Question on Voltage and Charge Carriers

[ParanoiA]

I'm not sure I get what you're aiming at here. Could you elaborate?

 

I will certainly try. I'm trying to conceptualize positive charge in terms of electron movement, say, in a battery cell. I can understand the "concept" of moving a positive charge against the field toward the positive source of the field, thereby increasing its potential energy. But I would only see a positive charge move against the field in a solution I guess, because you can't move ions in metals and that's the only way I know of a positive charge "moving" against a field. Otherwise, how does positive charge move? (In electrostatics positive charge redistributing itself was *actually* electrons redistributing themselves, so electrons are moving despite our concept of positive charge moving).

 

Ok, so that's inside the cell. Now, I have the same questions for outside of the cell, powering a circuit. If I have lots of positive charge, again, how am I moving positive charge over wire? We can't do that, and we know that electrons are moving, not positively charged ions nor protons. So I'm stuck again trying to figure out how electrons are moving using a concept that describes the movement of positive charge.

 

I guess that's what I'm struggling with now. Trying to work out electron movement in the context of positive charge movement. It's making it very difficult to grasp. But I can't just forget that electrons are moving around in metals, not positive charges, so I'm also a little frustrated. I'm guessing there's a good reason for all of this though....

 

 

Here's another quickie that's been bugging me: We can induce charge in two insulators by using friction - like a balloon and animal fur. If I remember correctly, rubber has more of an affinity for electrons than animal fur and thus takes on a negative charge while the fur, I would suppose is left with a positive charge. So...how do they lose their charge then? After I'm done sticking the balloon to the wall to wow the kiddos, how exactly does the balloon lose its charge and eventually fall to the ground? If only conductors allow electrons to move about their surface, and it took an act of friction to charge these insulators, then how do they casually become neutral again?

Edited March 9, 2011 by ParanoiA

[Cap'n Refsmmat]

The reason that classical circuit analysis treats electricity as moving positive charges is because they didn't know what electricity really was. It turns out that in the mathematics, though, an electron moving one way is the same as a positive charge moving the other way, even if there's no physical meaning to saying positive charges are moving in a conductor.

 

So positive charges aren't moving over wires. Rather, negative charges move the opposite direction. The moving positive charges are really just a lack of negative charges. If I move a lot of negative charges to one end of a wire, it becomes more negatively charged, and the other end becomes more positively charged, in exactly the same way as what would happen if I moved positive charges the opposite way.

 

Just think about things solely as electron motion and it should work out.

 

Here's another quickie that's been bugging me: We can induce charge in two insulators by using friction - like a balloon and animal fur. If I remember correctly, rubber has more of an affinity for electrons than animal fur and thus takes on a negative charge while the fur, I would suppose is left with a positive charge. So...how do they lose their charge then? After I'm done sticking the balloon to the wall to wow the kiddos, how exactly does the balloon lose its charge and eventually fall to the ground? If only conductors allow electrons to move about their surface, and it took an act of friction to charge these insulators, then how do they casually become neutral again?

Right, and the "positive charge" in the fur is just caused by a lack of electrons, not a gain in protons.

 

The key is that insulators are not perfect. Some charge will "leak" despite your best efforts. Humid air even conducts, very slightly.

[swansont]

The key is that insulators are not perfect.

 

You can take the view that everything is a conductor, it's just a matter of how good or bad it is. What we're doing is drawing an arbitrary line and saying everything on one side is an insulator, because we generally focus on really good conductors and really bad ones.

[ParanoiA]

The reason that classical circuit analysis treats electricity as moving positive charges is because they didn't know what electricity really was. It turns out that in the mathematics, though, an electron moving one way is the same as a positive charge moving the other way, even if there's no physical meaning to saying positive charges are moving in a conductor.

 

So positive charges aren't moving over wires. Rather, negative charges move the opposite direction. The moving positive charges are really just a lack of negative charges. If I move a lot of negative charges to one end of a wire, it becomes more negatively charged, and the other end becomes more positively charged, in exactly the same way as what would happen if I moved positive charges the opposite way.

 

Just think about things solely as electron motion and it should work out.

 

That does make sense. I'm working on that now too. I'm really enjoying this Physics Classroom site. So far, it's really good about restating ideas as they add on to them. It's really helped a lot with electrostatics, so I'm going to stay on this through to electric fields and potential and maybe it will spell out this electron movement for me. I don't mind carrying on with positive charge flow once I get a firm grip on this part.

 

 

Right, and the "positive charge" in the fur is just caused by a lack of electrons, not a gain in protons.

 

The key is that insulators are not perfect. Some charge will "leak" despite your best efforts. Humid air even conducts, very slightly.

 

You can take the view that everything is a conductor, it's just a matter of how good or bad it is. What we're doing is drawing an arbitrary line and saying everything on one side is an insulator, because we generally focus on really good conductors and really bad ones.

 

Both points noted. So, just to clarify, are we saying that a *perfect* insulator would not lose its charge without another act of friction?

 

Because I'm trying to imagine grabbing a piece of copper wire and touching it to the charged ballon and the other end to ground - and if the balloon were a perfect insulator, it should remain charged since we can't *transfer* charge that way. Even if, in reality, we may be able to neutralize the balloon that way.

 

The Physics Classroom has been good about mentioning humid air as a decent conductor, and how it normally absorbs most of our excess charges, like shuffling our feet across the carpet. I was trying to envision how air particles, or moisture, are interacting with the surface of the balloon in order to neutralize the charge.

[Cap'n Refsmmat]

Here's the way we thought of it in my electricity & magnetism course:

 

In an insulator, any charge you give it is trapped. You can deposit electrons on the surface of the insulator, giving it a negative charge, but those electrons will stay in a fixed location, and they will not conduct through the insulator to another location. However, if you put a wire on the surface of the insulator, the electrons there at the surface may be conducted out through the wire. The electrons not in contact with the wire, however, will be unable to move and will stay on the insulator.

 

Of course, in practice electrons with sufficient energy can move around if they want to, just with some difficulty, and so some electrons will move through the insulator. It's just that conductors make this far easier.

[ParanoiA]

Hey, your description matches nicely. This issue just so happened to be included on the bit on conductive charging...

 

The charging of an electroscope by contact with a negatively charged golf tube (or any charged insulating object) would best be described as charging by lightning. Rather than being a process in which the two objects act together to share the excess charge, the process could best be described as the successful effort of electrons to burst through the space (air) between objects. The presence of a negatively charged plastic tube is capable of ionizing the air surrounding the tube and allowing excess electrons on the plastic tube to be conducted through the air to the electroscope. This transfer of charge can happen with or without touching. In fact, on a dry winter day the process of charging the metal electroscope with the charged insulator often occurs while the insulator is some distance away. The dry air is more easily ionized and a greater quantity of electrons is capable of bursting through the space between the two objects. On such occasions, a crackling sound is often heard and a flash of light is seen if the room is darkened. This phenomenon, occurring from several centimeters away, certainly does not fit the description of contact charging.

 

Like you and Swansont have said, real objects are conductive and insulative on a sliding scale. It sounds like putting a wire on the surface of an insulator is tempting those electrons in that spot to make the jump for bigger real estate. I just need to keep in mind that I will not get a balanced system between the insulator and the conductor like I would between two conductors - the electrons will not redistribute uniformly so the two objects share the excess charge. This maintains our distinction between the two types of materials.

[ParanoiA]

Quick question...why does the direction of the electric field matter?

 

For instance, a positive source charge will direct the electric field "force" outward, yet negative charges will be attracted toward the positive source charge. Any description of force will be attractive, not repulsive, so what is the point of directionality here? (I realize a positive test charge will be directed outward from the positive source of the field, but again that seems to ignore field direction in favor of charge association - I don't see how field direction carries any consequence).

[swansont]

Quick question...why does the direction of the electric field matter?

 

For instance, a positive source charge will direct the electric field "force" outward, yet negative charges will be attracted toward the positive source charge. Any description of force will be attractive, not repulsive, so what is the point of directionality here? (I realize a positive test charge will be directed outward from the positive source of the field, but again that seems to ignore field direction in favor of charge association - I don't see how field direction carries any consequence).

 

It's by convention. The direction of the field is the direction of the force a positive charge would feel. You do have situations where positive charges move around (proton accelerator, alpha particles, positrons), and getting the sign consistently right is useful.

[Mr Skeptic]

The direction of the field tells you which way "positive charge" will flow. In the case of an induced electric field, there will be no initial charge separation so you can't do that by charge association.

[ParanoiA]

You all are really helping me out here, not sure if you realize that or not. Much thanks, sincerely.

 

 

Kind of going back to a question I had earlier that Swansont responded to, but now I have an example. Fairly easy I think...if I have a negatively charged field then a positive charge would be drawn toward the field. So, as I increase the distance I increase the potential energy of the positive charge. But this is what seems so odd, it's as if I can just increase this distance and just keep increasing the potential energy - as if positive charges in Europe are loaded with potential energy from negative fields in Alaska. At that distance, we should have some serious potential energy.

[Cap'n Refsmmat]

Kind of going back to a question I had earlier that Swansont responded to, but now I have an example. Fairly easy I think...if I have a negatively charged field then a positive charge would be drawn toward the field. So, as I increase the distance I increase the potential energy of the positive charge. But this is what seems so odd, it's as if I can just increase this distance and just keep increasing the potential energy - as if positive charges in Europe are loaded with potential energy from negative fields in Alaska. At that distance, we should have some serious potential energy.

This is true, but remember that the potential is given by:

 

[math]U=\frac{1}{4\pi \epsilon_0}\frac{q_1 q_2}{r}[/math]

 

where [imath]q_1[/imath] is the electric charge creating the field and [imath]q_2[/imath] the charge experiencing the field, and [imath]r[/imath] is the distance between them. The increase in potential decreases the farther away you get, if that makes sense, and eventually it's negligible.

[ParanoiA]

This is true, but remember that the potential is given by:

 

[math]U=\frac{1}{4\pi \epsilon_0}\frac{q_1 q_2}{r}[/math]

 

where [imath]q_1[/imath] is the electric charge creating the field and [imath]q_2[/imath] the charge experiencing the field, and [imath]r[/imath] is the distance between them. The increase in potential decreases the farther away you get, if that makes sense, and eventually it's negligible.

 

Thanks. I think that does make sense, yes. When thinking about a positive field, and a positive charge, it's easy to see the extreme for high PE being as close as you can get to the source. But when you change that to a negative field, then you're high PE is as far as you can get from the source, which isn't as easy to see in the extreme - the threshold is not so visibly obvious.

Edited March 15, 2011 by ParanoiA

[Mr Skeptic]

This is true, but remember that the potential is given by:

 

[math]U=\frac{1}{4\pi \epsilon_0}\frac{q_1 q_2}{r}[/math]

 

where [imath]q_1[/imath] is the electric charge creating the field and [imath]q_2[/imath] the charge experiencing the field, and [imath]r[/imath] is the distance between them. The increase in potential decreases the farther away you get, if that makes sense, and eventually it's negligible.

 

And, that's without the funny stuff that happens when you connect the two via a conductor. If you confine almost all the electric field in a 1 dimensional object, force won't decrease with distance along the wire (thus the electric field induced in a wire by a voltage difference is essentially the same along its length, which helps explain formulas like V=IR, so that the longer wire just has a weaker field along a longer path for the same total potential).

[ParanoiA]

Well, I can certainly "see" voltage now. I understand how amount of charge determines the strength of the field, and how the position of a charge placed in that field determines the electric potential on that charge, in that spot - which is measured in terms of potential energy per charge, Joules/Coulomb. One Volt equals one joule/coulomb.

 

I can see how pushing a charge against the field creates the "pressure" we exploit in circuits. The potential difference is the difference in electric potential between the initial and final locations in the field - the work performed by the electrochemical solution in the battery. The battery, a series of cells, is designed to create an electric potential difference between two points. A 12 volt battery is designed to require about 12 Joules/coulomb of work (energy) to push positive charge toward the high energy potential, providing for 12 Joules/Coulomb of work to an external circuit.

 

All of this has created a numerous list of questions I would pester a poor electronics/physics teacher to their death with....but I'm patient and will wait until I learn about the actual parts and operation of the battery before I go down that road.

 

 

 

One question that has bugged me from the beginning though....since we know a voltage requires a source field and reference point within that field, the position, then what is the reference point for the claim that shuffling our feet on the carpet creates thousands of volts of charge on our bodies? It's only that many volts with respect to a certain position in the field emanating from the charge on my body - where is this position they're assuming?

[Kris K.]

I love this stuff........it's not as exotic or vague as it seems.................simply...........voltage is exactly the VELOCITY of pushing the AMPERAGE down the line. The amperage is the muscle, that does the trick. WATTAGE is simply the measurement of HOW MUCH IS USED.

 

Elementary this way, my dear Watson...............lol

 

 

krist..............

[swansont]

I love this stuff........it's not as exotic or vague as it seems.................simply...........voltage is exactly the VELOCITY of pushing the AMPERAGE down the line. The amperage is the muscle, that does the trick. WATTAGE is simply the measurement of HOW MUCH IS USED.

 

 

No, and no.