The direction of flow of electrons

1. Jul 26, 2006

momomo_mo

I feel frustrated with the direction of flow of electrons in battery, external circuit and conductor.

1. Inside a conductor which is connected to a battery, an electric field is built up inside it. As we know electric field line starts from positive ions to negative ions. Positive ions with flow under columbic force. But electrons will flow in opposite direction. So, electrons will flow from lower potential to higher potential, is it? If yes, where does the energy comes from?

2. Can we say that a battery is a chemical cell? If yes, that means electrons will flow from negative pole to positive pole(from higher potential to lower potential).But I read some of the physics reference books, it stated that electrons flow from lower potential to higher potentail?Which one is correct? The concept is so abstract

3. When the electrons is forced out of the battery, it will flow from -ve to +ve(higher to lowerV) . Again, what’s the energy in moving the electron?

p.s. Actually I don't know why -ve pole is of lower potential and +ve pole is of higher potential? Is it random taking or because there is more electrons in the negative pole and so it is of lower potential?

I found that some of the reference books states that the pole with more positive ions is of higher potential are it correct?

Pp.s.from my point of view, that’s when I took chemical cell as a battery. It seems that electrons in the battery and in the external circuit flow in different direction, is it correct?

Sorry for asking such stupid questions

2. Jul 26, 2006

ZapperZ

Staff Emeritus
The designation of what is "high" and what is "low" is actually done by convention. Nature actually doesn't care what we call it. So assigning something to be + or - is also by convention (or in this case, tradition).

Would it help if you imagine positive charges to be globs of water, while negative charges to be air bubbles? The globs of water, such as water drops, will tend to fall to the ground, whereas air bubbles in water will tend to "fall" upwards.

The "energy" comes from the field, where the field does work on both + and - charges.

Zz.

3. Jul 26, 2006

momomo_mo

BUBBLES that's a good illustration.
HIgh and low is done by convention.so it is not related to the pole with more positive ions is of higher potential?

4. Jul 26, 2006

denni89627

This is a quote from a book i just happened to be reading on the subject.

"In physics, electrical current is considered to flow from the positive to the negative pole. This is known as theoretical current or conventional current. If you connect a light bulb to a battery, therefore, the theoretical current flows out of the positive terminal and into the negative terminal. But the electrons, which carry the charge, flow in the opposite direction, from negative to positive. This is the way engineers usually think about current."
Electricity Demystified by Stan Gibilisco

5. Jul 26, 2006

denni89627

after re-reading you're original post i realize you already knew what i just posted. sorry

6. Jul 26, 2006

BobG

The electrons do move from the negative terminal to the positive terminal, if you're talking about DC current. However, they can't move unless there's a 'hole' to move to. Once they move, the 'hole' has moved to the slot the electron used to occupy. Either way you look at it, a charge has moved. The movement of the 'holes' is what you normally track, since that's the actual electrical energy.

With an AC signal, the electrons don't even move from the negative to the positive. They just move back and forth.

The best way to think of it is the motion and energy transferred by a wave. For example, sound requires air or some other material in order to be transmitted as a wave. It's the movement of the wave you're interested in, not the movement of the air molecules.

7. Jul 26, 2006

momomo_mo

what is that holes?If electrons will flow to the holes,you said that the holes supplied the energy,how does it happen?
I am still frustrated will the term hignV and low V................with -ve terminal and +ve terminal

8. Jul 27, 2006

rcgldr

Some physics classes only use electron flow instead of "positive flow", which makes more sense, since that is what is happening in most cases.

9. Jul 27, 2006

ZapperZ

Staff Emeritus
I will try to answer this question, and a few of the ones you PM'ed me.

Let's say I have a uniform electric field in the +x direction such as this:

--------1-----------2-------->

I have a charge q at position 1, and at some point, it moves to point 2.

Let the potential at Point 1 be V1 and potential at point 2 be V2. Then the potential energy at Point 1 when there's a charge q there is U1=qV1 and the potential energy at Point 2 when there's a charge q there is U2=qV2.

Now, since the E field points in the direction given, we know that V1 > V2. So what is the change in potential energy of the charge in going from Point 1 to Point 2?

$\Delta(U) = U2 - U1$
$\Delta(U) = q(V2 - V1)$

For a positive charge, $\Delta(U)$ is negative because V2-V1 is negative. This means that a positive charge will lose potential energy (the same as a ball dropping in gravitationa field). It means that the field is doing work onto the charge. But what if the charge is negative? Then $\Delta(U)$ is positive because q has a negative sign. This means that it gained in potential energy. An external force has to do work AGAINST the field for you to move a negative charge from point 1 to point 2.

However, this also means that if you want to do the reverse, i.e. move q from point 2 to point 1, then a negative q can easily do this by letting the field does the work without the need of an external force.

So no, a negative charge does NOT gain in potential energy in going from "low" potential to "high" potential. We designated what's "low" and what's "high" by convention, and so is the assignment of the SIGN on a charge. The two of them will "conspire" to create a harmonious view of our universe.

Zz.

10. Jul 27, 2006

momomo_mo

That means is the battery supplied energy for electrons to travel along the circuit but there are no gain of energy of electrons?