity, and the charge that the battery can deliver is an extensive quantity. Double the flashlight, and there are two batteries and two bulbs,twice the charge, but the voltage is the same. Double the dimensions of the flashlight, the bigger battery has eight times the charge, but the voltage is STILL the same.
Extensive quantities include mass, charge, cost of a bag of potatoes. Intensive quantities include density, voltage, cost per pound of potatoes.
Voltage, in particular, is the ratio of two extensive quantities, stored electrical energy and stored electric charge, in the sense of taking a derivative of energy with respect to charge. Just like the cost per pound of potatoes, it's intensive.
The implications of this include another check you can perform on equations: you can't add or equate intensive and extensive quantities, just like quantities with different units.
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