My high school physics class was 45 years ago, and there isn’t a whole lot that I remember, most of which is probably obsolete by now anyway. But I do know that in an atom, protons are positive and electrons are negative.
But what exactly does this mean? Obviously it means that these two particles have an opposite charge, but what do the terms “positive” and “negative” refer to in this context? I understand our use of “positive” and “negative” regarding numbers, or temperatures on a particular scale, or plots on a graph . . . but in what sense can an entity be, in and of itself, “positive” or “negative”? IOW, if I encountered an electron, all by itself, not part of an atom, what about it would tell me that it’s “negative”?
Negative and positive are simply conventions. Under this convention, the electric charge of an electron is defined to be negative, while that of the proton is defined to be positive. You could just as easily call the proton’s charge negative and the electron’s positive (or for that matter, up and down, left and right, back and forth or any other arbitrary pair of opposites) and the nature of their interactions would remain the same; that is, two electrons or two protons will repel each other while an electron and a proton will attract. The point is, there is no quality about a given electric charge which inherently makes it negative or positive.
They’re simply naming terminologies. We could have said they were black & white or left & rigtht or innie & outie just as well.
The important point is that an electron has an intrinsic electrical charge and a proton has an intrinsic charge of the same size, and the two charges are opposite in their “flavor”. At bottom, that’s all the terms “positive” & “negative” really mean; they’re two words chosen because they’re opposite concepts.
Now charges sum across a batch of particles. E.g the net charge of 10 protons & 8 electrons is the same as 2 protons. the other 8 electrons & protons cancel each other out. And all charges are exactly the same size.
That fact means the integer number line you referred to is a pretty good metaphor for what’s going on with these charges. 2 + 3 = 5 & 5-4 = 1 and 7+(-4) = 3, etc. And fractional values don’t exist.
And that’s why “positive” & “negative” are preferred terms over innie & outie.
I assume you mean fractional values don’t occur when dealing with electrons and protons, which is correct. However, fractional values do exist; charged quarks come with charges in units of 1/3 and 2/3 that of the proton.
I know that the electron’s charge was defined as 1 long ago, well before quarks were thought of, let alone found.
But I’ve always been bothered by people saying that quarks have fractional charge. Aren’t the charges of the quarks 1 and 2, with the electron really having a charge of 3? In other words, quarks don’t really have fractional charge in anything more than a purely arbitrary historic sense.
You’re right, of course. You could just as easily assign quark charges as +/- 1 and 2 and the electron and proton would have charges of -3 and 3, respectively.
It wouldn’t be in appropriate to say that, necessarily, but the “elementary charge” of the electron or proton is the fundamental quantum of charge normally found in nature. Quarks are never found in the deconfined state–i.e. running around freely–except under extremely high pressure, high energy situations not found in anything like a normal environment; hence, fractional charges don’t, for all practical Earth-bound purposes, exist.
The fact that an electron is negative and a proton is positive is merely a convention, and one that is occasionally of confusion, as the electron current in a DC circuit is actually in the opposite direction of the conventional current. It’s also much, much slower; you can actually outrun it. Don’t think that’ll save you from being electrocuted, though; the electrons can stay right where they’re at and still conduct enough energy to fry your gizzards.
>Quarks are never found in the deconfined state–i.e. running around freely–except under extremely high pressure, high energy situations not found in anything like a normal environment
Is this really true? I think I heard somewhere recently that there are probably degenerate stars made of free quarks. If even a pretty small fraction of stars wind up this way, free quarks would be much more plentiful than quarks in planets, which historically is our baseline environment for thinking about the weird.
Also, correct me if anyone knows otherwise, but there may well be something that is affirmative about either “positive” or “negative” charge. In other words, one of these might be the presence of something, and the other the absence of it. We don’t know what that is, and to date we only have Franklin’s naming convention, but we actually don’t know that it’s arbitrary, either.
Finally, an interesting tidbit from the latest Scientific American. Neutrons are, of course, neutral in charge when considered as entire objects. But for some time it’s been thought that neutrons have their negative charge concentrated on the surface and their positive charge in the center. Now, however, there’s evidence that they have negative charge concentrated both on the surface and in the center, with the positive charge concentrated about halfway in. (I hope I didn’t swap + and - in this story but it’s about as nifty either way…)
If you accept that Franklin was pretty much observing behaviors of the “electron gas” whose movement is the basis for nearly everything we think of as electricity (I’m calling hole conduction and positive ion drift neglible players in the big picture here), then there really is an underlying sense to this, and Franklin guessed wrong when he named them. Wouldn’t we have to wait for Crooke tubes or Fleming valves or other electron tubes to come along and behave in a way that wasn’t symmetric, in order for a meaningful sign to be applied?
You can also get an asymmetry from the Hall effect, which might be a little easier to measure experimentally. But no experiment which Franklin would have had access to can distinguish between the cases, so he just had to guess and assign them arbitrarily.
My electronics teacher used to say that Franklin had a fifty-fifty chance of getting it right, and he blew it. The issue came up during a discussion of current flow conventions in electrical engineering, such as the arrows used in the symbols for diodes and transistors. The students wanted to know why the arrows were pointed in the wrong direction.
Just to make things even more confusing, in some materials, the charge carriers of a current are positive, rather then negative. So the arrows will follow the direction of motion the carriers in that case. (In case your curious, positive charge carriers in a material are electron holes, the absence of an electron.)
>in some materials, the charge carriers of a current are positive, rather then negative. So the arrows will follow the direction of motion the carriers in that case.
This is a somewhat synthetic curiosity. It may be useful to describe a place where you might expect an electron to be, but where it isn’t, as a “hole”, and then claim the current is being carried by holes. But the only physical objects that are moving around in this system are the electrons, and they are still going in the direction opposite to the direction engineers draw their arrows. Said another way, this is no more fundamentally true than claiming there are “negative dollars” and you have to take them from a cashier to buy something.
You can have positive ions drifting through an electrolyte, or you can have ionized hydrogen or other gasses in a vacuum chamber moving in a beam, and these are real cases of positive current flowing along the arrows that has a real physical embodiment. But holes aren’t things. They’re the opposite of things.