Electric fields and currents

[beenuseren]

Hi,

Could anyone help me with the electric fields.

Here is my doubt.

The electric force that exists between the negative charge and the positive charge, is that force responsible for the motion of electron which constitute current?

If so, there are force field lines which have certain shape and also they show the direction of force. Does the free electron takes the path depicted by the field line which is directed from negative plate of the battery to the positive plate?

IF that is right how is it correct? Because free electrons are present in the conductor wire and the wire can have many twists and turns.

At the end of it the main important thing is, Does the free electron takes the path depicted by the field line which is directed from negative plate of the battery to the positive plate?

kindly provide me correct theory.

Thanks,
Regards,
Beenu.

[sanctus]

Originally Posted by beenuseren

The electric force that exists between the negative charge and the positive charge, is that force responsible for the motion of electron which constitute current?

Yes, but sometimes it can be a bit more complicated (see superconduction, but eventually even that comes down to electromagnetic forces). But this electric force you describe I would call it the electric field. The force is the charge times the field, but if your charges are hold in place you can "see" no force but the field yes.

Originally Posted by beenuseren

If so, there are force field lines which have certain shape and also they show the direction of force.

Actualy, if you consider an electron in that field then it shows you the opposite direction of the force F=qE. and for the electron q<0.

Originally Posted by beenuseren

Does the free electron takes the path depicted by the field line which is directed from negative plate of the battery to the positive plate?

Why should it go anywhere else if there are no other forces/fields?

Originally Posted by beenuseren

IF that is right how is it correct? Because free electrons are present in the conductor wire and the wire can have many twists and turns.

Those materials who are good conductors have valence electrons which are not attached to the nuclei so there are some "free electrons" as you said; but those electrons are confined to rest in the conductor (ie. not attached strongly to a nucleus but attached to "the material"), so the twists are no problem.

Originally Posted by beenuseren

At the end of it the main important thing is, Does the free electron takes the path depicted by the field line which is directed from negative plate of the battery to the positive plate?

Again, why should it not, but I'm no more sure if the field direction goes from"+" to "-" or the other way around.

Originally Posted by beenuseren

kindly provide me correct theory.

I think it is correct, but ?

And by the way welcome to the forums.

[Tim_Lou]

yes, current in a wire (for example) is a result of electric field within the wire. (be it static or non-static)

and no, things do not necessary follow the field lines. the field lines only indicate force and hence acceleration when a charge is placed in that position. and acceleration does not necessary mean the direction of motion. for example, a particle can move in circle while the field is pointing toward the center of the circle.

as santus said, electrons in a wire are like particles bounding around. they are moving in very random fashion. when there is a field presented, they will still be bumping into things and colliding randomly. However, they will slowing tend to moving to the a higher potential (they are negatively charged). a great analogy is water falling from high ground to low ground in a river full of obstacles. (you can think of imaginary positive charges falling from high potential to low potential) people do not discuss about electrons moving in a wire, rather, they say positive charges moving in the oppositing direction. its just the way things are.

btw welcome to the forum.

[beenuseren]

Thanks Tim.

I always study as if negative charge is at higher potential when compared with the positive charge. That hepls me a lot. Because in reality the electron current flows from negative plate to the positive plate.

I have another doubt.

Let us consider two charges, one is negative and the other one positive charge, placed near one another. I believe that from each excessive electron on the negative plate there will be an electric force line directed towards the positive charge. So, the number of force lines is equal to the value of the negative charge. These force lines constitute the electric field.

And we represent these force lines as some form of curves. If these lines donot represent the electron or current path, then why do we have certain shapes for these curves and what do the shape of these curves represent?

Thanks,
regards,
Beenu.

Originally Posted by Tim_Lou

yes, current in a wire (for example) is a result of electric field within the wire. (be it static or non-static)

and no, things do not necessary follow the field lines. the field lines only indicate force and hence acceleration when a charge is placed in that position. and acceleration does not necessary mean the direction of motion. for example, a particle can move in circle while the field is pointing toward the center of the circle.

as santus said, electrons in a wire are like particles bounding around. they are moving in very random fashion. when there is a field presented, they will still be bumping into things and colliding randomly. However, they will slowing tend to moving to the a higher potential (they are negatively charged). a great analogy is water falling from high ground to low ground in a river full of obstacles. (you can think of imaginary positive charges falling from high potential to low potential) people do not discuss about electrons moving in a wire, rather, they say positive charges moving in the oppositing direction. its just the way things are.

btw welcome to the forum.

[Tim_Lou]

well, electric field lines always point away from positive charges direct toward negative charges. since electric field is defined as

field lines do not mean much physically, it is just a visual interpretations of things. relatively speaking, field lines sort of represent "electric field density" (dont take it too literally). the closer the field lines are, the greater the electric field(magnitude). the thing is, lets say you have a positive charge and you want to draw the field lines around it. You can draw 10, 5, 20, or 100 field lines, it wouldn't matter, you will always end having having less field lines further away from the charge (lesser electric field magnitude). its just for visual interpretations.

these field lines are good for understanding certain theories, like gauss's law. (the number of field lines going out an enclosed area - the number of fields lines going in this enclosed area are directly proportional to the charge enclosed--this is gauss law in terms of "field lines").

[beenuseren]

Thanks for the reply. I think I understood.

Here is my another doubt. This is with respect to transistor.

In CE configuration, how does the increase in base current reduces the width of depletion region in the CB junction?

Regards,
beenu

[Qfwfq]

Well, depletion region sounds more like a FET than a BJT, so it does't fit in with base current and CE (common emitter) configuration. Which kind do you mean to ask about?

[beenuseren]

I am talking about BJT, not FET. BJT does have the depletion region at the BE and CB junction.
My doubt is when we increase the base current in BJT(CE config), how does the Vce voltage reduces, there by increasing the current?

thanks,

[Qfwfq]

Originally Posted by beenuseren

I am talking about BJT, not FET. BJT does have the depletion region at the BE and CB junction.

A depletion region forms on each side of an inversely polarized junction, this is the mechanism of a JFET, but the BE junction of a BJT isn't inversely polarized in CE configuration. The other juction is inversely polarized except in saturation, but this in't really central to the functioning of the stage.

Originally Posted by beenuseren

My doubt is when we increase the base current in BJT(CE config), how does the Vce voltage reduces, there by increasing the current?

Cart before the horse. The charge carriers from the emitter, which reach the CE junction before recombination and are hence majority carriers for the base, are what cause the collector current because they cross the CB junction as if it were directly polarized. The voltage on the collector terminal depends on what it's connected to, but usually the stage is designed so there's a resistance and therefore a change in collector current causes a change in voltage at the terminal.

[Qfwfq]

I was typing too fast!

Originally Posted by Qfwfq

The charge carriers from the emitter, which reach the CE junction before recombination and are hence majority carriers for the base

should have been:

The charge carriers from the emitter which reach the CE junction before recombination, and are hence minority carriers for the base,

Sorry.