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Kirchhoff's Current Law states that the net current through a node is always 0. AFAIK this derives from conservation of charge principle. My question is, is KCL applicable to any electrical component? For example is it applicable to transistors, integrated circuits, etc.

My thought is that it should be applicable, because otherwise, the component would be accumulating charge over time, which I presume is not a stable or desirable (in general) condition. Another possibility would be that the component would be "leaking charge". For example, the component would be "throwing charge into air" etc. In this case, the component is not accumulating charge but charge is being moved out of the circuit. I guess this doesn't happen in general as well.

So my question is, is Kirchhoff's Current Law is applicable to any circuit element? For example, if I add up the currents through pins of an integrated circuit at a given time by taking current directions into account, will I get 0 amperes? Similarly for any other circuit elements. Are there any cases where the net current is not 0 amperes?

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4  
Even in your "leaking" case, the net flow through the node is 0, the leak is just another exit path. Note that this is about current flowing through nodes, not about the components (you have e.g. capacitors where you can stuff charge into and it won't come out for a while) – PlasmaHH Jan 15 at 12:18
    
Leaking to the air happens all of the time in a sense: Heat – David Hoelzer Jan 16 at 14:03
up vote 16 down vote accepted

You are exactly right: due to the conservation of charge, which is a direct consequence of the gauge symmetry of electrodynamics and therefore an unbreakable (according to all current knowledge) law of nature, the sum of current over all possible paths summed over all time is always exactly zero. In the case where the current doesn't go through discrete conductors, it's known as Gauss's Law.

For real life electronic components, Kirchoff's current law is exact to the accuracy that all the current flows through the devices pins. This is usually a very good approximation, since any imbalance in charge tends to get balanced due to electric attraction. Some components though, such as an electron gun, break this on purpose, and therefore from a circuit perspective explicitly break Kirchoff's law. Of course if you account for the stream of electrons coming out, the current law holds again.

Now there's a small but important caveat here: the charge only has to be conserved in the end, not at each moment of time separately. That means that if there's a component that stores net charge, the current can enter there, wait for some amount of time as a charge, and the exit only later. However, no practical component stores appreciable net charge for any appreciable amount of time. This is also true of capacitors and batteries: a capacitor stores an equal amount of positive and negative charge on its plates, whereas a battery has positively charged and negatively charged ions which flow (as electric current) to meet each other when the circuit is in operation. In both cases, the net charge is zero at all times, and so the total charge is constant, and Kirchoff's current law still holds. The same also holds for Flash memories, that is, the charge stored is balanced by a hole in the semiconductor.

However, as the The Photon points out in his answer, for components such as antennas, there may be a small but finite time delay between the current entering a component and exiting it.

Nonetheless, for all practical electronics purposes, for example a complicated IC as specifically mentioned by the OP, Kirchoff's current law holds exactly.

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So when I measure the net current through the pins of an integrated circuit (or any other type of component) at a given time, I should get net 0 amperes right? – Utku Jan 15 at 12:24
    
@Utku For all practical purposes, barring exceptions such as the electron gun, yes. – Timo Jan 15 at 12:31
    
I'd like to add that there is one important exception: The sum of all currents into a point equal the change in stored charge at that point – Brog Jan 15 at 12:43
    
@Brog You're of course right. I added an explanation concerning that point to my answer. – Timo Jan 15 at 20:21
    
Don't floating-gate transistors (used in flash memory) store very small amounts of charge for a long period of time? – immibis Jan 16 at 0:58

Kirchoff's circuit laws apply to circuits of lumped elements.

If your circuit contains distributed elements, such as transmission lines and antennas, you can't count on KCL applying absolutely.

For example, in a transient analysis current may flow into an antenna momentarily, without flowing out to any other circuit node, at least until 1/2 a cycle later. If we were to do a full electromagnetic analysis of the situation, we could presumably identify a displacement current from the antenna to the surrounding ground and other circuit elements, but usually such an analysis is too complicated to be tractable.

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Kirchoffs laws assume that we can divide our circuit into "components" where all charge enters and exits components through a pin and that components have no net charge.

This is only an approximation of reality. All real-world components have capacitance to each other and the universe in general. When voltages change this stray capacitance must be charged or discharged which means a net transfer of charge between components. When components physically move the capacitance between them changes and a net charge movement is needed to keep the voltages the same.

Will that affect be measurable? that very much depends on the speeds at which your circuit works and the size of your components.

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