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Let's say you have an LED and a resistor as part of a series circuit. Why does the order of the two not matter? Why does the resistor not have to be between the negative terminal and the LED to reduce the current before it reaches the LED?

I understand that Kirchhoff's Current Law states that current is equal at all points in a series circuit, but I want to know why.

Ian Randman
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  • Because the LED drops the voltage regardless of where it is. – Ignacio Vazquez-Abrams Sep 26 '16 at 01:54
  • The current is equal at all points in a series circuit is because it is the Law. Simply due the law of conservation of mass, since there is no branches for the current flow to escape. – Ale..chenski Sep 26 '16 at 02:14
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    You're probably getting hung up in the common misconception about current flowing through a circuit. Current does not start flowing from the source and adjust it's behavior as it meets various components along the way. The current flows simultaneously through all parts of the circuit as soon as voltage is applied. The entire circuit must be solved from start to finish before calculating the current. – Dan Laks Sep 26 '16 at 02:49
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    current is in a loop not a cascaded waterfall. – Tony Stewart EE75 Sep 26 '16 at 02:59

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Current is equal because neither LED's nor resistors nor wires, nor any other circuit elements can store or destroy or create charge. This is the conservation of charge principle.

So for every charge that enters a wire, there is one at the far end which exits. Likewise for the LED and resistor and capacitor and inductor. This is why it does not matter.

Conservation of charge is the basic principle behind Kirchhoff's current law.

user57037
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    @Andreas, you are very much mistaken. Capacitors obey conservation of charge and satisfy Kirchhoff's current laws instantaneously and at all times exactly the way other circuit elements do. If you are talking about propagation delay due to EM wave, well, that affects all components, not just capacitors. – user57037 Sep 26 '16 at 03:05
  • I have deleted a comment with a thought (about conservation of charge in capacitors) that came to me late at night, probably while I was dizzy. Mkeiths answer to this comment is correct. – Andreas Sep 26 '16 at 08:43
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You ask why current is equal at all points in a series circuit.

Charge is conserved, like water in a perfect pipe.

And that is backed up by either the very unsatisfactory, or the very deep, 'because that's the way we model an ideal series circuit!', or 'that's what an ideal series circuit means'.

In an ideal series circuit, it's small enough that electrical effects propagate instantaneously from one point to another, all components are ideal, and as this is a series circuit, there are no shunt components.

If we are modelling no shunt paths, then there is nowhere for charge to 'leak out', through a shunt resistance, or a hole in the pipe. There is nowhere for charge to 'accumulate', into a shunt capacitor, or a balloon-like section of the pipe.

Real components might not be so simple. A real resistor has stray capacitance to ground, which can charge up, so more charge enters than leaves. But this is not a simple series circuit, it's something more complicated with a shunt component. Not only that, the effect of this stray capacitance is completely negligible in a battery+resistor+LED circuit, we can ignore it completely.

And this is the way we start to learn circuits, ideal components with instantaneous propagation.

99% of people (probably an underestimate) never need to look beyond this simple model.

When you do need to consider finite speed propagation, Kirchoff's rules still apply, it's just that we have a much more complicated ideal circuit, with harder rules to apply.

Neil_UK
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See the series connection as a road with wide and narrow sections. All cars driving on that road ( The current) must pass both the wide and the narrow sections. There is no other road. In electrical terms the serial chain offers no escape to the flowing current. Here it is not important where the wide and narrow sections are positioned. The same with the resistor and the led.

Decapod
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