When does water analogy to electricy break down?

crutschow

Joined Mar 14, 2008
38,936
You can't get it backwards if it's the same thing. The both exist yin-yang as they rise out of charge. Field lines always exist from a point charge, it's only when another point charge is near that a difference in charge (voltage potential) between the two points exists.
(With apologies to RB, I will make one last comment to the 0.01%, since I fully realize I am not changing anyone's mind.) :rolleyes:

Of course the electron charge is the source of the E field. I should have more precisely said that the gradient in the electric field is a result of the externally applied voltage across the resistor. My point is that it's not necessary to invoke the E-field when discussing voltage, since the E-field is not used for calculating the current flow in simple circuits, only voltage. Voltage can be viewed as simple pressure on the electrons. The source of the pressure can be electro-chemical, as with a battery, or magnetic, as with a generator. Of course the electrons are kept apart by their own E-field, just as water molecules are.

I fully understand that the water analogy can only be applied to simple DC or AC circuits and I previously noted its limitations. I think it can be helpful at the start for newbies, without undue confusion when more advanced concepts are introduced. You don't. We can agree to disagree. ;)
 

GetDeviceInfo

Joined Jun 7, 2009
2,286
I'll throw in my 2 cents, as a Millwright and Electrician. I will occasionally use a hydraulic/electrical analogy to help mechanics understand basic electrical concepts, but it's typically contained to 'pressure = flow over resistance', and 'power in = power out'. I don't use an electrical analogy to explain mechanics to Electricians, cause they wouldn't get it anyways.
 

BreadCrum6

Joined Aug 17, 2011
18
The water analogy serves it's purpose just as finger counting does when introduced to basic math. In my opinion water analogy > conventional flow.
 

cabraham

Joined Oct 29, 2011
82
I didn't say it had to be massless (any more than electrons are required to be massless). It's just that it's not necessary to consider the mass of the water or kinetic energy in the analogy (as you implied in your previous post).
The problem is, you have it backwards. It's the voltage that generates the electric field not the field that generates the voltage. :rolleyes:

................................
I don't think so. Voltage is defined as an integral of the E field over a specified path. Voltage does not generate E fields. E fields are an inherent property to all charged bodies.

Claude
 
The problem is, you have it backwards. It's the voltage that generates the electric field not the field that generates the voltage.
From my little knowledge i know that voltage is a word that is defined to make our life easier. It is defined after constraining our self to work with components in which
1) rate of change of charge inside our component is zero
2) rate of change of magnetic flux outside our component is zero

Voltage do not produce electric field. Charge produce electric field. And to move a charge against an electric field require some "pressure". This "pressure" is voltage. V = W/q

This is all theoretical things i read in books. Maybe i am wrong. That are my inference from the texts.
 

THE_RB

Joined Feb 11, 2008
5,438
(With apologies to RB, I will make one last comment to the 0.01%, since I fully realize I am not changing anyone's mind.) :rolleyes:
...
No apologies needed to me, I wasn't offended in any way by your clear and well written explanations.

The reason for my exaggerated "dismay" is that this kind of thing happens a lot and does not really need to.

Here's my take; This is an electronics forum. We work with voltage, we measure it with a voltmeter. We work with conventional current flow, we measure that with ammeters or measuring voltage drops etc.

We DON'T work with the insides of components using electron microscopes examining holes. That's in physics, chemistry and semiconductor design. Separate field, we don't do that here any more than we do astrophysics or medicine.

Most of these arguments take a side, either the physics side of what happens at an atomic/subatomic level inside a bit of silicon, or the more practical electronics side of what happens on pinB when we apply 1v on pinA. The trouble is both sides can be mostly right on a subject but still be diametrically opposed, for the simplest example electron flow vs conventional current flow.

So if both sides are right, but opposed, and one side represents the topic of the forum *general electronics* then the right answer is the electronics answer; that current flows from + to -, no argument.

If someone wants to argue with that then go to the chemistry or physics forum, talk all you like about individual electrons there and stop scaring the electronics newbies. ;)
 

nsaspook

Joined Aug 27, 2009
16,515
We DON'T work with the insides of components using electron microscopes examining holes. That's in physics, chemistry and semiconductor design. Separate field, we don't do that here any more than we do astrophysics or medicine.

Some of us do work in physics, chemistry and semiconductor design and see the seperation of "electronics" from the basic physics of electricity as wrong becauses it's a synergy that requires a basic knowledge of the field to really understand circuit operation beyond anything other than cookie-cutter level even when EM waves as commonly thought of are not directly involved. To know that voltage and current are abstract simplications with very real limitations and what that limitations are enables a good tech the skills to see and fix the problems that others see as weird or strange.
 

JMW

Joined Nov 21, 2011
138
The water analogy is very good to use with non technical people, such as customers. Cabraham is correct on this point. We are Engineers/techs/hobbyists and need to be as exact as possible to describe what we are trying to do. The O.P. has already discovered its shortcomings.
 

crutschow

Joined Mar 14, 2008
38,936
I don't think so. Voltage is defined as an integral of the E field over a specified path. Voltage does not generate E fields. E fields are an inherent property to all charged bodies.
Of course it's the electron charge that generates the E field. But, as I noted in post #22 it's the voltage (pressure) applied to the electrons that generates the gradient in the E field. You don't generate the gradient to provide the voltage.
 
We DON'T work with the insides of components using electron microscopes examining holes. That's in physics, chemistry and semiconductor design. Separate field, we don't do that here any more than we do astrophysics or medicine.
As I wrote in my last post
From my little knowledge i know that voltage is a word that is defined to make our life easier. It is defined after constraining our self to work with components in which
1) rate of change of charge inside our component is zero
2) rate of change of magnetic flux outside our component is zero
that to make our easier we have imposed certain restrictions on our self. But there are certain times we need to go beyond the restrictions (if you have taken this course, "Electromagnetic Waves and Radiating Systems") and so we need to understand to field theory as well.

Here are few lines from my book on Antenna

Despite the power and usefulness of the circuit approach the communications engineer concerned with microwaves or with radio transmission problems quickly becomes aware of its limitations.
Of course voltages and currents are the end results in which the engineer is interested, but the intermediate step, the electromagnetic filed, is now a necessary one.
and its not intimidating to newbies. I as a newbie, love this kind of conversation.
 
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