# What is (+,−)?September 18, 2005 1:36 PM   Subscribe

What does (+,−) mean?

I've seen it used in connection with electricity. Maybe something to do with batteries? Besides that, I'm not sure.
posted by Count Ziggurat to Technology (10 answers total)

Positive and negative.
posted by iconomy at 1:39 PM on September 18, 2005

They refer to the positive/negative terminals on batteries. From HowStuffWorks:

If you look at any battery, you'll notice that it has two terminals. One terminal is marked (+), or positive, while the other is marked (-), or negative. In an AA, C or D cell (normal flashlight batteries), the ends of the battery are the terminals. In a large car battery, there are two heavy lead posts that act as the terminals.

Electrons collect on the negative terminal of the battery. If you connect a wire between the negative and positive terminals, the electrons will flow from the negative to the positive terminal as fast as they can (and wear out the battery very quickly -- this also tends to be dangerous, especially with large batteries, so it is not something you want to be doing). Normally, you connect some type of load to the battery using the wire. The load might be something like a light bulb, a motor or an electronic circuit like a radio.

Inside the battery itself, a chemical reaction produces the electrons. The speed of electron production by this chemical reaction (the battery's internal resistance) controls how many electrons can flow between the terminals. Electrons flow from the battery into a wire, and must travel from the negative to the positive terminal for the chemical reaction to take place. That is why a battery can sit on a shelf for a year and still have plenty of power -- unless electrons are flowing from the negative to the positive terminal, the chemical reaction does not take place. Once you connect a wire, the reaction starts.

posted by purephase at 1:40 PM on September 18, 2005

Well, in a Battery, the + symbol is for the positive voltage, where the current flows "from", and the negative is where it flows "to". I'm sure that simple explanation just made some physicists cringe, so maybe reading up on the physics of Capacitors would help.

In math/statistics, the +/- or ± sometimes indicates the estimated error. So, if I measured the height of a tree with simple trigonometry, I might indicate that I don't know exactly how high it is by expressing my answer as 10 feet +/- 5, ... so I'm pretty sure the tree is no higher than 15, nor shorter than 5.
posted by odinsdream at 1:41 PM on September 18, 2005

It's a bit hard to know what you're fishing for. Sometimes, "±" is used in equations to shorten equations. Instead of e.g. "A+B = C+D and A-B = C-D", you can write "A±B = C±D".
posted by springload at 2:22 PM on September 18, 2005

If you're referring to a specific symbol you've seen and not + and - and general, you need to give more context.
posted by cillit bang at 2:32 PM on September 18, 2005

"+, -" probably means "both positive and negative current".

This is distinct from the "+/-" and "±" constructions, which refer to size tolerances. In mechanical drawing, a dimension called out to one- thousandth of an inch is often toleranced as "±0.005 inch," meaning that a part that is within five-thousandths of the stated dimension is acceptable.
posted by Kirth Gerson at 2:52 PM on September 18, 2005

+ and - are generally used to represent the positive and negative terminals of a power supply (e.g. a battery). "Conventional current" flows from positive (a lack of electrons) to negative (a surplus of electrons).

That might sound weird, but when you realise that the charge on each electron is negative, it kinda makes sense. That is, negative electrons flowing in one direction is equivalent to current flowing in the other direction.

You might think of it better as not giving me £5. Each time you don't give me £5, you're £5 better off.
posted by alby at 3:26 PM on September 18, 2005

Electrical Engineers talk about two things: electrons, which you are probably familiar with, and holes, which you aren't.

Atoms are composed of a core that contains protons (+ charge) and neutrons (no charge) surrounded by a cloud of electrons (- charge). Electrons are ordered according to their energy levels, and in some elements the highest energy electrons will actually break away from the atom. This free, negative electron carries energy with it and that's what most people picture when they think of electricity.

But electrical engineers have to ask "what's left behind?" The answer is an atom no longer in balance and carrying a positive charge (because it now has one more proton than electron). Sometimes, an electron from a neighboring, neutral atom will jump over to this positively charged atom. This fills the original hole but creates a new one. Conceptually, you can think of this positive hole moving around the material.

Some people are going to hate holes and claim that I'm crazy: it's just another movement of electrons. My freshman year I thought my professor had gone off the deep end. And yet, holes and electrons together have revolutionized the way the world works. This understanding has led to diodes and transistors which are basic building blocks of computers and everything electronic.

Your batteries are polarized: electrons come from the negative (-) end and holes from the positive (+). We mark current direction with the flow of holes because early in electrical history the flow of positive charge mattered more to the scientists. A little funny and eternally confusing to new electrical engineers. If you want to make it in the field you need to think in a very real way about the flow of both negative and positive charges.
posted by sbutler at 4:33 PM on September 18, 2005

sbutler: huh? if you are learning device physics, maybe you have to think about holes and electrons, but after that it seems to me the two are equivalent.

after all, the arrows on the symbols for diodes and transistors are all oriented to make sense for hole flow. after learning about electrons as the actual subatomic particle, we can go right back to reasoning about holes instead, as it follows the paradigms which dominate electronics (which were established before anyone knew anything about subatomic physics.)
posted by joeblough at 4:56 PM on September 18, 2005

There's also a band called +/- but I guess that's not what you're going for...
posted by nebulawindphone at 11:01 AM on September 19, 2005

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