Why is voltage constant in a wire?

[StandardsGuy]

It isn't, except in arm waving and sometimes misleading 'explanations'. Voltage Is Not A Force; the Voltage (Potential Difference) across any circuit element (including the connecting wires) tells you the Energy Needed to move charges through it. If they will go through without dissipating energy then the PD is Zero. We approximate the PD along a length of wire to zero but, once the wire is long enough or thin enough (or made of something other than Copper or Silver) there may be a significant amount of energy dissipated in it and we have to include the Voltage Drop.

Really? EMF (electromotive force) is measured in the units Volts. Consider a battery only. It has an EMF which is analogous to pressure in a water pipe. Considering the OP's education level your post may be confusing.

 

[Integrand]

Really? EMF (electromotive force) is measured in the units Volts. Consider a battery only. It has an EMF which is analogous to pressure in a water pipe. Considering the OP's education level your post may be confusing.

EMF is one of those tricky historical terms we have to cope with despite their potential to mislead. I believe the point being made is that despite the name, electromotive force is not to be equated in any way with mechanical force.

 

[sophiecentaur]

EMF is one of those tricky historical terms we have to cope with despite their potential to mislead. I believe the point being made is that despite the name, electromotive force is not to be equated in any way with mechanical force.

Electromotive Force is a term from way back before anyone had a clue about what Electricity is. It is an exception which has to be used with care. "emf" is a term that refers to the Potential available from a voltage source when no current passes. It is unfortunate that it has passed into modern usage because it does nothing but cause confusion - in threads like this one. Voltage is not a force. Energy and Force are two different things. As for 'Pressure' - just how could that be allowed into any serious discussion? As has been regularly stated on PF, analogies are very risky and are seldom received by the uninitiated in the way that's (albeit helpfully) intended.
I would say that the OP's apparent level means that the casual use of a Water Analogy could be harmful without a very strong caveat - which has not been included.
It has already been necessary to put the idea that the Kinetic Energy of Electrons is involved in the transfer of Electrical Energy, which is another problem when we try to use Mechanics to explain Electricity.

 

[litup]

Hey! I have a question and a drew up a diagram to help out. I drew up a circuit that includes just a voltage source and a resistor. I know the voltage is constant in each of the circled portions of the circuit. As in, as you move through the wiring, the voltage doesn't change from one point to the next. Really, the voltage only changes when you move through the load. This much I know, haha.

The source of my confusion is I don't know why this happens. If voltage is supposed to be the driving force that moves the current, how can the electrons move from one point to the next if the voltage isn't changing? After all, there isn't a difference in energy states from one point to the next within the wiring, so what's causing the motion? View attachment 107191

This has been what's bothering me. Any and all help is appreciated!

I'm surprised nobody has mentioned the 4 point probe concept. If you measure a low resistance with a DVM you just get a minimum reading and for my meters they are all say 0.1, 0.2 ohms, something like that because they just can't measure resistance less than 1/10th ohm very well. A 4 point probe is different, it has 4 tips all in a row usually and the outer two tips inject a small current into the wire or whatever, and the two inner tips separated by maybe 1 mm measure the voltage developed across those tips. It can measure very tiny voltages and the instrument converts those into ohms readings, as low as one micro ohm and believe me, it will measure very well the resistance of any wire and those wires are definitely not zero ohms so there is a voltage drop across those wires and the higher the current in the wire the larger the voltage drop and that voltage drop leads directly to heating the wire. That is why an extension cord gets hot if you say, run a toaster taking 1500 watts with a small household extension cord, the toaster might work but some portion of the energy used to heat the elements of the toaster are also heating the wires used to power it which means the toaster is now not getting 1500 watts because there is a voltage drop across the extension cord which lowers the energy available to the toaster and the energy not used in the toaster is now being used directly to heat the wires of the extension cord and the smaller the diameter of the wires used for that job the more the wires heat up and the less is available for the toaster to the point where the wires will melt and maybe cause a fire which is why you use high current extension cords for big jobs.

 

[sophiecentaur]

@litup: Your example is a good one to demonstrate that we have a choice whether or not to include the very low resistance of connecting wires in our circuit calculations. When the resistance of the cable is near the same magnitude as the resistance of the components in a circuit, then it is vital to take it into account and its effect can be easy to find (as in your 'four point probe'). It would be another matter to try to use a four point probe on the copper strips between components of a CMOS circuit. It is often very reasonable to assume connecting wires to have zero resistance and we all start off that way in School. But with Meters that give only 3 sig figs and when component values are deliberately chosen by the teacher for the results to 'come out right', we don't even consider it - except when we find we need to jiggle the connectors a bit, to eliminate dirty contact resistance.

 

[OmCheeto]

...
Considering the OP's education level your post may be confusing.

Just checked his profile: Degree in Chemical Engineering (undergraduate?)

Yah, I know what Sheldon said about them; "Hello, Oompa Loompas of science." [ref]
Having only gotten a tad past halfway through my attempt at a BS in EE, I found that comment hilarious; "Wait? What? What does that make me?"

I would attempt to make more funny jokes about this thread, but it's a very interesting topic, as, I've discovered over the last few years, that I don't know what a "Volt" is, and am very interested in seeing a solution.

ps. Spent the whole morning trying to figure this out, only to prove, mathematically, that, at the atomic level, electricity doesn't work.
pps. Unless you drop down to the milliamp ranges, and then, electricity works again. Whew!

 

[sophiecentaur]

Just checked his profile:

Assuming people's qualifications can be a risky business.
Yesterday, I dusted off some of my certificates.
O Level Latin, to name but one!

 

[sophiecentaur]

Nil carborundum.