To the Ineffable all,

While reading some of the questions and answers in this forum, I deduce that there seems to be a bit of confusion about just what voltage is.

I disagree, most people here try to give their idea of voltage which they through experience have found to explain it better. Most of the times, when someone asks for a clear definition of voltage, they want to know of something they can relate to in everyday life(well, everyday life of an electronic geek).

Electrical charges come in two flavors, positive and negative. Two or more like charges do not like to get together. They repel one another. To gather them together, energy has to be applied to the charges. So now the charges have this accumulated potential energy (P.E.)that was applied to the system to force the charges together. Now we can build up the same potential energy by bringing many charges somewhat close together, or we can bring few charges very close together. In both cases, we can have the same P.E. involved, but different voltages. Why is that? Well, the reason is that voltage is the energy density of the charge. In other words, it is the P.E. associated with the charges, divided by the number of charges involved. In MKS units, that is joules/coulomb. So voltage is not P.E., but it is proportional the P.E. associated with the charge. Twice the P.E., twice the voltage. When charge flows, thereby causing current to exist through a resistance, it dissipates its P.E. in the form of heat, and the voltage (energy density per charge) drops.

However, that is one great definition/explanation of voltage IMHO. Most of the times its good to start back from basics than to develop on analogies to get a better understanding of a concept.
I recall learning something similar in 9th standard (perhaps known commonly as 9th grade out there.) I never had trouble understanding those, as a matter of fact I am more comfortable with this definition. As a matter of fact I never came across the water analogy.(actually I have more trouble relating head,pressure,velocity head etc.).
So I'll add your contribution is very much appriciated , thanks.

Edit: I was wondering if voltage is not a vector then how come the sum of voltage in three phase circuits equal zero?
how come a voltage leads or lags the other by 120 degree?

 

Edit: I was wondering if voltage is not a vector then how come the sum of voltage in three phase circuits equal zero?
how come a voltage leads or lags the other by 120 degree?

Now we're getting something truly useful out of this. Voltage itself is not a vector quantity. What this means is that at any point in time voltage has some value only. As opposed to something like velocity that has a magnitude and direction.

The reason that a 3 phase system adds to zero voltage is that at any point in time, each phase has some voltage on it, but because of the phase relations in time, they always cancel each other out.

Phasors describe sinusoids in a steady state situation, but don't discuss exact points in time.

 

Caveman,

I'm not a mathematician, therefore I am happy with that.

OK so you are. But you said "A vector to a mathematician is a matrix with only one column or only one row." I do not think a mathematician would be happy with that.

Still got to define zero. That's two points. It's all relative to a reference.

Yes, voltage is measured with respect to a reference. But I was talking about a gradient, which is defined at one point. Just like a derivative is.

Any text that uses pressure to explain voltage is not necessarily wrong.

How can you explain something with defining it? And physical pressure is surely not the same as the density of electrical energy per charge.

Physics is optional in many programs. You sometimes must explain it even simpler, like by analogy.

Still, most folks know what energy is even without physics training. They have to know what energy is, otherwise they would not know what the power dissipation of a resistor means.

No one has said that your statement of a definition of voltage is wrong, just that there are others that are equally valid,

I think that my definition is more direct and simple than the other definitions. Ratch

 

thingmaker3,

I concur with Caveman. Defining "Voltage" as "Joules per Coulomb" is indeed accurate. But so is defining "Voltage" as "Amps per Ohm."

Voltage is NOT amps per ohm. Perhaps you mean amps times ohms. But even then you would be wrong, because voltage can be present between two points without any amps being present. Now if you think of voltage as the energy density of a charge, then amps and ohms would not enter into the definition and you would not be confused.

The real problem behind this little communications glitch is the definition of "definition." I suggest you look it up, Ratch.

The problem is not communication. It is acceptance and understanding. Ratch

 

At the risk of feeling this thread is nothing more than a cursory discussion with little direction, could you please explain (with examples) where there has been some factually inaccurate misconceptions of what voltage is?

Dave

 

recca02,

I disagree, most people here try to give their idea of voltage which they through experience have found to explain it better. Most of the times, when someone asks for a clear definition of voltage, they want to know of something they can relate to in everyday life(well, everyday life of an electronic geek).

They try, but they don't always succeed.

However, that is one great definition/explanation of voltage IMHO. Most of the times its good to start back from basics than to develop on analogies to get a better understanding of a concept.
I recall learning something similar in 9th standard (perhaps known commonly as 9th grade out there.) I never had trouble understanding those, as a matter of fact I am more comfortable with this definition. As a matter of fact I never came across the water analogy.(actually I have more trouble relating head,pressure,velocity head etc.).
So I'll add your contribution is very much appriciated , thanks.

Thank you, I appreciate your gratitude.

I was wondering if voltage is not a vector then how come the sum of voltage in three phase circuits equal zero?
how come a voltage leads or lags the other by 120 degree?

Those are phasors you are talking about--having magnitude and phase. They have some vector like properties like addition/subtraction, but they are not vectors. Ratch

 

Dave,

At the risk of feeling this thread is nothing more than a cursory discussion with little direction, could you please explain (with examples) where there has been some factually inaccurate misconceptions of what voltage is?

Are you asking me? You only have to look at the very first posting of this thread to answer your question. Ratch

 

Dave,

Are you asking me? You only have to look at the very first posting of this thread to answer your question. Ratch

Yes I am. There are no examples of where there are problems in peoples existing explanations. People use the hydraulic analogy because it provides a simple physical analogy that allows new students to this subject to conceptualise the notions and interrelationships of voltage and current in circuit applications. It is not intended to explain the concepts in exact terms, not least that once you get beyond resistive circuits it doesn't work.

The implication of this thread was there was "confusion about just what voltage is", so I ask for examples of this. Otherwise the premise of this "discussion" is based around semantic analysis, and hence this is not really an electronics discussion at all.

Dave

 

m4yh3m,

I hope this thread doesn't confuse any of the neophytes...

It shouldn't. Matter of fact, it should clear things up. Only those old and set in their ways might have difficulties.

with all the "redefining" of words with each post, some of the readers might not take it with a grain of salt like the veterans, especially if they want to try and be "up to speed" with what they might assume to be "today's standards".

What words have been redefined? Concepts have been examined and evaluated.

I say dump it into the off topic area

But this thread is on topic. Voltage is just about as "on topic" as you can get. Ratch

 

beenthere.

The above is contradictory.

In what way?

Of course voltage is not energy.

Correct.

It is a measure of potential energy.

Tilt! No, it is proportional to the potential energy of the charge, but it is not potential energy or a measure of potential energy.

Or is there something in your statement that implies that a difference in potential between two points (a voltage difference) will not cause current charge to flow in a conductor that joins those points?

As long as there is a conduction path between two points of different voltages, charge will flow. Nothing I said contradicts that.

If voltage is not a measure of PE, then how do you explain the measurement of electrical power being defines as Volts times Current? If voltage can't measure energy, then it can't be used to measure power.

Good question, easily answered. V x I = joules/charge x charge/time = joules/time = power

If you want to measure energy, then energy = V x Q = joules/charge x charge = joules = energy

Ratch

 

I will bring it to your attention that some questions have been posed and not answered. The next question is: why should this topic remain open?

 

Volts = Joules/Coulomb = Amp-Ohms = Newton-meter per Amp-seconds = kg*m^2/(A*s^3).

All of this is true. The last one is the actual SI definition of a volt.

This describes the point. Why don't you just "go back to basics" with the units? Because it makes no bloody sense! That's why! You try to simplify to something that does.

A kid looks at a battery and a resistor and you tell him that the 1.5V on the battery is the potential energy per charge on that battery, he glazes over. So you give him some more direction. "Let's say we have one electric charge on the positive terminal and we put a resistor in the way. It has potential energy like when a ball is up high and it has energy that it can lose by falling. It will convert its potential energy to heat by going through the resistor, then reach the negative terminal of the battery." So the kid scratches his head and asks, "Then it's done? That charge just sits there?" You say, "No, it gets pushed up by the chemical reaction of the battery to the positive terminal and goes again." The kid scratches his head, and says, "Oh! It's kind of like a water pump and it is pumping current through the wire." "Well no, it isn't. Pressure is a vector not a scalar like voltage."

Give me a break. It works as an analogy. He got the idea.

 

Dave,

The implication of this thread was there was "confusion about just what voltage is", so I ask for examples of this. The implication of this thread was there was "confusion about just what voltage is", so I ask for examples of this.

I am thinking of all the times I have run across posts in threads here and elsewhere where I felt that voltage was not understood or explained wrong. It is too much work to review all that I read, so I will only give two examples.

http://forum.allaboutcircuits.com/showthread.php?t=10025 ,post #1

This thread, post #18, see answer #24

Otherwise the premise of this "discussion" is based around semantic analysis, and hence this is not really an electronics discussion at all.

If we talk about voltage, we are talking about electronics. Ratch

 

I can't believe I didn't look this up before, guess what?

Pressure is a scalar quantity!

Not only that pressure can be defined as Energy Density. In this case it is defined as Potential Energy per unit volume.

This is very analogous to voltage as Potential Energy per unit charge. Now do you agree the model is valid?

 

caveman,

This describes the point. Why don't you just "go back to basics" with the units? Because it makes no bloody sense! That's why! You try to simplify to something that does.

Because I am trying to describe something qualitatively, not quantitatively. I am concerned with a electrical concept, not the units that describe it. I only use the common units that everyone if familar with, not the SI equivalency cooked up by a standards committee.

A kid looks at a battery and a resistor and you tell him that the 1.5V on the battery is the potential energy per charge on that battery, he glazes over. So you give him some more direction. "Let's say we have one electric charge on the positive terminal and we put a resistor in the way. It has potential energy like when a ball is up high and it has energy that it can lose by falling. It will convert its potential energy to heat by going through the resistor, then reach the negative terminal of the battery." So the kid scratches his head and asks, "Then it's done? That charge just sits there?" You say, "No, it gets pushed up by the chemical reaction of the battery to the positive terminal and goes again." The kid scratches his head, and says, "Oh! It's kind of like a water pump and it is pumping current through the wire." "Well no, it isn't. Pressure is a vector not a scalar like voltage."

I would say, "look kid, we are going to make a electrical circuit which will move some charge through this resistor. The charge will not move by itself, because the resistor will try to impede the charge flow. This battery will supply the energy to circulate the charge through the resister and the battery, thereby enabling an electrical circuit to exist. This battery is 1.5 volts, and it will have enough power to support a finite amount of current for this value of resistance. If I had a 6 volt battery which has 4 times the voltage, I would be able to push 4 times the current through the circuit. Or if I reduced the resistance by one-half, I would have twice as much charge flowing. If you want, I can show you how to calculate the current you can expect if you know the value of the resistor and the voltage of the battery. Now, lets hook things up and see if the resistor gets warm. Do you want to know why that happens?"

Get the idea? All it takes is a little imagination. Ratch

 

For those of you who say that the water analogy falls apart after resistors and capacitors, I guess you've never heard water pipes "hammer" when the washing machine valves cut off. This is precisely analogous to what happens when current is cut off in a resistive/inductive/capacitive circuit. The "hammering" occurs due to the LC time. It decays due to the resistance of the water flowing in the pipes.

If you suggest there is no water analogy to a transistor, there are some sailors who may disagree with you. There are bow thrusters, used for steering ships' bows (the pointy end) in tight quarters which are an analogy to giant "water transistors". I couldn't find a photograph of the exact type I was looking for, but here's a picture of a similar type:

Water is pumped in by an impeller from the bottom. Water jets on either side at the "Y" control whether the main flow out is to port or starboard (left or right). This is directly analogous to a transistor with two bases and two emitters that could be selected by application of current to one or the other base; a small current controlling a larger current.

 

caveman,

I can't believe I didn't look this up before, guess what?

Pressure is a scalar quantity!

No, not really. See below.

Not only that pressure can be defined as Energy Density. In this case it is defined as Potential Energy per unit volume.

PV is well known in piston physics and gas equations in chemistry. But force per area does not completely cancel out the volume multiplier, so the numerator is transformed from a force which is a vector into a energy unit which is not. That is not the same as (energy/charge) x charge which completely cancels out the denominator and thereby allows voltage to be called a energy charge density. Ratch

 

I can't prove to you that I am right because you won't believe me, but:

The Department of Physics and Astronomy at Georgia State University disagree with you: http://hyperphysics.phy-astr.gsu.edu/Hbase/press.html#ed

They also happen to agree with the water analogy.
http://hyperphysics.phy-astr.gsu.edu/Hbase/electric/watcir.html#c2

Nasa also disagrees with you. In fact they have a section on it:
http://www.grc.nasa.gov/WWW/K-12/airplane/pressure.html

Here is an excerpt from a thermodynamics book that mentions it:
http://www.lightandmatter.com/html_books/0sn/ch05/ch05.html

Pressure is a scalar. And I'm not the only one that thinks so.

 

Dave,

I am thinking of all the times I have run across posts in threads here and elsewhere where I felt that voltage was not understood or explained wrong. It is too much work to review all that I read, so I will only give two examples.

http://forum.allaboutcircuits.com/showthread.php?t=10025 ,post #1

This thread, post #18, see answer #24

For your examples: the linked example post #1 is a question asked by someone who didn't understand something (what one would expect from an OP in a thread). The answers provided discussing voltage were valid to that question - unless tere is some distinct limitation of the advice offered that you wish to share? The other example doesn't count as it came after you started this thread!

I think your OP approach this thread from the wrong angle, and I have yet to see a suitable debunking of other voltage models to suggest that any other model (including yours) is a superior way to visualise voltage from both a conceptual and practical perspective.

If we talk about voltage, we are talking about electronics. Ratch

Not when we are prattling around in the world of "you say pot-ay-to and I say pot-ah-to". Ok lets move on with the thread and see where we end up...

For those of you who say that the water analogy falls apart after resistors and capacitors, I guess you've never heard water pipes "hammer" when the washing machine valves cut off. This is precisely analogous to what happens when current is cut off in a resistive/inductive/capacitive circuit. The "hammering" occurs due to the LC time. It decays due to the resistance of the water flowing in the pipes.

If you suggest there is no water analogy to a transistor, there are some sailors who may disagree with you. There are bow thrusters, used for steering ships' bows (the pointy end) in tight quarters which are an analogy to giant "water transistors". I couldn't find a photograph of the exact type I was looking for, but here's a picture of a similar type:

Water is pumped in by an impeller from the bottom. Water jets on either side at the "Y" control whether the main flow out is to port or starboard (left or right). This is directly analogous to a transistor with two bases and two emitters that could be selected by application of current to one or the other base; a small current controlling a larger current.

Nice one Wook. I'm not a hydraulics man, but it is interesting to see the RLC and BJT equivalent. I suppose the point with the water analogy is that is a visual simplification that helps novices understand the basics. Would it be any easier explaining to someone the functioning of a BJT or a bow-thruster?

Obviously there are limitations to the water analogy - I would be surprised if there were a FET equivalent, or any other component that relies on action at a distance as a control mechanism.

Dave

 

Caveman,

Pressure is a scalar. And I'm not the only one that thinks so.

OK, now I am beginning to see the light. It is a matter of perspective. I searched the net and came up with the following

*1) The pressure at a given depth is independent of direction -- it is the same in all directions. This is another statement of the fact that pressure is not a vector and thus has no direction associated with it when it is not in contact with some surface.

*2) The pressure on a submerged object is always perpendicular to the surface at each point on the surface.

OK, the first statement gives an explanation of why they do not consider pressure to be a vector. I can see why pressure not acting on anything does not have a direction to be a vector.

Now the second statement specifies a direction (perpendicular) if there is an object for pressure to act upon. And using vector calculus to find the total force on a submerged object would require pressure to be thought of as a vector. Otherwise, one would not know the direction of the force.

So it is a matter of perspective and we each are half right and half wrong. Ratch