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Current is calculated according to the resistances in a circuit. Then, why is the current same everywhere in a series circuit? Shouldn't a greater current pass through the smaller resistance? I am wondering why the total current within a circuit is calculated based on Req, if resistances don't affect the current? I=V/R?

I know similar questions has been asked, but this has a minor detail which was neither asked nor answered. I am wondering why the total current within a circuit is calculated based on Req, if resistances don't affect the current? I=V/R? Current is charge per unit time, but nobody talks about the time factor affecting the current, I know that the number of particles within a series circuit will be the same, but why is the current same?Do the particles know that there will be resistance before they even reach it?

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    $\begingroup$ The elements in the series combination will each have different voltage across them. $\endgroup$
    – The Photon
    Sep 25 '18 at 20:36
  • $\begingroup$ Current is defined as the amount of charge flowing past a point per unit time. If truly is a series circuit (i.e., no branches), then where else is there for the current to go? $\endgroup$
    – user205719
    Sep 25 '18 at 20:39
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    $\begingroup$ In a series circuit, there is only one path for the current to flow. Due to the fact that charge must be conserved, every circuit element in a series circuit MUST have the same amount of current passing through it. $\endgroup$ Sep 25 '18 at 20:39
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    $\begingroup$ Think of it like a hose. All the water that goes in one end had to go out the other. $\endgroup$
    – zeta-band
    Sep 25 '18 at 21:06
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    $\begingroup$ Possible duplicate of Current in series resistors and voltage drop in parallel resistors $\endgroup$ Sep 25 '18 at 22:45
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The two fundamental rules of circuit theory are Kirchoff’s voltage law and Kirchoff’s current law. If you apply KCL at the node between two elements in series then they must have the same current. The value of their resistance or whether they are inductors, capacitors, batteries or any other type of circuit element is irrelevant. They must have the same current from first principles.

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Then, why is the current same everywhere in a series circuit?

Two circuit elements are series connected if they have identical current through, i.e., all of the current through one circuit element is (necessarily) through the other.

The defines series connected.

Now, I understand that you might not be satisfied with this. You might wonder how one can be certain that all of the current through one circuit element must be through the other and, another answer points out, the answer is to apply KCL.

Regardless, the current is the same in a series circuit because, to say that a circuit is a series circuit is to say that all of the circuit elements have identical current through.


if the definition for current is the amount of charge flowing past a point per unit time, then shouldn't current decrease after each resistor in a circuit, since the particles would slow down?

Stipulate for the sake of argument that the current out of a resistor is less than the current into the resistor. What would happen?

Electrons must accumulate within the resistor. But this electron accumulation would repel the electrons flowing into and out of the resistor (electrons repel each other).

That is, any accumulated electrons would act to decrease the current into the resistor and to increase the current out of the resistor thus reducing the accumulated electrons until the current out equals the current in.

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Current is the name we give to the derivative $\frac{\partial Q}{\partial t}$. It represents the change in charge as time passes. You can look at it as the "flow" of charge through the circuit.

Here is a diagram of a simple series circuit with resistors (taken from Wikipedia):

simple series circuit

If in a series circuit $x$ electrons leave the battery in a time $\Delta t$, then, as current flows counterclockwise through the circuit, you must have the same number $x$ of electrons entering the battery again in the same amount of time $\Delta t$. That means that the number of electrons entering the first resistor in time $\Delta t$ must also be $x$, and $x$ electrons must also leave the first resistor to pass on to the second, et cetera, until arriving again at the battery.

If this were not true, electrons would have to be disappearing from the circuit without explanation, but this would violate conservation of charge.

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