are kirchoff laws useless?

[IanB]

The obvious case is where current flows into an antenna and disappears out of the end as radio waves.

It certainly doesn't! 

[Cerebus]

The obvious case is where current flows into an antenna and disappears out of the end as radio waves.

It certainly doesn't! 

Well of course it doesn't actually disappear, but in the naïve view where there is only KCL and KVL at least some of current into an antenna (aka a wire that just ends and doesn't close a loop) just goes 'somewhere else'. It only makes sense once you include a equivalent circuit for an antenna that implicitly hauls the world of electromagnetism into the nodes and vertices of the KCL/KVL world.

And I think a naïve view is appropriate here when the OP asks "are kirchoff laws useless?"(sic). If you want to try explaining exactly what's really going on to the OP such that he properly understands it and uses no simplistic or naïve examples, I will gladly observe with, depending on how well you do, awe or amusement.

[IanB]

Well of course it doesn't actually disappear, but in the naïve view where there is only KCL and KVL at least some of current into an antenna (aka a wire that just ends and doesn't close a loop) just goes 'somewhere else'. It only makes sense once you include a equivalent circuit for an antenna that implicitly hauls the world of electromagnetism into the nodes and vertices of the KCL/KVL world.

This is just not correct. KCL is observed perfectly in an antenna, it has to be. The current law is an expression of the conservation of charge, and that is a fundamental and inviolable law of physics. Unless charge leaks out of the antenna by corona discharge or some other mechanism, current is conserved.

If you were to pick a point on the wire feeding the antenna and then integrate the current flow across that boundary over time, then the total would approach zero over a sufficiently long time interval.

[bson]

It only makes sense once you include a equivalent circuit for an antenna

All circuits are equivalent circuits.

[photon]

Well of course it doesn't actually disappear, but in the naïve view where there is only KCL and KVL at least some of current into an antenna (aka a wire that just ends and doesn't close a loop) just goes 'somewhere else'. It only makes sense once you include a equivalent circuit for an antenna that implicitly hauls the world of electromagnetism into the nodes and vertices of the KCL/KVL world.

This is just not correct. KCL is observed perfectly in an antenna, it has to be. The current law is an expression of the conservation of charge, and that is a fundamental and inviolable law of physics. Unless charge leaks out of the antenna by corona discharge or some other mechanism, current is conserved.

If you were to pick a point on the wire feeding the antenna and then integrate the current flow across that boundary over time, then the total would approach zero over a sufficiently long time interval.

What is lost is not the current (I) but the energy (QV). Since charge (Q) is conserved, the energy loss is from heat dissipated from the battery and the other matter in the antenna.

[IanB]

What is lost is not the current (I) but the energy (QV). Since charge (Q) is conserved, the energy loss is from heat dissipated from the battery and the other matter in the antenna.

The energy loss from heat dissipation is a parasitic loss, which ideally would be as little as possible. The energy dissipation of value is of course by electromagnetic radiation.

[Cerebus]

Well of course it doesn't actually disappear, but in the naïve view where there is only KCL and KVL at least some of current into an antenna (aka a wire that just ends and doesn't close a loop) just goes 'somewhere else'. It only makes sense once you include a equivalent circuit for an antenna that implicitly hauls the world of electromagnetism into the nodes and vertices of the KCL/KVL world.

This is just not correct. KCL is observed perfectly in an antenna, it has to be. The current law is an expression of the conservation of charge, and that is a fundamental and inviolable law of physics. Unless charge leaks out of the antenna by corona discharge or some other mechanism, current is conserved.

If you were to pick a point on the wire feeding the antenna and then integrate the current flow across that boundary over time, then the total would approach zero over a sufficiently long time interval.

I think you're missing the point here and also being unnecessarily pedantic - you've passed the test, you're an engineer. The whole sub-thread here is the result of a throwaway explanation of a throwaway reference to Maxwell. The point, if there is any, is that KCL and KVL do not fully account for electromagnetics. You can worry away at the details of a throwaway example for as long as you like but it won't alter the fact that (1) in context, it doesn't really matter, (2) I don't really care if I was pedantically, exactly correct in the precise technical detail of a throwaway remark that was meant to merely illustrate something.

[albert22]

For an EE Kirchoff laws are useful and unavoidable. Provided that can be applied;
quote from wikipedia

Both of Kirchhoff's laws can be understood as corollaries of the Maxwell equations in the low-frequency limit. They are accurate for DC circuits, and for AC circuits at frequencies where the wavelengths of electromagnetic radiation are very large compared to the circuits.
...
Limitations

KCL and KVL both depend on the lumped element model being applicable to the circuit in question. When the model is not applicable, the laws do not apply.
...

For a carpenter they are useless.

[IanB]

The point, if there is any, is that KCL and KVL do not fully account for electromagnetics.

I'm disagreeing with you because I think they do fully account for electromagnetics.

I think KCL expresses a general rule about conservation of charge, and KVL expresses a general rule about the conservative nature of electric fields. As such they are (when accumulation terms and propagation delays are neglected) axiomatically true. They are true in transformers, in antennas and anywhere else that you can measure currents and voltages without disturbing the circuit.

Since we seem to be disagreeing about this point it appears we need another expert person to arbitrate.

[photon]

What is lost is not the current (I) but the energy (QV). Since charge (Q) is conserved, the energy loss is from heat dissipated from the battery and the other matter in the antenna.

The energy loss from heat dissipation is a parasitic loss, which ideally would be as little as possible. The energy dissipation of value is of course by electromagnetic radiation.

Energy_in = power supply. Energy_out = EM waves + losses. Losses = heat. No loss of charge or current.

[IanB]

Energy_in = power supply. Energy_out = EM waves + losses. Losses = heat. No loss of charge or current.

Of course. We are in complete agreement on that. Maybe I misunderstood your earlier comment when you seemed to imply that the energy losses were only to heat.

[mrpackethead]

beetroot

[rfeecs]

How about just reading the Wikipedia article:
https://en.wikipedia.org/wiki/Kirchhoff%27s_circuit_laws#Kirchhoff.27s_voltage_law_.28KVL.29

Both of Kirchhoff's laws can be understood as corollaries of the Maxwell equations in the low-frequency limit. They are accurate for DC circuits, and for AC circuits at frequencies where the wavelengths of electromagnetic radiation are very large compared to the circuits.

Limitations
KCL and KVL both depend on the lumped element model being applicable to the circuit in question. When the model is not applicable, the laws do not apply.
KCL, in its usual form, is dependent on the assumption that current flows only in conductors, and that whenever current flows into one end of a conductor it immediately flows out the other end. This is not a safe assumption for high-frequency AC circuits, where the lumped element model is no longer applicable.[2] It is often possible to improve the applicability of KCL by considering "parasitic capacitances" distributed along the conductors.[2] Significant violations of KCL can occur[3] even at 60Hz, which is not a very high frequency.
In other words, KCL is valid only if the total electric charge, \scriptstyle Q , remains constant in the region being considered. In practical cases this is always so when KCL is applied at a geometric point. When investigating a finite region, however, it is possible that the charge density within the region may change. Since charge is conserved, this can only come about by a flow of charge across the region boundary. This flow represents a net current, and KCL is violated.
KVL is based on the assumption that there is no fluctuating magnetic field linking the closed loop. This is not a safe assumption for high-frequency (short-wavelength) AC circuits.[2] In the presence of a changing magnetic field the electric field is not a conservative vector field. Therefore the electric field cannot be the gradient of any potential. That is to say, the line integral of the electric field around the loop is not zero, directly contradicting KVL.
It is often possible to improve the applicability of KVL by considering "parasitic inductances" (including mutual inductances) distributed along the conductors.[2] These are treated as imaginary circuit elements that produce a voltage drop equal to the rate-of-change of the flux.

[Macbeth]

in the time ive been in electronics, never in my life ive seen someone using kirchoff laws analysis to design a circuit or repairing something.
are they useless? by this i mean they have no application at the time of designing something, or have you used them?

Daves EEVBlog #819 had over 2000 thumbs up "if you found this useful"

and



EEVBlog #820 had nearly 2000 thumbs up. So I would say there are only a handful of naysayers say they are useless.(Seriously, who thumb downed these videos?)

[uncle_bob]

Hi

Ok, so here we all have gone for darn near two pages of agreeing with each other. Not a single peep I can see from the original poster of the thread. Makes one sort of wonder about the original post and it's context. Either we have scared him away (quite possible), or the post was in jest (also possible), or something else altogether (like he's in jail ....Who knows ...

Bob

[Karlo_Moharic]

James Clerk Maxwell

I know who Maxwell is , but I don't understand what he meant by saying that Maxwell is to blame for Kirchhoff laws breaking down ?

[uncle_bob]

James Clerk Maxwell

I know who Maxwell is , but I don't understand what he meant by saying that Maxwell is to blame for Kirchhoff laws breaking down ?

Hi

Fundamentally the basic Kirchoff laws do not consider currents in anything other than "conductors". Current is nicely contained and thus can be analyzed on that basis. Conductors may be wires, or volumes of materiel, they still contain the current.

As soon as you have propagating waves in free space generated by currents (or currents induced by waves), they no longer are nicely constrained. If you break down the constraint, you break down the applicability of that particular law. The filed of electromagnetic waves is pretty much all based on Maxwell's famous equations.

Yes, there are other "AC" things that you need to consider in circuits. Energy storage can take place in a number of ways. It does need to be accounted for. The fundamental basis for all this still is mesh and node equations. Without that, your analysis stops dead. Without Kirchoff, no mesh and node ....

Bob

[rstofer]

in the time ive been in electronics, never in my life ive seen someone using kirchoff laws analysis to design a circuit or repairing something.
are they useless? by this i mean they have no application at the time of designing something, or have you used them?

You can't even power an LED without both KVL and KCL.
The LED spec: 20 mA If at 2.0V
The battery spec:  12 VDC 0 Ohms internal impedance

KVL gets us the fact that the resistor needs to drop 10V => 12V at battery - 2V at LED => 10V across the resistor
KCL gets us the fact that the current through the resistor is 20 mA => the current in the loop is the same everywhere so the resistor current is exactly equal to the LED current
Ohm's Law gets us the fact that the resistor should be 500 Ohms - R = E / I = 10V / 0.02A = 500 Ohms

The thing about these Laws is that they are frequently applied, like in the case of the LED, without even thinking about the laws themselves.  It just becomes second nature.  But the fact that the are intuitively applied doesn't make them useless.  They aren't simply suggestions, they are LAWS!

[onlooker]

I  guess the OP can ask the question better with a more defined scope. For example, one may ask: does anyone still explicitly use Kirchhoff's laws to solve circuitry problems involving  more than, say, 5 or 10 variables in the design work?

In other words, I guess, the proper question is about how often Kirchhoff's laws are consciously used; It is not about the correctness of Kirchhoff laws in a particular type of EE work.

There are many alternatives to explicitly using Kirchhoff's laws. Examples are simulation software, relying on experiences, using application notes or similar design, or simply by trial and error.

The often mentioned example of LED serial with a resistor is a good case in point. I believe most people knows how the current can be calculated long before aware of the name, Kirchhoff and his laws. The point is, for simpler situations, there are simplified rules. They also likely existed before Kirchhoff's laws even if they now can be regarded as a special application of Kirchhoff' laws.

[uncle_bob]

Hi

About the only person who has *not* posted to this thread more than once is the original poster ...

Bob

[T3sl4co1l]

Well of course it doesn't actually disappear, but in the naïve view where there is only KCL and KVL at least some of current into an antenna (aka a wire that just ends and doesn't close a loop) just goes 'somewhere else'. It only makes sense once you include a equivalent circuit for an antenna that implicitly hauls the world of electromagnetism into the nodes and vertices of the KCL/KVL world.

It's naive to think KCL applies everywhere, globally, at all instants.

Realize that a schematic is an abstraction of the real world.  There are no true capacitors, nor inductors.  An RLC network responds instantaneously to an input, something no real circuit can do.

KCL works because, in an abstract circuit, there is no concept of a speed of light.  Therefore KCL is true globally at all times.

When we include EM, KCL doesn't cease being useful, but it is necessarily restricted to infinitesimal points, or point-like areas for the purposes of analysis.

An example of a "point like area" is a transmission line port: for frequencies below the higher modes of the TL, it is true that current into one terminal of a port equals current out the complementary terminal of that port.  (A TL is a four terminal, two port component.  A port is simply a pair of terminals at one end of the line: +/- for twisted pair, signal and ground for coax.  We ignore the common mode or shield current for this purpose, as well.)

For an antenna, the current into the base is balanced by displacement current into the EM field.  Just as it is for any resonant circuit, except the "capacitor" happens to be a propagating EM wave, instead of a bunch of E bottled up in a lot of dielectric.

Tim

[munkeyman1985]

They get used all of the time. You might not be thinking about it, but any one building, testing, or troubleshooting electronics must be using them.

[T3sl4co1l]

Fundamentally the basic Kirchoff laws do not consider currents in anything other than "conductors". Current is nicely contained and thus can be analyzed on that basis. Conductors may be wires, or volumes of materiel, they still contain the current.

This is, at best, a misinterpretation of nature, and at worst a gross corruption of the underlying laws.

Consider:
1. If KCL applies only within conductors, then transmission lines wouldn't be possible.  The current would leave the far port, the instant it arrives at the near port.
2. TLs exist.
3. Therefore KCL doesn't apply within conductors.

Further consider:
1. If KCL doesn't apply to fields, then there need be no conservation of displacement currents.
2. Displacement currents are necessarily conserved.  (Faraday's law and electric induction.)
3. Therefore KCL does apply.

As I said before, you can't apply KCL over an arbitrary volume, because the currents will in general be unequal over time or space.  But it is necessarily locally true, because the fields are not discontinuous.

Tim

[uncle_bob]

Fundamentally the basic Kirchoff laws do not consider currents in anything other than "conductors". Current is nicely contained and thus can be analyzed on that basis. Conductors may be wires, or volumes of materiel, they still contain the current.

This is, at best, a misinterpretation of nature, and at worst a gross corruption of the underlying laws.

Consider:
1. If KCL applies only within conductors, then transmission lines wouldn't be possible.  The current would leave the far port, the instant it arrives at the near port.
2. TLs exist.
3. Therefore KCL doesn't apply within conductors.

Further consider:
1. If KCL doesn't apply to fields, then there need be no conservation of displacement currents.
2. Displacement currents are necessarily conserved.  (Faraday's law and electric induction.)
3. Therefore KCL does apply.

As I said before, you can't apply KCL over an arbitrary volume, because the currents will in general be unequal over time or space.  But it is necessarily locally true, because the fields are not discontinuous.

Tim

Hi

Any analysis of a transmission line that is based *only* on KCL and *not* on Maxwell will be a failure. The most fundamental thing on the first day of any transmission line course is Maxwell. To say that currents flow through vacuum (without Maxwell) simply is not in any way correct.

Bob

[EEVblog]

I've seen Dave talk about the principle when he did the OpAmp video. IIRC it was in relation to input bias current. He may not have mentioned it by name.

Correct. Kirchoff's laws are so fundamental and second nature, you don't even realise you are using them.
It's "obvious" that current into a node equals the total current out the node, but that's Kichoff's laws for you. It's the 2nd thing you usually learn in DC circuit theory after ohms law.
Hardly anyone goes around saying "because of Kirchoff's Law, the current into the opamp junction must flow this way", because there is simply no need to repeat it because it's so obvious. Just like I don't always say "because of ohms law..."