Electric current

anpanman SG

An electric current flows through the resistor as shown in the diagram above. What is the amount of charge passing through point "b" in 1 second?

This question seems weird. I can't apply the charge = current X time formula directly because the current will definitely decrease after passing through the resistor. You can't apply the Work done = Charge X Voltage formula or V = RI formula either.

I'd be glad if someone could help. Lots of thanks.

Twincat SG

Current is the same throughout a series ciruit.

eagle SG

because the current will definitely decrease after passing through the resistor

This is a wrong statement

As Twincat has put it, current is the same throughout the series circuit.

Audioboxing SG

Hi I would like to take this chance to ask some of the more educated people here (no scarcasm implied)

Question: Why is the current constant throughout?

16/f/lonely SG

Hi I would like to take this chance to ask some of the more educated people here (no scarcasm implied)

Question: Why is the current constant throughout?

Because current is the flow of electric charge. So you can say it measures the number of electrons flowing past a point.

Thus said, current then has to be the same, measured at any point of a series circuit, if you understand the above sentence.

And if you understand that, then you'll understand why the same is not true for parallel circuit.

Audioboxing SG

Yes, but when in an electric field the electrons accelerate and generally the acceleration is not constant. Furthermore, the collision of the electrons with the atoms of the conductor would have then reduced the momentum of the electrons. So that would imply that the current is not constant.

I'm confused.

16/f/lonely SG

I'm confused.

Consider this:

the electrons are bouncing all over within the wires.

The only way for them to move in ONE direction TOGETHER, is to apply a potential difference that would create an electric field.

This would cause all of them to move as a whole in ONE direction while still moving randomly. In other words, although they still bounce around, the electric field will cause their movements to be translated to the direction dictated by the field while still colliding and moving randomly.

So if they all move in that direction at the same speed (we look only at the velocity of the elctrons in that direction), then the current will have to be same throughout a series circuit.

eagle SG

I'm confused.

Don't worry about your first post. It is a very good question.

Generally, how electrons move in a wire/conductor is very different from how electrons move in an electric field (without any dielectric I mean). In a wire/conductor, electrons do not move straight, but rather, in haphazard directions. However, the general direction is still in the direction of the current.

As to why when we see electricity at work immediately after we flipped the switch on, it's because of the repelling force of electrons; the electrons move a little, and it will repel the neighbouring electrons, and so on, throughout the circuit. This repelling force, if I remember correctly, should travel at the speed of light.

16/f/lonely SG

Don't worry about your first post. It is a very good question.

The speed of the general drift of electrons in a circuit is rather slow if I recall correctly.....in the order of millimetres per second.

eagle SG

The speed of the general drift of electrons in a circuit is rather slow if I recall correctly.....in the order of millimetres per second.

That's right. It's called drift velocity.

Audioboxing SG

I see... that's very insightful. However what about the problem with the acceleration of the electrons? Since the electrons will move faster (or slower) at some time so wouldn't that also imply that the current is not constant? I never understood this...

edit: Could it be due to the fact that there is no electric field inside a conductor? Hence the constant current? However, considering resistance, wouldn't a resistance be a dielectric? Again leading to the conclusion that current is not constant. Hmm, don't get where I got it wrong...

eagle SG

think of this

if the charges are being 'pushed out' by the e.m.f. at a certain rate, then they have to move through the circuit at that rate no matter there's resistance a not.

just like.... when a EM wave passes from an optically less dense medium to an optically denser medium, the frequency of the wave doesn't change... this EM wave thing is just an analogy hor.... not 100% related...

Audioboxing SG

But wouldn't that mean that the frequency of the 'electron emission' is constant, and not the current?

I think an example to illustrate my point is to consider a distribution of positive charges in a volume, and a single negative charge (say an electron) is placed at a certain velocity in the electric field of the positive charges. So the electron will accelerate because there is a net force on it. Imagine that the electron is constrained to move in a possible closed path in the region near the positive charges. That would imply that the current is not constant, since the electron is faster at some time and slower at some time...

Surely you wouldn't say that the 'current' of a string of marbles sliding down a curved slide in a playground is constant? Since the marbles do not have constant speed... ?

eagle SG

Your first statement is already wrong...

If the frequency of 'electron emission' is constant, then the 'emitted electrons' will 'push' the next electrons to move on, and so on so forth throughout the whole wire. In that sense, the charges are moving at the constant frequency. And because of that, the current is constant. That's why when we switch off the battery/circuit, immediately, current is zero. This is because the 'electron emission' out of the battery is now zero.

Edit: last statement deleted.... a bit vague... let me go think about it

Audioboxing SG

Your first statement is already wrong...

Edit: last statement deleted.... a bit vague... let me go think about it

Looks like you don't sleep either, lol

Anyway, I still don't think the constant 'frequency of electron emission' explains the constant current... I might be very wrong though. =D

Electric charges do move only because of an electric field... one of the fundemental interactions of nature.

eagle SG

Err.... electric charges do not only move because of electric fields....

And the "electron emission" thingy.... still stands...

Anyway for your question... let's see it in this simplified way. In a resistor, it sorts of "slow down" the movement of electrons until they move at a constant speed. The higher the resistance, the more it "slows down" the movement. The lower the resistance, the lesser it "slows down"

Take this simple analogy... It's just like slow moving constant traffic in high traffic roads vs slightly faster moving constant traffic in lesser traffic roads vs very fast moving constant traffic in zero traffic roads.

Or rather, take the analogy given in schools... Battery is like a motor, and electrons is like the water that the motor is driving through the pipe. If the motor turn at a constant speed, is the flow of water going to be the same? If the rate of flow of water gets faster, it will "push" the motor to turn even faster.... yet if the rate of flow of water gets slower, it will be "pushed" from behind by other water... doesn't make much sense right?

It's quite hard to explain over the net... There's a need for a lot of graphics drawing and visualisation... You should ask your teacher or tuition teacher to draw out for you... I'm a bit reluctant to add more info without some form of drawing because it might confuse you even more...

Sry for being blunt, but I think you are just confusing yourself with your question... Your question is like.... why does an electron contains charge? Why does moving charges give a current? Things like that... Chances are, your question isn't found in most uni physics textbooks... If you are interested, you could come over to around YCK road there and I could lend you a few of such textbooks to read through :D

Audioboxing SG

hmm...I guess I will ask my teacher about this 'emission' thingy. Thanks for responding the question though! Appreciate it very much.

eagle SG

it's not really emission.... that's why without graphical visualisation... more and more confusing things will come out -.-"