Why is the current the same in a series circuit?

[zippyhippy]

Hey guys,

So in class I learned that the current has to have a constant value in a series circuit. We later went on to learn that if the current were not the same, charges would clog up in certain parts of the circuit. I'm having trouble understanding this. I've been scouring the internet for a simulation of this, but I can't find one! Please help me.

To clarify, this is what we learned (maybe a bit off):

We have two resistors linked in a series circuit, one wider than the other (so the wider one has less resistance). At the junction between the wider resistor and the narrower resistor, charge builds up as it tries to enter the narrower resistor. As the charges build up, their electric field pushes against the original electric field of the wider resistor and the electric field of the charges that go through add to the electric field of the narrower resistor.

But how does this mean that the current is the same? What if you had a narrower resistor first, linked to a wider one? Would it be the same?

I also tried to explain it this way:
The charges in the wider resistor are traveling at a slower rate than those in the narrower resistor. However, there is more charge/are more electrons passing a point in the wider resistor than there are in the narrower resistor (more lanes vs. less lane). The extent to which these increase or decrease result in a constant current. The wider resistor has more electrons but a slower speed, while the narrower resistor has less electrons but a greater speed.

Thanks for reading through my essay. I hope you can help me out.

 

[Evil Bunny]

So in class I learned that the current has to have a constant value in a series circuit. We later went on to learn that if the current were not the same, charges would clog up in certain parts of the circuit.

There is no if... There is no "clogging". If you have a complete circuit and you have loads in series on this circuit, the current is identical through the loads. Period. No ifs ands or buts. If something got "clogged" I would imagine you have an open and no current flow.

We have two resistors linked in a series circuit, one wider than the other (so the wider one has less resistance). At the junction between the wider resistor and the narrower resistor, charge builds up as it tries to enter the narrower resistor. As the charges build up, their electric field pushes against the original electric field of the wider resistor and the electric field of the charges that go through add to the electric field of the narrower resistor.

It doesn't really "build up". It's a closed loop. Circulating around and around... nothing has changed as far as "flow" goes. You've squeezed through all the "wider" and "narrower" resistors and now you have a set "speed" that everything is traveling based on the combination of these resistors.

The charges in the wider resistor are traveling at a slower rate than those in the narrower resistor. However, there is more charge/are more electrons passing a point in the wider resistor than there are in the narrower resistor (more lanes vs. less lane). The extent to which these increase or decrease result in a constant current. The wider resistor has more electrons but a slower speed, while the narrower resistor has less electrons but a greater speed.

You could look at it that way if it makes it easier to understand the concept, but in actuality there is no speed change going on anywhere. Everything is moving at exactly the same rate.

 

[nasu]

You could look at it that way if it makes it easier to understand the concept, but in actuality there is no speed change going on anywhere. Everything is moving at exactly the same rate.

The drift velocity does not have to be the same all over the circuit. It may change from point to point even for a series circuit.

 

[Evil Bunny]

Yeah, I guess any single electron could be moving a little (or a lot) faster or slower than any other single electron, but the "net" movement of the group is the same. Hence, the same current...

 

[Naty1]

Because current is proportional to drift velocity, which is, in turn, proportional to the magnitude of an external electric field...,

http://en.wikipedia.org/wiki/Drift_velocity

So with two resistors of different value in series the electric field (potential) across them is different and so the drift velocity is different...but the current is the same...

Zippy posted:

The charges in the wider resistor are traveling at a slower rate than those in the narrower resistor.

yes because the resistance is lower...hence the potential...

However, there is more charge/are more electrons passing a point in the wider resistor than there are in the narrower resistor (more lanes vs. less lane).

nope...

Think of a two lane highway feeding into a four lane highway...just because there are four lanes doesn't mean there are any more cars going by...

 

[nasu]

Yeah, I guess any single electron could be moving a little (or a lot) faster or slower than any other single electron, but the "net" movement of the group is the same. Hence, the same current...

The drift velocity refers to the net movement due to the electric field.
The individual velocity of the electrons contains the thermal motion component (the larger one) and the extra velocity induced by the field (drift velocity).
The rest was answered already by Naty1.