batteries, electric potential and current

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mikau16
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batteries, electric potential and current

Postby mikau16 » Thu Feb 19, 2009 5:59 pm UTC

So I understand that the purpose of a battery is to set up an electric potential difference between its terminals, and the end with higher potential is marked + and lower potential is marked -,

thus the electric potential energy increases as a positive test charge is moved from - to +, and thus negative work is done on the test charge, which means the electric field must be directed opposite to its path from - to +. So the electric field is directed from + to -.

BUT! this means that an electron (a negative charge) placed in this field would flow opposite the direction of the field, which is from - to +, and as far as I understand, electrons are supposed to flow from + to -

where is the flaw in my reasoning?

my only suspicion is the electric potential between the terminals of a battery is defined to be the amount of work required to move negative test charge from one to the other.

suppose the battery does positive work on a (negative) test charge moving from a to b, then there is a negative change in electric potential from a to b and so Vf - Vi < 0, so Vf < Vi, labeling Vf as - and Vi as + gives an initial terminal of + and a final terminal of - thus the electric field does positive work on an electron moving from the + to the - terminal

makes sense, but that's just a guess.

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meat.paste
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Re: batteries, electric potential and current

Postby meat.paste » Thu Feb 19, 2009 6:18 pm UTC

Way back when they were standardizing what + meant in electricity, they didn't know about the electron. So, the sign convention was adopted that shows current flowing from the + terminal to the - one. This current is carried by positively charged holes. These holes do not have a grounding in a physical sense. The holes are simply where the electrons aren't. The actual electrons are flowing from the negative terminal to the positive terminal.

I hope this doesn't cause more confusion.

douglasm
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Re: batteries, electric potential and current

Postby douglasm » Thu Feb 19, 2009 6:36 pm UTC

The designation of which of a proton and electron has a positive or negative charge is largely arbitrary. As long as you are consistent, the math works out just fine with electrons as positively charged and protons negative. Also, most effects do not readily distinguish between positive charges moving in one direction and negative charges moving in the opposite direction.

It just so happens that whoever originally came up with the labels for such things guessed wrong about the direction charged particles were moving in and by the time someone discovered a way to tell the difference the mislabeling was too well established to change. What is generally labeled as electric current moving in a particular direction is actually electrons moving in the opposite direction. Electrons do, in fact, flow from - to +.

mikau16
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Re: batteries, electric potential and current

Postby mikau16 » Thu Feb 19, 2009 7:17 pm UTC

My thanks to both of you for the quick and helpful response. I understood you both completely.

but something's screwy.. my book claims that when you connect a capacitor consisting of two parallel plates, to either ends of a battery, with plate H connected to the positive terminal, and plate L connected to the negative terminal, plate H becomes positively charged, and L negatively charged. If the flow of electrons is in fact from - to + then plate H connected to the positive terminal, should definitely become NEGATIVELY charged, no?

in fact, my book includes this exact sentence:

The field drives just as many electrons from the negative terminal of the battery to capacitor plate L


this clearly contradicts what you two are saying, does it not?

Moreover, if I assume + and - to be defined as the terminals with higher and lower electric potential, respectively, (as the book did) then as I argued earlier, this implies a flow of electrons from - to + which agrees with the convention you two have stated.

so is the book wrong or what?

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danpilon54
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Re: batteries, electric potential and current

Postby danpilon54 » Thu Feb 19, 2009 7:41 pm UTC

Electrons flow from the negative terminal to plate L, making it negatively charged. The positive terminal "pulls" electrons out of plate H making it positively charged. From what I can tell the book is correct, but worded in a confusing manner.
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mikau16
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Re: batteries, electric potential and current

Postby mikau16 » Thu Feb 19, 2009 7:49 pm UTC

unless i am horrribly sleep deprived, what you just said implies electrons flow from positive to negative.

mikau16
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Re: batteries, electric potential and current

Postby mikau16 » Thu Feb 19, 2009 8:19 pm UTC

*spits up breakfast* hold on a second! does the electric field inside a battery look something like this?

let | | be the battery and >> denote the direction of the electric field:
>> | >> | >>
(my original assumption)

or this:
<< | >> | <<
(my new suspicion)

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Gammashield
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Re: batteries, electric potential and current

Postby Gammashield » Thu Feb 19, 2009 8:49 pm UTC

mikau16 wrote:unless i am horribly sleep deprived, what you just said implies electrons flow from positive to negative.


Sadly, you're horribly sleep deprived. :p

Think of it this way. The negative terminal has an overabundance of electrons, far more than an electrically neutral bit of metal, while the positive terminal has too few electrons: more positive atomic nuclei charges than electrons. So it's a net 'positive'.

If we had a simple battery-and-wire setup, no capacitor or anything, electrons would flow *out* of the negative terminal, move through the whole wire, and flow *into* the positive terminal. So, from negative, to positive.


Now, in a circuit with a capacitor in it, electrons flow *out* of the negative terminal, along the wire... and hit the L plate, where there's a dead-end. They pile up there continuously, making a bigger and bigger negative charge until finally shoving in any more electrons takes too much energy, and electricity stops flowing and we reach a steady state with no current, and a negatively charged L-plate.

In the meantime, this large negative charge on the L plate "pushes away" electrons on the H plate which flow along their wire *into* the positive terminal (which has room for more electrons after all) so we see current on that wire... until finally enough electrons have moved away from the H plate so that there's a strong positive charge (not enough electrons after all), and flowing into the battery's positive terminal no longer makes sense, energetically.

So, if we look at the big picture, we have electrons flowing *out* of the negative terminal of the battery, and *into* the positive terminal (at least until enough charge piles up at the capacitor to stop further flow). So, electrons flow from negative, to positive.

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Gammashield
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Re: batteries, electric potential and current

Postby Gammashield » Thu Feb 19, 2009 8:52 pm UTC

mikau16 wrote:*spits up breakfast* hold on a second! does the electric field inside a battery look something like this?

let | | be the battery and >> denote the direction of the electric field:
>> | >> | >>
(my original assumption)

or this:
<< | >> | <<
(my new suspicion)



Your 'new suspicion' is right. Lines always flow *out* of one terminal, and *into* the other one.

Though, as a practical matter there's usually insulation between the two halves of the battery, so no electrons can actually move from one terminal to the other except via an external wire connecting them. So, while the field *is* like your second case, electrons can't actually flow along the 'inner' battery route.

mikau16
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Re: batteries, electric potential and current

Postby mikau16 » Thu Feb 19, 2009 9:36 pm UTC

Ahhh! Now I see!

The flaw in my reasoning was I assumed the electric field inside the battery was the same as that in the wire outside the battery. You'd think it would be the case, since charge is flowing through it.. hinky!

But reversing the direction of the electric field inside the battery fixes everything, all the definitions and equations make sense now.

My thanks to you, Gammashield, and the rest of you for clarifying. You were all very helpful.

ONWARD TO THE CENTER OF THE EARTH!


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