[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.