Thread: Voltage and Water pressure analogy question

I'm having difficulty conceptually understanding voltage.

Having read the search query, I've noticed many comparisons between voltage and water pressure.

The question that immediately comes to mind then; if increasing pressure in a hose increases the speed of current, then are electrons traveling faster through a wire when voltage is increased and current and resistance remain constant?

Yes. The electrons themselves still move pretty slowly though.

Edit: Reading this more closely, no that wasn't correct. The current will increase if voltage increases and resistance stays the same, so your question describes an impossible situation.

Hi can I join in too?(I always found electricity hard as well)

If the voltage is increased from 100 volts to 200 volts and the current is passing through a length of copper wire,say 1 metre in length and 1 millimetre in radius What would be the increase in speed of the electrons individually ?

I suppose there would be a very large number of electrons filing through at a slowish rate ?-like an extremely wide queue of shoppers trying to get into a football stadium?

So speed itself would be low but the amount of electrons moving a centimetre ,say per second would be very high?

Myself, I'm not sure. I'm thinking that cross-sectional density of electrons is not constant regardless of current.

In both electronics and hydraulics, more current means more items passing a certain point in a conductor/conduit. Do we know that a wire is saturated with electrons regardless of the current — even zero current?

On the other hand, not to confuse the issue, there's skin effect, where high voltage sends the electrons (all repelling each other) to the surface of the conductor, so you have electrons pretty much flowing near its surface instead of throughout its cross section as one might intuitively think.

Originally Posted by jrmonroe

...
On the other hand, not to confuse the issue, there's skin effect, where high voltage sends the electrons (all repelling each other) to the surface of the conductor, so you have electrons pretty much flowing near its surface instead of throughout its cross section as one might intuitively think.

ergo stranded wire (more surface area) carries higher voltage with less heat loss?

Drift velocity - Wikipedia, the free encyclopediaThe formula for evaluating the drift velocity of charge carriers in a material of constant cross-sectional area is given by:^[1]

where v is the drift velocity of electrons, I is the current flowing through the material, n is the charge-carrier density, A is the area of cross-section of the material and q is the charge on the charge-carrier.

Originally Posted by sculptor

Originally Posted by jrmonroe

...
On the other hand, not to confuse the issue, there's skin effect, where high voltage sends the electrons (all repelling each other) to the surface of the conductor, so you have electrons pretty much flowing near its surface instead of throughout its cross section as one might intuitively think.

ergo stranded wire (more surface area) carries higher voltage with less heat loss

The conductance is proportional to cross-sectional area, not surface area, to a close approximation for ordinary applications. Skin effect does become important at high frequency.
Stranded wire is more flexible and works better with crimp type connectors which is why it's used.
Electrical resistance and conductance - Wikipedia, the free encyclopedia
Skin effect - Wikipedia, the free encyclopedia

Originally Posted by jrmonroe

On the other hand, not to confuse the issue, there's skin effect, where high voltage sends the electrons (all repelling each other) to the surface of the conductor, so you have electrons pretty much flowing near its surface instead of throughout its cross section as one might intuitively think.

Ach! You have confused the issue by unnecessarily bringing up the skin effect (which is largely irrelevant to the OP), and then providing a very badly mangled explanation. Skin effect has nothing whatever to do with voltages, high or otherwise. It has everything to do with Faraday's law of induction, however.

Since you brought it up, let's see what skin effect actually is, and how it arises: Let an isolated conductor carry a time-varying current (that's the key part). That time-varying current will generate a time-varying magnetic field. A time-varying magnetic field, in turn, induces a current (just as in a classic transformer). If one traces how that induced current is distributed through the conductor ("right-hand rule" and all that), one finds that the induced current opposes the original current in the interior of the wire, but reinforces it near the surface. So, the current distribution is altered from a uniform one at DC, to a nonuniform one when the conductor carries alternating current. The higher the frequency, the more pronounced the effect.

Originally Posted by rayznack

The question that immediately comes to mind then; if increasing pressure in a hose increases the speed of current, then are electrons traveling faster through a wire when voltage is increased and current and resistance remain constant?

You've described a situation that is almost never encountered in practice. If we are talking about "normal" regimes in which the drift velocity of the electrons is small compared to the thermal velocity, then the conductor will obey Ohm's law. In that case, an increase in voltage will necessarily result in an increase in current if the resistance is constant.

An increase in current will, indeed, imply an increase in the drift velocity of electrons. If you actually compute what that velocity is for typical conductors, you will obtain numbers that typically surprise first-timers to the calculation. The velocities are much, much, much lower than c, for example. They are typically much lower than the speed of a human on a stroll.

Mein Gott! I have embarrassed myself. I was remembering — trying to — what a power generation and distribution professor taught us many years ago. He said the electrons traveled near the surface because "like charges" repel. I didn't know if "skin effect" was the right term, but it seemed so. I am somewhat antiquated. He also confused us by talking about the conductor sizes by bird names, such as two Sparrows equal a Cardinal, three Cardinals equal a Bluebird, or whatever. Ach du Lieber, Professor!

You are probably thinking about the principle where a charged conductive sphere has the charges residing entirely on the surface.

Originally Posted by tk421

Originally Posted by rayznack

The question that immediately comes to mind then; if increasing pressure in a hose increases the speed of current, then are electrons traveling faster through a wire when voltage is increased and current and resistance remain constant?

You've described a situation that is almost never encountered in practice. If we are talking about "normal" regimes in which the drift velocity of the electrons is small compared to the thermal velocity, then the conductor will obey Ohm's law. In that case, an increase in voltage will necessarily result in an increase in current if the resistance is constant.

An increase in current will, indeed, imply an increase in the drift velocity of electrons. If you actually compute what that velocity is for typical conductors, you will obtain numbers that typically surprise first-timers to the calculation. The velocities are much, much, much lower than c, for example. They are typically much lower than the speed of a human on a stroll.

I suppose I should have said keeping current constant and increasing resistance.

But in that case, is Ohm's Law analogous to Bernoulli's principle?

Eg., Keeping voltage constant, and increasing resistance, reduces the amount of electron flow, but increases their velocity?

Edit: I came across this which reinforces my confusion attempting to conceptually understand voltage:

http://openstudy.com/updates/507ae1d7e4b035d93135aed1

Originally Posted by rayznack

I suppose I should have said keeping current constant and increasing resistance.

If you keep current constant and vary resistance, the voltage will vary.

Eg., Keeping voltage constant, and increasing resistance, reduces the amount of electron flow, but increases their velocity?

Constant voltage + increasing resistance = less current flow. Their velocity should stay the same (same voltage potential to accelerate them) but the overall flow will decrease (i.e. number of electrons per second.)

Originally Posted by rayznack

I suppose I should have said keeping current constant and increasing resistance.

But in that case, is Ohm's Law analogous to Bernoulli's principle?

Eg., Keeping voltage constant, and increasing resistance, reduces the amount of electron flow, but increases their velocity?

You're inconsistent in setting your conditions, so it's hard to figure out what your actual question is. In the first line above, you have constant I, and increasing R. Then two lines later, you have V constant and increasing R. These are two different problem statements.

But no matter. Let me give you an equation that will allow you to answer your own question, once you've decided what your question actually is.

At low field strengths, the drift velocity of carriers is proportional to the electric field (the proportionality constant is conventionally called the mobility, but that's not important here). Mobility is material-dependent. That is, vel = mu*E, where mu is mobility.

Next, for a simple arrangement (e.g, wire diameter and composition constant throughout), the electric field will be constant at a value equal to the total voltage divided by the wire length: E = V/length.

Combining these two, we get vel = mu*V/length.

So let's assume that you are keeping the voltage constant, while increasing R. If you increase R by increasing the length, then the electric field will go down (same voltage, greater length = smaller E field). If mobility is fixed, then the velocity will go down.

If you increase resistance by choosing a more resistive (lower mobility) material, but keep the length constant (meaning E is constant), then the velocity will also go down.

Etc. Hope that helps.

(As for Bernoulli, I recommend not spending too much time worrying about analogies, since there are too many cases where the analogies fail. Keeping track of the exceptions is more work than simply learning about the phenomenon of interest directly.)

Edit: I came across this which reinforces my confusion attempting to conceptually understand voltage:

http://openstudy.com/updates/507ae1d7e4b035d93135aed1

I'd stay away from online exchanges like that. Not all who offer an opinion are able to offer a good one.

Forget Bernoulli's principle in the analogy. That is due to the fact that the flowing water has kinetic energy, whereas electrons have such low mass that the kinetic energy is negligible.
Bernoulli's principle - Wikipedia, the free encyclopedia
If you raise the water pressure, the flow will increase, just like raising the voltage in Ohm's law. Think of raising the water level in a tank with a hole in the side. As the water level increases, the pressure increases and water flows faster out the hole.
Raising the resistance while keeping flow constant would be analogous to reducing the size of the hole in the tank. To keep the flow constant through a smaller hole, you'd have to increase the velocity. The only way to do this is increase the pressure.

Originally Posted by billvon

Originally Posted by rayznack

I suppose I should have said keeping current constant and increasing resistance.

If you keep current constant and vary resistance, the voltage will vary.

Eg., Keeping voltage constant, and increasing resistance, reduces the amount of electron flow, but increases their velocity?

Constant voltage + increasing resistance = less current flow. Their velocity should stay the same (same voltage potential to accelerate them) but the overall flow will decrease (i.e. number of electrons per second.)

Yes, that's where the analogy falls apart, but where I become confused.

I don't understand how constant electron speed, higher resistance and current reduction results in constant voltage.

The analogy with water pressure would result in greater speed of water flowing from the nozzle if you reduce flow (increase resistance).

So how is increased resistance able to maintain voltage w/o changing the speed of the electrons while also reducing current?

Originally Posted by rayznack

Originally Posted by billvon

Originally Posted by rayznack

I suppose I should have said keeping current constant and increasing resistance.

If you keep current constant and vary resistance, the voltage will vary.

Eg., Keeping voltage constant, and increasing resistance, reduces the amount of electron flow, but increases their velocity?

Constant voltage + increasing resistance = less current flow. Their velocity should stay the same (same voltage potential to accelerate them) but the overall flow will decrease (i.e. number of electrons per second.)

Yes, that's where the analogy falls apart, but where I become confused.

I don't understand how constant electron speed, higher resistance and current reduction results in constant voltage.

The analogy with water pressure would result in greater speed of water flowing from the nozzle if you reduce flow (increase resistance).

So how is increased resistance able to maintain voltage w/o changing the speed of the electrons while also reducing current?

Spend a few moments studying my answer. Your questions are all taken care of there.

Originally Posted by rayznack

Yes, that's where the analogy falls apart, but where I become confused.

I don't understand how constant electron speed, higher resistance and current reduction results in constant voltage.

V=IR. Lower current and higher resistance results in the same potential - and the potential is what drives electron speed.

The analogy with water pressure would result in greater speed of water flowing from the nozzle if you reduce flow (increase resistance).

Here's where the analogy starts to fall apart.

So how is increased resistance able to maintain voltage w/o changing the speed of the electrons while also reducing current?

V=IR, so voltage (which is what drives electrons) remains the same.

Originally Posted by billvon

Originally Posted by rayznack

Yes, that's where the analogy falls apart, but where I become confused.

I don't understand how constant electron speed, higher resistance and current reduction results in constant voltage.

V=IR. Lower current and higher resistance results in the same potential - and the potential is what drives electron speed.

The analogy with water pressure would result in greater speed of water flowing from the nozzle if you reduce flow (increase resistance).

Here's where the analogy starts to fall apart.

So how is increased resistance able to maintain voltage w/o changing the speed of the electrons while also reducing current?

V=IR, so voltage (which is what drives electrons) remains the same.

What I'm not understanding is why Resistance and Voltage are proportional *if* higher resistance does not result in increased electron speed at constant Voltage.

It's like closing the nozzle of a garden hose to restrict flow and the water comes out at the same speed regardless.

Originally Posted by rayznack

Originally Posted by billvon

Originally Posted by rayznack

Yes, that's where the analogy falls apart, but where I become confused.

I don't understand how constant electron speed, higher resistance and current reduction results in constant voltage.

V=IR. Lower current and higher resistance results in the same potential - and the potential is what drives electron speed.

The analogy with water pressure would result in greater speed of water flowing from the nozzle if you reduce flow (increase resistance).

Here's where the analogy starts to fall apart.

So how is increased resistance able to maintain voltage w/o changing the speed of the electrons while also reducing current?

V=IR, so voltage (which is what drives electrons) remains the same.

What I'm not understanding is why Resistance and Voltage are proportional *if* higher resistance does not result in increased electron speed at constant Voltage.

It's like closing the nozzle of a garden hose to restrict flow and the water comes out at the same speed regardless.

Try to forget about the speed of the water and think about the volume. If you have a hose pipe of a certain crossectional area (conductor size) fed from a tap at a certain pressure (voltage) then that will result in a certain volume of water (current) passing through the hose. Now if you reduce the size of the hose (resistance) and the water pressure is constant then there will be a reduced volume of water passing through the hose. So if you want to increase the volume of water (current flow) you have two options, increase the pressure (voltage) or increase the hose pipe size (reduce resistance). Now there are a couple of things to consider, one is if the hose pipe is too small and the pressure too great then it can burst!

The problem with analogies like the my example above, is that they are nothing more than a visual representation. In reality things are quite a bit more complex and there are many other factors to consider. Most of which have been covered by some excellent replies in this thread!

Originally Posted by rayznack

What I'm not understanding is why Resistance and Voltage are proportional *if* higher resistance does not result in increased electron speed at constant Voltage.

If you insist on the water analogy, so be it. Just don't complain when you reach a point where the analogy hurts more than it helps.

If voltage is analogous to pressure, then maintaining constant flow (current, in gallons per minute) should logically require a higher pressure if you increase the resistance. I don't see the problem here.

It's like closing the nozzle of a garden hose to restrict flow and the water comes out at the same speed regardless.

First, current is not the same as speed (did you even bother to read what I wrote? It's a bit rude not to even acknowledge it). Current, here, is gallons per minute.

But that turns out to be a diversion. Your analysis is not even correct within the domain of the water analogy. If you restrict the nozzle of the hose, you have raised the resistance. To maintain the same gallons per minute flow, you will most certainly have to increase the pressure (voltage). Again, where is the problem?

Originally Posted by rayznack

What I'm not understanding is why Resistance and Voltage are proportional *if* higher resistance does not result in increased electron speed at constant Voltage.

It's like closing the nozzle of a garden hose to restrict flow and the water comes out at the same speed regardless.

Because wires are not quite like hoses. If they were identical, then as you restricted flow the speed of the water would remain the same; the flow would just decrease.

Using pressure and volume to understand the concepts of electricity works on basics. But does getting more electrons through a wire serve any real perpose if no work is getting done? If you concidered a river a mile wide turning a paddle wheel at 5 revolutions per minutes you could measure some horsepower. Now take a river 10 feet wide and you are turning a paddlewheel under a 40 foot water fall (voltage) turning a paddlewheel 200 rpm measure the horsepower. Higher voltage is very advantagious in getting work done. Amperage creates restrictions (wire sizing). Ohms is the work you need to get done.

Originally Posted by keeseguy

Using pressure and volume to understand the concepts of electricity works on basics. But does getting more electrons through a wire serve any real perpose if no work is getting done? If you concidered a river a mile wide turning a paddle wheel at 5 revolutions per minutes you could measure some horsepower. Now take a river 10 feet wide and you are turning a paddlewheel under a 40 foot water fall (voltage) turning a paddlewheel 200 rpm measure the horsepower. Higher voltage is very advantagious in getting work done. Amperage creates restrictions (wire sizing). Ohms is the work you need to get done.

No. Most of what you've written is "not even wrong," I'm sorry to say.

Ohms is not work. We do have, for DC circuits, the relation P = VI. Notice the absence of ohms. So, volts and amps are what gets work done. Not ohms.

In most of the the discussion so far (being basic ohms law referenced) it appeared that flow and resistance to flow being discussed I introduced the concept of power (watts law). To say resistance doesn't liken to work could be argued as untrue, since resistance is what needs to be overcome before voltage can push current. Watts law and power need to be realized to understand why pressure and voltage are not interchangable in the understanding of electricity and water.

Originally Posted by keeseguy

In most of the the discussion so far (being basic ohms law referenced) it appeared that flow and resistance to flow being discussed I introduced the concept of power (watts law). To say resistance doesn't liken to work could be argued as untrue, since resistance is what needs to be overcome before voltage can push current. Watts law and power need to be realized to understand why pressure and voltage are not interchangable in the understanding of electricity and water.

Well you do have resistive power loss of course, which is i²R. But this is not work, it is power loss as heat, which is something else, as we know from thermodynamics. So yes the overcoming of resistance requires power to be expended, but this is not doing work.

Originally Posted by keeseguy

Using pressure and volume to understand the concepts of electricity works on basics. But does getting more electrons through a wire serve any real perpose if no work is getting done?

Yes. MRI magnets for example, or power line distribution.

If you concidered a river a mile wide turning a paddle wheel at 5 revolutions per minutes you could measure some horsepower.

Not unless there was some resistance to the motion.

Now take a river 10 feet wide and you are turning a paddlewheel under a 40 foot water fall (voltage) turning a paddlewheel 200 rpm measure the horsepower. Higher voltage is very advantagious in getting work done. Amperage creates restrictions (wire sizing). Ohms is the work you need to get done.

Higher voltage increases potential. Amperage does not create restriction; resistance does. Work can be done by electric motors, which use current to create magnetic fields, then do work with those fields. Although electric motors have resistance the resistance does NOT have anything to do with the work being done.

Originally Posted by keeseguy

In most of the the discussion so far (being basic ohms law referenced) it appeared that flow and resistance to flow being discussed I introduced the concept of power (watts law). To say resistance doesn't liken to work could be argued as untrue, since resistance is what needs to be overcome before voltage can push current. Watts law and power need to be realized to understand why pressure and voltage are not interchangable in the understanding of electricity and water.

The energy delivered by a voltage or current source is an explicit, definite quantity. Some of that energy goes into heat (in the wire, as well as in the source), and some is delivered to the load.

Pressure and voltage are perfectly interchangeable in the understanding of electricity in terms of a water analogy, provided that you properly circumscribe the domain of applicability of the analogy, and take care to state your assumptions explicitly. Your attempts at elucidation are based on a faulty understanding, so I'm afraid that you've only muddied the waters further. Conflating resistance with work is just plain wrong. Please don't argue this point further.

Originally Posted by billvon

Higher voltage increases potential. Amperage does not create restriction; resistance does. Work can be done by electric motors, which use current to create magnetic fields, then do work with those fields. Although electric motors have resistance the resistance does NOT have anything to do with the work being done.

Disagree. In the real-world everyday electrical conductor carrying current, that being considered to be described by quantity of electrons, the resistance is not CONSTANT, but varies to some degree by the QUANTITY of electrons involved, the intensity of the current flow if you will, thus Amperage level causes, resistance to change due to the heating effect, no?

Why in the world is everyone so concerned with the speed of the electrons involved, anyway?? jocular

Originally Posted by jocular

Disagree. In the real-world everyday electrical conductor carrying current, that being considered to be described by quantity of electrons, the resistance is not CONSTANT, but varies to some degree by the QUANTITY of electrons involved, the intensity of the current flow if you will, thus Amperage level causes, resistance to change due to the heating effect, no?

Amperage does not change resistance. Other things that are a result of amperage (like heating, or arcing, or melting) can - but again, that's not a result of current, that's a result of heat added. You could do the same thing by putting the conductor in an oven.

Why in the world is everyone so concerned with the speed of the electrons involved, anyway?? jocular

We are conditioned to think that fast=higher energy due to our experience in the world of mechanics.

[QUOTE=jocular;514866]

Originally Posted by billvon

Disagree. In the real-world everyday electrical conductor carrying current, that being considered to be described by quantity of electrons, the resistance is not CONSTANT,

I disagree with your disagreement. In real-world conductors, the resistance does not vary until the electron velocity approaches thermal velocities. Until you get to that extreme regime, the resistance of a typical metal is exceptionally well-approximated as constant. If it were not so, then your stereos would distort horribly. The everyday observation of low distortion tells us that conductors obey Ohm's law very, very, very well.

but varies to some degree by the QUANTITY of electrons involved, the intensity of the current flow if you will, thus Amperage level causes, resistance to change due to the heating effect, no?

Here you're talking about a second-order effect. If you're going to enlarge the discussion to include every possible effect, then we'll lose sight of the OP's question. Are you proposing to consider things like physical strains (which can have larger effects than thermal effects in some materials)?

Why in the world is everyone so concerned with the speed of the electrons involved, anyway?? jocular

I don't know about "so" concerned. But electron velocity matters to separate the very regime you are discussing (one in which resistance varies with current), and the ordinary, "Ohmic," regime, in which resistance does not vary.

Don't confuse first-order dependencies with second-order ones.

Forgive my remarks earlier which were in accurate, perhaps I have been out of the classroom too long. A few points have been made which I must have missed or just misinterpreted. Is velocity a variable? I thought amperage was a volume. Also I thought I read earlier about the skin effect of conductors, this is not heat producing but does effect electron flow, right? I have had the garden hose discussions to explain electricity in the past, but, I have always felt it muddied the waters when you deal with A/C, induction, neutral wire issues, 2 hot wire single and 3 phase.

Originally Posted by keeseguy

Also I thought I read earlier about the skin effect of conductors, this is not heat producing but does effect electron flow, right?

It increases resistance by reducing the amount of copper cross-section available to carry current. It is frequency dependent.

Originally Posted by keeseguy

Forgive my remarks earlier which were in accurate, perhaps I have been out of the classroom too long. A few points have been made which I must have missed or just misinterpreted. Is velocity a variable? I thought amperage was a volume. Also I thought I read earlier about the skin effect of conductors, this is not heat producing but does effect electron flow, right? I have had the garden hose discussions to explain electricity in the past, but, I have always felt it muddied the waters when you deal with A/C, induction, neutral wire issues, 2 hot wire single and 3 phase.

There is an equation for fluid flow, known as the Hagen-Poiseuille equation, that is precisely isomorphic to Ohm's law. Pressure <--> voltage; flow rate <--> current; viscosity <--> resistance. [As long as viscosity is independent of flow (as it will be in the laminar flow regime), that is.] So in that case, the analogy works extremely well.

Amperage is not a volume. It is a flow rate (e.g., so-and-so coulombs per second). Velocity is very much a component of flow rate. Since current depends on other parameters, so too does velocity.

Skin effect is heat-producing in the same sense that all resistance is heat-producing. As billvon says, skin effect depends on frequency. Higher frequency results in less "skin", and thus higher resistance (and, therefore, more heat for the same current). The skin effect is normally presented as a radio-frequency problem, but it also constrains the design of power lines. There's no reason to make conductors very much thicker than about 1cm at 50-60Hz, because the added interior metal would just add weight, but carry little additional current.

And as to how far one should take the garden hose analogy, I agree with you that its limitations make it unsuited for more sophisticated situations, even if one could devise rigorously correct explanations. Better to spend that effort on the situation directly, rather than on an analogy. After all, how hard is V = IR, really?

Originally Posted by tk421

I disagree with your disagreement. In real-world conductors, the resistance does not vary until the electron velocity approaches thermal velocities. Until you get to that extreme regime, the resistance of a typical metal is exceptionally well-approximated as constant. If it were not so, then your stereos would distort horribly. The everyday observation of low distortion tells us that conductors obey Ohm's law very, very, very well.


Here you're talking about a second-order effect. If you're going to enlarge the discussion to include every possible effect, then we'll lose sight of the OP's question. Are you proposing to consider things like physical strains (which can have larger effects than thermal effects in some materials)?

Why in the world is everyone so concerned with the speed of the electrons involved, anyway?? jocular

I don't know about "so" concerned. But electron velocity matters to separate the very regime you are discussing (one in which resistance varies with current), and the ordinary, "Ohmic," regime, in which resistance does not vary.

Don't confuse first-order dependencies with second-order ones.

Possible to disagree with the disagreement of a disagree? It appears that what you imply means I squared R losses, which present as wattage dissipated, do not produce heat. Sure, in well-designed applications, those losses are not sufficient to significantly increase conductor temperature. The point I'm making is that in a given "everyday" electrical system having conductors and termination points, connections if you prefer them called, often local resistances become present which most definitely increase in temperature, often to extreme degree. From the practical standpoint, localized heat can, and often does, create temperatures high enough to cause chemical change to the conducting medium's structure, oxidation for example, which increases resistance at that location, which then in turn increases dissipated local heat, which then.....and so on, a "snowball" effect seen very frequently.

Why has use of aluminum conductor material been banned from use in mobile homes?

It seems you use high-falutin' terminology which essentially aims toward agreement with what I proposed, though they are on differing levels of consideration. At any rate, Ohm's Law is most definitely a most applicable tool, one fact which seems agreed upon. jocular

Originally Posted by tk421

Amperage is not a volume. It is a flow rate (e.g., so-and-so coulombs per second).

Please explain how a given number of electrons "moving" past some point per unit time are not a volume of electrons. jocular

Originally Posted by jocular

Why has use of aluminum conductor material been banned from use in mobile homes?

Because it is difficult to terminate well. Poor terminations increase resistance. The increased resistance generates more heat, which then causes frequent expansion and contraction, which in turn loosens the connection further. High temperatures can also cause local alloys (steel/aluminum) to form; these are less conductive.

Copper/aluminum terminations are even worse since the dissimilar metals involved cause rapid corrosion.

However, properly terminated aluminum wires work well (and are common in longer runs where cost is an issue.)

Originally Posted by jocular

Please explain how a given number of electrons "moving" past some point per unit time are not a volume of electrons. jocular

10 gallons a minute flow through a hose into a bucket. What is the volume of water in the bucket?

Originally Posted by billvon

Originally Posted by keeseguy

Also I thought I read earlier about the skin effect of conductors, this is not heat producing but does effect electron flow, right?

It increases resistance by reducing the amount of copper cross-section available to carry current. It is frequency dependent.

A paradox for you to solve for us all now! "Skin effect" is frequency dependent, as you state, thus explaining that as frequency increases (of an A.C.), the conductor's "current density" across it's cross-section becomes higher as distance from the center of the conductor increases. So, kind of, at extremely high frequencies, almost no current flow exists in the center of the cross-section, correct?

How then, does an excess of electrons present as "static charge" exist on the surface of a conductive material when the "frequency" is zero? Faraday Effect.

Please also tell us why current density becomes higher toward the surface of a conductor as voltage becomes very high. Conductors used for power transmission are most often not solid, but rather tubing, hollow, to save weight and cost, since little current flow would exist in the center, if it were solid. jocular

Originally Posted by jocular

Originally Posted by tk421

Amperage is not a volume. It is a flow rate (e.g., so-and-so coulombs per second).

Please explain how a given number of electrons "moving" past some point per unit time are not a volume of electrons. jocular

Please explain how it is. I don't understand how you could possibly equate a velocity-related quantity with a static one. It doesn't make any kind of sense at all. It's rather like asking how a mile per hour is not a pound.

Originally Posted by jocular

Originally Posted by billvon

Originally Posted by keeseguy

Also I thought I read earlier about the skin effect of conductors, this is not heat producing but does effect electron flow, right?

It increases resistance by reducing the amount of copper cross-section available to carry current. It is frequency dependent.

A paradox for you to solve for us all now! "Skin effect" is frequency dependent, as you state, thus explaining that as frequency increases (of an A.C.), the conductor's "current density" across it's cross-section becomes higher as distance from the center of the conductor increases. So, kind of, at extremely high frequencies, almost no current flow exists in the center of the cross-section, correct?

Yes. That's not a paradox, though.

How then, does an excess of electrons present as "static charge" exist on the surface of a conductive material when the "frequency" is zero? Faraday Effect.

That's not a paradox, joc. I don't know why you think it is. The skin effect has to do with alternating current. You are now presenting a totally different scenario with the word "static" explicitly signaling to you that it's different.

So let's break this down into a few parts, logically speaking.

First, that the skin effect refers to current preferentially flowing on the surface does not logically preclude charges on the surface in a static scenario. Again, I don't know why you think they do.

In the static situation, the wire is simply acting as one terminal of a capacitor. There is no current flow.

Please also tell us why current density becomes higher toward the surface of a conductor as voltage becomes very high. Conductors used for power transmission are most often not solid, but rather tubing, hollow, to save weight and cost, since little current flow would exist in the center, if it were solid. jocular

You've again conflated two things in a muddled way. As I've already posted several times in connection with skin effect, conductors used for power transmission are sized precisely in acknowledgment of skin effect. A skin depth of about 1cm at power line frequencies means that a solid conductor that is substantially thicker buys little resistance reduction, but costs greatly in weight. To get more skin without more weight, one may use tubular conductors. The increase in surface current density in power lines is not due to high voltage. If anything, it's due to the magnetic field induced by the current. I explained the skin effect in some detail a few posts back in this thread, I think.

Originally Posted by jocular

Originally Posted by tk421

Amperage is not a volume. It is a flow rate (e.g., so-and-so coulombs per second).

Please explain how a given number of electrons "moving" past some point per unit time are not a volume of electrons. jocular

Amps relate to rate of flow, that is volume per unit time. The electrical analogue of volume is electric charge, measured in coulombs. Amps are coulombs per second.

Originally Posted by jocular

Originally Posted by tk421

I disagree with your disagreement. In real-world conductors, the resistance does not vary until the electron velocity approaches thermal velocities. Until you get to that extreme regime, the resistance of a typical metal is exceptionally well-approximated as constant. If it were not so, then your stereos would distort horribly. The everyday observation of low distortion tells us that conductors obey Ohm's law very, very, very well.


Here you're talking about a second-order effect. If you're going to enlarge the discussion to include every possible effect, then we'll lose sight of the OP's question. Are you proposing to consider things like physical strains (which can have larger effects than thermal effects in some materials)?

Why in the world is everyone so concerned with the speed of the electrons involved, anyway?? jocular

I don't know about "so" concerned. But electron velocity matters to separate the very regime you are discussing (one in which resistance varies with current), and the ordinary, "Ohmic," regime, in which resistance does not vary.

Don't confuse first-order dependencies with second-order ones.

Possible to disagree with the disagreement of a disagree? It appears that what you imply means I squared R losses, which present as wattage dissipated, do not produce heat.

Joc, I'm sorry but I'm having a tough time with you because you combine stubbornness with a lack of logic. Pleaes focus. My statements are quite clear and correct.

Let me repeat: The resistance of an ordinary conductor -- like copper -- is, for practical purposes -- not a function of current. If you disagree, simply present what that function is. You will also have to explain how, despite the nonlinearity that a current-dependent resistance would present, a stereo amplifier does not produce horrible distortion. If you do not understand why distortion is a necessary consequence of your proposed current-dependent resistance, post back and I will explain it to you.

Next: You seem to be under the impression that the mere presence of heating somehow supports your claim. It does nothing of the kind. Heating is just a sign that there exists some resistance, which is not in question. You then somehow leap to the conclusion that the presence of heating also means that the resistance is changing. Think carefully about that chain of reasoning. If you do, you will see that the conclusion does not follow at all logically from the premise.

Sure, in well-designed applications, those losses are not sufficient to significantly increase conductor temperature. The point I'm making is that in a given "everyday" electrical system having conductors and termination points, connections if you prefer them called, often local resistances become present which most definitely increase in temperature, often to extreme degree. From the practical standpoint, localized heat can, and often does, create temperatures high enough to cause chemical change to the conducting medium's structure, oxidation for example, which increases resistance at that location, which then in turn increases dissipated local heat, which then.....and so on, a "snowball" effect seen very frequently.

All well and true, but quite off the point of the OP. You've moved the goalposts several times here, with the result of muddling the issues quite a bit. No one is saying that heating effects don't exist. No one is saying that metals have no resistance change with temperature. But to say that metals therefore have a current-dependent resistance is to confuse the OP. You cannot write, for example, an equation that relates resistance change to velocity, without taking into account a thermal model. What that tells you is that the velocity sensitiivity isn't fundamental, which is the whole context of the OP. You might as well argue that cosmic rays also affect resistance. That's true, too, but an unenlightening diversion from the main point.

Why has use of aluminum conductor material been banned from use in mobile homes?

Everyone knows the answer (and it's not just mobile homes). What the blazes does that have to do with the OP? You've gone a little bonkers, joc. The banning of aluminum has nothing to do with resistance variation with current. It has everything to do with the resistance of aluminum oxide.

It seems you use high-falutin' terminology which essentially aims toward agreement with what I proposed,

No. See the foregoing. We are not in agreement.

though they are on differing levels of consideration. At any rate, Ohm's Law is most definitely a most applicable tool, one fact which seems agreed upon. jocular

You can't have it both ways. Ohm's law is a linear relationship between voltage and current, with resistance as a constant that is independent of both. You are proposing that the resistance is a function of current. Joc's law and Ohm's law are two totally different things.

Originally Posted by tk421

Please also tell us why current density becomes higher toward the surface of a conductor as voltage becomes very high. Conductors used for power transmission are most often not solid, but rather tubing, hollow, to save weight and cost, since little current flow would exist in the center, if it were solid. jocular

You've again conflated two things in a muddled way. As I've already posted several times in connection with skin effect, conductors used for power transmission are sized precisely in acknowledgment of skin effect. A skin depth of about 1cm at power line frequencies means that a solid conductor that is substantially thicker buys little resistance reduction, but costs greatly in weight. To get more skin without more weight, one may use tubular conductors. The increase in surface current density in power lines is not due to high voltage. If anything, it's due to the magnetic field induced by the current. I explained the skin effect in some detail a few posts back in this thread, I think.

You have stated above, in many more words and to greater extent, precisely the same thing I did, while denouncing my statement. Only one conclusion may be reached for that. I shan't argue that black is white with you. joc

Originally Posted by jocular

A paradox for you to solve for us all now! "Skin effect" is frequency dependent, as you state, thus explaining that as frequency increases (of an A.C.), the conductor's "current density" across it's cross-section becomes higher as distance from the center of the conductor increases. So, kind of, at extremely high frequencies, almost no current flow exists in the center of the cross-section, correct?

Right.

How then, does an excess of electrons present as "static charge" exist on the surface of a conductive material when the "frequency" is zero? Faraday Effect.

Excess electrons are present throughout a conductor in the case of a static charge. Capacitive effects tend to concentrate the charge near the surface when there is another conductor nearby with a positive charge. However, in a case where a conductive material is inside a container that is at the same potential, the charges will be more or less evenly distributed.

Please also tell us why current density becomes higher toward the surface of a conductor as voltage becomes very high.

Capacitive coupling (at DC) and skin effect (at AC.)

Conductors used for power transmission are most often not solid, but rather tubing, hollow, to save weight and cost, since little current flow would exist in the center, if it were solid. jocular

Depends on the frequency (higher = thinner skin depth) and the environment (underwater cables = more capacitive coupling, so more charge on the surface.) Most elevated transmission lines are solid since skin effect is not very deep (about 10mm) at 60Hz. That means you'd need a cable about an inch thick before skin depth became much of an issue.

Originally Posted by billvon

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Conductors used for power transmission are most often not solid, but rather tubing, hollow, to save weight and cost, since little current flow would exist in the center, if it were solid. jocular

Depends on the frequency (higher = thinner skin depth) and the environment (underwater cables = more capacitive coupling, so more charge on the surface.) Most elevated transmission lines are solid since skin effect is not very deep (about 10mm) at 60Hz. That means you'd need a cable about an inch thick before skin depth became much of an issue.

Thank you for this! The H.V. conductors carrying power from Hoover Dam are indeed hollow; they sell (or did, years ago, I bought a couple) two-inch long segments of the actual conductor used. The stuff is about 1-1/4-inch outside diameter, with a wall thickness of perhaps 3/16 of an inch. Curiously, it's not tubing per se., but rather narrow, interlocking segments of copper about 1/4 inch wide, by 3/16 thick, having a sort of tongue and groove interlock method. It was explained by the Tour Guides that this allowed the long spans to flex more easily in heavy winds, thus minimizing work-hardening (I suppose). Those Guides always amazed me with their knowledge and quick answers when asked quite technical questions.

I highly recommend the tour of that magnificent structure! joc