What happens to the current when the circuit is closed?

Question

Electronics

Good afternoon, I would like to understand in more detail what happens at the moment the circuit is closed and why, when resistors are connected in series, we have a constant current and total resistance

Let's have the following circuit


Let the resistors have a resistance of 300 Ohm, 200 Ohm, 100 Ohm

When the circuit is open, there is no current , there is no voltage, there are electrons

When the circuit is closed, the electrons begin to move and the following questions arise: the
electrons begin to move at all points in the circuit at the same time, that is, before and after the resistors, and between them. as if the part that goes through the resistors should move faster because there are no obstacles in its way, and the part that goes through the resistors goes slower. It's right?
If true, then it turns out that the current gradually stabilizes and then already has a constant value throughout the circuit, but at the beginning it should be unstable and not have a fixed value?

I really can’t find information on this moment, or I don’t understand something in what is happening.
I would like to understand at least where to dig, otherwise everything that is everywhere is constant current, resistance adds up, why it adds up and not, for example, the largest is taken, it’s not clear

Answer 1

It's too big a rabbit hole to just dive into.
1. It is necessary to distinguish between a real-life circuit and its mathematical model. In a real-life circuit, there are no ideal resistors and wires - the resistor will, for example, have a parasitic inductance, the key that we close at the beginning of the experiment will not close immediately, but will bounce in the form of several current pulses, and so on.
But the influence of all these effects is either extremely short-lived or small in magnitude, so it is not taken into account.
Having drawn a circuit only from resistors and a key, you immediately make assumptions that the resistors are ideal (have no parasitic inductance), that the whole design is ideal (there is no so-called "capacitance" mounting), that the switch turns on instantly, and there is simply no transient.
If you need to take into account everything discarded above, you will actually get a different circuit (capacitance and inductance will be added), and you will analyze not direct current, but a transient process, which will require compiling vector equations for complex current.
Why do resistors add up resistances - in short, because, on the basis of observations made by scientists, it was established (by Georg Ohm in particular) that substances behave like this. For electrical calculations, it makes little sense to delve into the molecular physics of what is happening, but if you really want to:
you can imagine the resistor as an RVZ-6 tram, which has one door at the beginning of the body and one door at the end. Charge carriers really need to run the tram strictly from one door to another (there is a new challenge in tiktok). The number of charge carriers that ran through the tram per unit of time is directly by definition the strength of the current in the circuit - it is directly proportional to the property "crowding of the tram with passengers" - the more passengers, the less runners-charges per unit of time the tram can pass through itself. Thus, by placing several trams in a row and conducting a challenge, it is possible to establish the law of the speed of tiktokers running through the tram, which will be identical in form to Ohm's law.
Moreover, the tram model also allows you to simultaneously reproduce the Joule-Lenz law - shoving tiktokers will increasingly enrage the audience in the tram - generating hat (and the resistor generates heat - heat). And with a large stream of tiktokers, the tram will go berserk (and the resistor will burn out)

Answer 2

electrons begin to move at all points in the circuit at the same time

yes, all electrons start moving almost instantly.
when a potential difference is applied through an electrically conductive circuit , an electric field instantly (at the speed of light) propagates .
an electric field gradient arises and under the influence of this gradient, the electrons begin to change their movement due to thermal energy, a current appears, the movement of electrons under the action of an electric field - a "current" proportional to the gradient of the electric field, i.e. voltage of the electric field in this area, and inversely proportional to the resistance of the movement of electrons in this area.
the speed of thermal motion of electrons at a temperature of 27 degrees 1.1 * 10^5 m / s = 1100 km / s
the speed of orderly movement of electrons with current10 ^ 7 A / m2 \u003d 10 A / mm 2 (very high current density) is equal to 7.8 * 10 ^ -4 m / s \ u003d 0.78 mm / s, as you can see, the difference is
proof
by large orders. ~10^8 for the speed of thermal motion and difference ~10^13 from the speed of light

Answer 3

as if the part that goes through the resistors should move faster because there are no obstacles in its way, and the part that goes through the resistors goes slower. It's right?

No. The speed of electrons (the amount of charge passing through the wire section per unit time) is constant. To be precise - after closing the circuit becomes constant very quickly, the transient, as a rule, can be neglected.
An obstacle in the path of electrons (resistor) is expressed in the fact that a large potential difference (voltage) acts on the corresponding section of the circuit. U1, U2, U3 are proportional to the resistor values ​​(according to Ohm's law).

Answer 4

Here it is clearly
https://www.youtube.com/watch?v=6Hv2GLtnf2c

Answer 5

Do not be shy, such questions arose in almost everyone who begins to comprehend electrical engineering. That's how they taught us all this, about 50 years ago.
1. Any operating electrical circuit has the form of a ring consisting of series-connected sources of an electric field (electrophysicists say - EMF) and everything else on which this electric field is distributed (resistors, switches, etc.). All this can be present in any quantity, and then the action of the same type of chain elements will have to be summarized.
2.If there is an open switch in the circuit, then the circuit can still be considered as annular, just one of the elements included in it has an infinitely large resistance, and therefore the sum of the electric fields of all sources is distributed to it. Due to the infinite total resistance of the circuit, there is no current in it.
3. If you close the switch, then the electric field propagating at the speed of light is instantly redistributed in proportion to the resistance of the circuit sections. In this way, the circuit "knows" how much current it will receive (after all, it is the same in a series circuit throughout the ring). Electrons begin to slowly push through the crystal lattice of metal wires, and giving them their energy, heat them up.
4.The simultaneity of the movement of electrons is influenced by the local properties of individual sections of the circuit - inductance, capacitance. They are able to store the energy of the field for a while (that's why the pros call them reactivities), and this to some extent changes the uniform distribution of the field. In steady state operation, this is not important, since there is a limited time, but if someone is interested in rapid changes in current, then this has to be taken into account separately.
5.Above, I described the simplest mode of operation of the circuit - steady state. But electrical engineering is inexhaustible, amazing things happen in it - for example, after playing a little with the so-called. "nonlinear elements" and reactivity, it is possible to obtain sections in a series circuit in which the current will many times exceed the average that flows throughout the rest of the circuit. But you do not need this now, and I mentioned it only to interest.