[Potironette]

I think I was having trouble coming to terms with what voltage dropping means...does "all elements" include the battery?

So far, what I think I understand is that in an imaginary circuit with one resistor, say a lightbulb, and where the wires are perfect conductors, the change in voltage, or the potential difference, across the lightbulb will be exactly that of the battery, because in V = IR, the current doesn't have to be split up among more than one resistor's resistance. It's like the wires didn't exist...so:

On the other hand, when the wires do contribute to the total resistance of that circuit, then the current is affected by all "resistors," which includes the wires. This means voltage keeps dropping steadily all the way to the end.

I'm still confused about the battery though. Setting the negative terminal to zero, and moving across to the positive terminal, the voltage increases.

Although, it has to increase in order for the circuit to move electrons around. I guess chemical stuff inside the battery creates a potential difference from the negative end to the positive end mostly through the wires and mostly not within the battery, and that provides the potential difference required for resistors to turn potential energy of electrons into heat/light/etc. Except isn't it more like potential difference creates some sort of potential energy for electrons --> electrons move and so that's kinetic energy --> electron kinetic energy turns into heat/etc. in resistors?

And I guess that whole thing is related to series circuits (my class started on that immediately after the quiz) which says that the current across the entire circuit is the same, and the circuit's resistance (equivalent resistance) is made up of all the resistors' resistances, and the battery voltage is split up among all the resistors.

Err, so from what I remember when voltage drops it just means electrons came up to the resistor, and slowed down. Now there's one side of the resistor that's more negative, and one side that's more positive, and so now there's a difference..? But how does the battery and overall current fit into this? The difference in voltage across the entire circuit just happens to be that of the battery because that's what's connecting the battery terminals? With the voltage drop being greater on resistors having more resistance because they just take more potential energy to power and slow electrons down more like that so the difference created across those resistors is greater? And all the while the current just accommodates for the resistors?

Also, for parallel circuits, whatever the "equivalent resistance," and the current and voltage for the circuit as a whole is, am I basically treating all the resistors in series as one resistor for the circuit? And it just so happens that because I_b = I_1 + I_2 +..., and I = V/R, V_b = V_1 + V_2+..., it works out that 1/R_eq = 1/R_1 + 1/R_2 +...?

And, do parallel circuits affect everything just because if resistors aren't very resistant (whatever that mean) or incredibly not-resistant (whatever that means), and metals have free moving electrons, then electrons in all wires will be affected?

[Coda]

Originally Posted by Potironette

I think I was having trouble coming to terms with what voltage dropping means...does "all elements" include the battery?

No. If you INCLUDE the battery, then the net voltage drop across the circuit must be zero (because the battery has a negative voltage drop because it's a source of potential).

So far, what I think I understand is that in an imaginary circuit with one resistor, say a lightbulb, and where the wires are perfect conductors, the change in voltage, or the potential difference, across the lightbulb will be exactly that of the battery, because in V = IR, the current doesn't have to be split up among more than one resistor's resistance. It's like the wires didn't exist...so:

This looks correct.

The thing that might be throwing your intuition is the idea that the same resistor is causing a different voltage drop depending on the voltage of the battery. The resolution to that is to realize that it means that the current (that is, I) is affected -- less electricity is flowing through the circuit altogether because of the resistance. What the resistor does is it inhibits the ability for that voltage to be translated into work elsewhere in the circuit. (This is a very desirable trait, because otherwise you're shoving excess power through things that AREN'T designed to deal with it -- connecting an LED directly to 5V is a good way to blow it out, but if you put a load resistor ahead of it in the circuit then the voltage feeding into it will be lower.)

On the other hand, when the wires do contribute to the total resistance of that circuit, then the current is affected by all "resistors," which includes the wires. This means voltage keeps dropping steadily all the way to the end.

Yep.

I'm still confused about the battery though. Setting the negative terminal to zero, and moving across to the positive terminal, the voltage increases.

This goes back to correlations not being a cause-and-effect relationship, which we were discussing earlier. Yes, you observe that the voltage is higher, but that doesn't explain anything about WHY that's true.

Although, it has to increase in order for the circuit to move electrons around. I guess chemical stuff inside the battery creates a potential difference from the negative end to the positive end mostly through the wires and mostly not within the battery, and that provides the potential difference required for resistors to turn potential energy of electrons into heat/light/etc. Except isn't it more like potential difference creates some sort of potential energy for electrons --> electrons move and so that's kinetic energy --> electron kinetic energy turns into heat/etc. in resistors?

Ideally, NONE of it within the battery -- that would cause batteries to discharge themselves when not connected to anything, which is definitely not what you want to have happen. :P

And yes, you are completely correct about the energy conversions.

And I guess that whole thing is related to series circuits (my class started on that immediately after the quiz) which says that the current across the entire circuit is the same, and the circuit's resistance (equivalent resistance) is made up of all the resistors' resistances, and the battery voltage is split up among all the resistors.

Yup. Goes along with what I said above.

Err, so from what I remember when voltage drops it just means electrons came up to the resistor, and slowed down.

Mm... Vaguely, but mostly no.

The entire circuit is already populated with electrons before you ever connect a power source. They're just not really moving. So when you add a voltage, that's applying a force to one end, but you can't really say that the electrons have to slow down when they get to the resistor, because they can't pile up on each other -- there's only so much room for them. This is why the current through the entire circuit is constant: the electrons can only move through the circuit as fast as they can move through the slowest part, because they're always being pushed into the flow.

Think about a circular tube filled completely up with marbles. If you push on one of the marbles, that's applying a voltage, and you can feel the resistance of friction, but all of the marbles move at the same speed. If you were to put a narrower bit of tubing in there somewhere, you'd have to apply extra force to shove a marble through it, but that wouldn't change the fact that they all have to stay at the same speed -- it just slows the whole thing down.

The friction in that narrower tubing has a normal force that acts against your attempts to move the marbles. The net force available to move marbles through the rest of the tube is, correspondingly, exactly that much less. That's the voltage drop of our marble-circuit resistor. And just like the normal force of friction is proportional to the force being applied, the voltage drop across the resistor is proportional to the voltage being applied to the whole circuit!

Now there's one side of the resistor that's more negative, and one side that's more positive, and so now there's a difference..? But how does the battery and overall current fit into this? The difference in voltage across the entire circuit just happens to be that of the battery because that's what's connecting the battery terminals? With the voltage drop being greater on resistors having more resistance because they just take more potential energy to power and slow electrons down more like that so the difference created across those resistors is greater? And all the while the current just accommodates for the resistors?

The battery is the reason that one side is more negative and one side is more positive. The difference in voltage across the entire circuit is equal to the voltage of the battery BECAUSE the voltage is defined by the work necessary to push a charge across the circuit! It's not a coincidence; it's a definition.

But yes, the resistors use up energy, leaving less energy available to do other things.

Also, for parallel circuits, whatever the "equivalent resistance," and the current and voltage for the circuit as a whole is, am I basically treating all the resistors in series as one resistor for the circuit? And it just so happens that because I_b = I_1 + I_2 +..., and I = V/R, V_b = V_1 + V_2+..., it works out that 1/R_eq = 1/R_1 + 1/R_2 +...?

I'm not quite sure what the question is. Can you try rephrasing it?

And, do parallel circuits affect everything just because if resistors aren't very resistant (whatever that mean) or incredibly not-resistant (whatever that means), and metals have free moving electrons, then electrons in all wires will be affected?

When you're mixing parallel and serial circuits, you can treat multiple elements on a branch of a parallel circuit as a single resistor, and you can treat the whole of a parallel group of elements as a single resistor in a bigger serial circuit.

So yes, that means that the effective resistance across the parallel part of the circuit DOES affect the current of the whole thing.

[Potironette]

For parallel circuits, can I also pretend that all the resistors in parallel merge together into one resistor? And it just so happens from how the math works out, the resistance for the parallel circuit can be calculated using: 1/R_eq = 1/R_1 + 1/R_2 +...
(I think based on what I learned in class today, I am treating parallel circuits that way. As for the equation, I do remember how to find it, though I wonder why that's the equation on my reference sheet and not (R_1 * R_2)/(R_1 + R_2) = R_eq )

Err..so voltage measures change in potential energy based on current and time, and when voltage drops it means some potential energy got converted into some other energy across a resistor?
And all the while the battery has its own voltage, potential isn't the only thing that matters...so resulting from the potential is electrons moving--which is kinetic energy. But, just like how if I slide a book across the table, the kinetic energy of the book is lost to friction/heat/etc. and it stops, the electrons are sort of "stopping"--except there is a constant "force." Therefore, just as pushing a book across a table vs across a rug, more KE is lost to heat across the rug and so it is slower, so too does more resistance mean the current is slower across a circuit with more resistance?

And is that why Power = Work/time = ΔPE/time? Because somehow, change in PE is sort of Force (and distance)...P = W/t --> W = Fd = ΔPE for electric fields = qEd = qΔV --> P = qΔV/t --> P = VI?
Why doesn't distance matter in a circuit? Does it really just not exist as a value? But at the same time, I can't very well say P = F/t, because then P = IE.


EDIT: Random question-how does a voltmeter measure voltage? I assumed that the greater the current passing through the voltmeter, the greater the voltage measured. But apparently the ideal voltmeter has infinite resistance? Doesn't that mean no current passes through the voltmeter? And if there is no current, how does it measure anything?

[Coda]

Originally Posted by Potironette

For parallel circuits, can I also pretend that all the resistors in parallel merge together into one resistor?

In terms of how the rest of the circuit sees it? Yes.

(I think based on what I learned in class today, I am treating parallel circuits that way. As for the equation, I do remember how to find it, though I wonder why that's the equation on my reference sheet and not (R_1 * R_2)/(R_1 + R_2) = R_eq )

Because they're equivalent (it's pretty simple manipulation to prove it), and 1/R_eq = 1/R₁ + 1/R₂ is less work to evaluate. :P

Err..so voltage measures change in potential energy based on current and time, and when voltage drops it means some potential energy got converted into some other energy across a resistor?

That's one way to look at it, yes.

And all the while the battery has its own voltage, potential isn't the only thing that matters...so resulting from the potential is electrons moving--which is kinetic energy. But, just like how if I slide a book across the table, the kinetic energy of the book is lost to friction/heat/etc. and it stops, the electrons are sort of "stopping"--except there is a constant "force." Therefore, just as pushing a book across a table vs across a rug, more KE is lost to heat across the rug and so it is slower, so too does more resistance mean the current is slower across a circuit with more resistance?

Precisely.

And is that why Power = Work/time = ΔPE/time? Because somehow, change in PE is sort of Force (and distance)...P = W/t --> W = Fd = ΔPE for electric fields = qEd = qΔV --> P = qΔV/t --> P = VI?

Looks like you've basically got it down.

Why doesn't distance matter in a circuit? Does it really just not exist as a value? But at the same time, I can't very well say P = F/t, because then P = IE.

Distance doesn't matter in a circuit for the same reason that current is constant across a circuit: it doesn't matter how far the other end of the wire is, when you push on the electrons on the near end, it forces electrons out on the other end. While the individual electrons don't move very fast, the energy propagates through the wire at an appreciable fraction of the speed of light, so you can more or less disregard it at human scales.

EDIT: Random question-how does a voltmeter measure voltage? I assumed that the greater the current passing through the voltmeter, the greater the voltage measured. But apparently the ideal voltmeter has infinite resistance? Doesn't that mean no current passes through the voltmeter? And if there is no current, how does it measure anything?

The ideal voltmeter has infinite resistance so that it doesn't interfere with the circuit being measured by using up some of the current. (After all, by applying a voltmeter to a circuit, you're creating a parallel circuit!) The conflict you're trying to resolve HAS no resolution because an ideal voltmeter is impossible. :P You're completely correct: it HAS to have SOME current flowing through it in order to measure anything. From that point, it's just V=IR, and you know the resistance of the voltmeter and you measure the current flow so you can determine the voltage.

[McSwiggins]

Ahh! Mathy stuff! -scurries away frantically-

[Potironette]

Is another way to look at it that voltage measures the difference in the joules (of PE?) used to move charge at two different points?

At human scales?

--
Lol, technically it's science?
--
But I do have an English question from writing my history paper:

Is it more proper to write "white middle-class Americans" or "white, middle-class Americans" / "dark, greasy film" or "dark greasy film" or "dark and greasy film" ?

[Coda]

Hmm. I suppose that's not bad as a rough visualization.

At any scale that humans can meaningfully interact with without technological aid, the speed of light might as well be infinite, because any time elapsed for light to get from point A to point B is so small that it's not going to influence the significant figures of your measurements. The speed of electrical propagation is likewise beyond meaningful consideration. The speed of sound is even fast enough to be irrelevant in most cases. (And the speed of sound is a particularly relevant metaphor, because it determines how long it takes for the far end of an object to respond to a force applied to the near end.)

As for the English question: Both are grammatically correct, but they have different meanings. If you can swap around the adjectives without influencing the meaning, include the comma. If swapping them makes the phrase mean something different, then leave the comma out.

So for example, if you have multiple greasy films and you want to refer to the dark one, "dark greasy film" is correct. However, if you only have one film, and it happens to be both dark and greasy, then "dark, greasy film" and "greasy, dark film" are equivalent.

In the case of "Americans" the distinction is particularly significant. Are you referring to the set of middle-class Americans who are white, or are you describing a group of Americans as being both white and middle-class?

[Potironette]

Is voltage the joules of PE changed to...some other energy...as charge moves from one point to another?

Is sound how humans perceive kinetic energy?

My exact sentences were:
"After the war, a booming economy that made televisions a household staple made white middle-class Americans more vulnerable to food advertisements."
I guess I was referring to the set of middle-class Americans who were white.
I was told that it was without the comma, but I don't know what the difference is.

"A dark and greasy plastic film called “Saran” was developed by the Dow Chemical Company during the war for soldiers to spray on fighter planes to protect against sea spray."
I was describing one type of film. So, I suppose it's with a comma?
But is there a connection between this and "white, middle-class Americans"?

[Coda]

No, change in energy per unit time is power, which is measured in watts. It's the P in P=IV. Voltage is change in energy per unit charge.

Yes, humans perceive changes in the kinetic energy of the eardrum as sound. However, if a tree falls in the wood, and no one is around to hear it, it still produces sound waves, so it's imprecise to describe sound as the way humans perceive kinetic energy.

The second sentence is the easier one. You're listing independent descriptions of the film. You could describe it as a "dark film" or as a "greasy film" without being wrong, therefore commas are appropriate.

The first sentence is more complex to analyze. The two descriptions aren't completely independent. You can drop "white" without damaging the sentence -- saying "middle-class Americans" would still be correct. However, if you were to say "white Americans" were affected, then you would be wrong, because poor white Americans (here the comma MUST be omitted to be correct!) were not affected due to not having TVs, and rich ones (notice that the pronoun "ones" replaces "white Americans" as a whole unit) were not affected due to already having access to everything they want.

I apologize for being imprecise. It doesn't require that there be more than one kind; that was meant as an example.

[Potironette]

So I can sort of say that voltage is the difference between joules of PE used to move charge in Coulombs at two different points. But what does it mean that joules of PE get used? I mean, in a circuit, isn't PE used up as kinetic energy for electrons all the time? Or is it that because voltage is the difference in energy being used up, it means that only when there is a resistance, more PE is required to move electrons along because electron energy gets lost, then there's a voltage?


So..commas are used if two descriptions of something are independent?
Why would "middle-class Americans" be fine? Is it that if I use "middle-class" it implies white so I don't need to add it?

[Coda]

Originally Posted by Potironette

So I can sort of say that voltage is the difference between joules of PE used to move charge in Coulombs at two different points. But what does it mean that joules of PE get used? I mean, in a circuit, isn't PE used up as kinetic energy for electrons all the time? Or is it that because voltage is the difference in energy being used up, it means that only when there is a resistance, more PE is required to move electrons along because electron energy gets lost, then there's a voltage?

Voltage is the difference (in joules) in potential energy available to one coulomb of charge before and after moving through a part of the circuit.

Again, thinking of things in terms of cause-and-effect relationships isn't always accurate. A good scientist shouldn't think about the how or the why when making a measurement, because that can introduce bias in the results. The measurement simply is what it is. It's an empirical observation; it's data. That data can then be used in support of a hypothesis about the causes and the effects, but that data is not itself a description of either.

To be more concrete: don't necessarily think of it as "using" potential energy, because it could also be GAINING it -- if you force a charge backwards against the potential difference (for example, if you're recharging a battery) then that charge now has potential energy that can be released to flow back through the circuit.

To actually DISCUSS the hows and whys here: For a given, fixed voltage, it doesn't matter how much resistance there is; that voltage will always be capable of moving that coulomb of charge. You can see this in the examples you've been working: the voltage drop across a single resistor varies according to the total resistance of the circuit. Resistance affects how long it takes that unit of charge to pass through the circuit: V=IR shows us that.

So..commas are used if two descriptions of something are independent?

This isn't WRONG, but it's insufficiently precise. To get into the nitpicky details, there are two ways to use adjectives: coordinate adjectives, and cumulative adjectives.

Coordinate adjectives each modify the noun separately. You can rearrange them without changing the meaning. These are written with commas. If you can replace the comma with the word "and" without making it sound weird, then this is the type of use you're looking at.

A cumulative adjective effectively creates a new compound noun, and as a result, you can't separate the adjective from the noun without changing the meaning. For example, the "hot" in "hot rod" is being used as a cumulative adjective. If you compare "the yellow hot rod" and "the yellow, hot rod" you will see they mean different things -- while both are brightly-colored, the first one is a fast vehicle; the second one is a heated bar. Apply the tests: "hot yellow rod" doesn't work to describe a car, nor does "yellow and hot rod".

There's a gray area in between, and I think the "white middle-class Americans" example is one of them even though it leans more towards cumulative. To make things more abstract... If I had a lot of marbles, and I asked you for the large, blue marble, then it could go either way, and the nuance is mostly one of grouping. I could be asking you "of the blue marbles, I want the large one," or I could be asking you "of all of the marbles, I want the one that is large and blue." The former (which wouldn't have a comma) would be more intuitive if the marbles were already sorted out by color, or if all of the marbles were blue in the first place, or for whatever other reason I considered "blue marble" to be a specific type of thing, and then I was being more specific within that grouping.

Why would "middle-class Americans" be fine? Is it that if I use "middle-class" it implies white so I don't need to add it?

Not so much the implication as the generalization: black middle-class Americans were also affected the same way, I would imagine. But proportionally speaking the overwhelming majority of middle-class Americans were white, so if you took a snapshot of the affected people, you would see that most of them are white and middle-class.

[Potironette]

Oh, so voltage is a measurement. Err, so at one point of a circuit voltage and potential energy is something. At another point in comparison to that point it might be something else. Voltage is a measurement of potential...but what is potential energy a measurement of?

Why is cross-sectional area used to compare with resistivity in R = pL / A? Is circumference ever used?

Ohh, I understand the different now. If there's a comma then the last bit without the comma is what is being described. If there's no comma then all the words after the first word is one thing. Thank-you!

Oh, but that means if someone says "bright yellow sun" it technically(?) means a bright yellow-sun :/ ? What about a bright-yellow sun? Do I just use a dash?

[Coda]

Originally Posted by Potironette

Oh, so voltage is a measurement. Err, so at one point of a circuit voltage and potential energy is something. At another point in comparison to that point it might be something else. Voltage is a measurement of potential...but what is potential energy a measurement of?

Not quite. Voltage is a measurement of potential difference. You don't measure voltage at a single point. You measure voltage across two points.

Potential energy as a general concept is a measurement of how much energy could be released if the subject were free move on its own. It is, in a sense, stored energy. You're familiar with gravitational potential energy. The other kinds are no different.

Why is cross-sectional area used to compare with resistivity in R = pL / A? Is circumference ever used?

The cross-sectional area is used because not all wires have a circular cross-section. Obviously for a cylindrical conductor you could use R = pL / (pi*r²), but cross-sectional area is a more general concept.

Ohh, I understand the different now. If there's a comma then the last bit without the comma is what is being described. If there's no comma then all the words after the first word is one thing. Thank-you!

Oh, but that means if someone says "bright yellow sun" it technically(?) means a bright yellow-sun :/ ? What about a bright-yellow sun? Do I just use a dash?

Usually the distinction DOESN'T matter, because "(bright yellow) sun" and "bright (yellow sun)" are both valid parses of the sentence. If the distinction truly matters, then yes, you can use a hyphen (not a dash, they're technically different). Though usually if the distinction matters you just reword the sentence ("The sun was bright yellow as it crossed the sky" instead of "the bright yellow sun crossed the sky").

That said, "bright yellow" is a somewhat unusual case -- technically it ought to be "brightly yellow" if you're treating it as a modified adjective (because adjectives don't modify other adjectives), but instead it's actually a modified noun ("yellow" as a thing instead of a description) being used as an adjective.

[Potironette]

Ohh, it hadn't occurred to me that a wires could be not circular.

Hmm, so when there is PE, it basically means if there were charge at a certain position with PE, it would move some way. This means voltage measures the difference in how much a charge would get pulled at different points? But isn't current the same throughout the entire wire? Why would there be a difference in PE? Oh actually, I think I'm getting messed up between potential different and change in potential energy.
So..voltage measures difference in required potential energy per charge between two points? And as electrons move along, they require more potential energy per charge because of resistors using up energy? Or if electrons are forced the other way around..more PE per charge is required the other way around?
..Am I talking about cause-and-effect again? But without talking about cause-and-effect what does PE per charge mean?

If "brightly yellow sun" means the sun is brightly yellow, then does "starry blue sky" mean that technically, the sky is starry blue? Although, maybe people normally put a comma between starry and blue :/

[Coda]

Originally Posted by Potironette

Hmm, so when there is PE, it basically means if there were charge at a certain position with PE, it would move some way. This means voltage measures the difference in how much a charge would get pulled at different points?

No, voltage measures how much work is done BY a unit charge going through those points (or conversely, how much work must be done to force the charge to go through those points in the opposite direction).

But isn't current the same throughout the entire wire? Why would there be a difference in PE? Oh actually, I think I'm getting messed up between potential different and change in potential energy.

No, that's not the confusion you're having, because there ISN'T a difference.

So..voltage measures difference in required potential energy per charge between two points?

It's either "required potential energy" or "difference in available potential energy".

Take some time to reflect on the points I've made above, and if you still have questions, ask again, but I think the change in perspective might answer a lot for you.

If "brightly yellow sun" means the sun is brightly yellow, then does "starry blue sky" mean that technically, the sky is starry blue? Although, maybe people normally put a comma between starry and blue :/

"Starry" is an adjective, not an adverb. If it were "starrily blue" then you'd be right, but I just made that word up right now.

[Potironette]

So..is it like this?
Voltage is either the work it takes or would take to move one coulomb from one place to another.
Work is a convenient thing that relates force and distance to each other.
Hence, work in voltage is how much force and distance it takes to reach a certain point in space.
PE is how much work it would take for a given amount of charge reach that certain point in space.
It takes force and distance, or work, to move negative charges across a resistor because..how else does something move? Or, in the case of charging a battery, it takes work to move negative charges the other way around for the same reason, maybe.

Ohh, starry isn't even an adverb :/. I'd forgotten that things like "blue green" were different from "bluish green."