I read that if size of a circuit is bigger then wavelength, Kirchhoff laws can't be applied on that circuit. So, my question is: which wavelength? I know that wavelength is wave parameter, but voltage and current are not actualy waves, and that is confusing me a little.

 

Where did you read that?

 

The wavelength of an electromagetic wave at the operating frequency.

Bob

 

Wavelength is a measurement of Radio waves or Lightwaves, it is the Inverse of Frequency..

 

Thank you very much. Does it means that when I increase frequency, voltage is "delayed" in circuit ( because wavelength of electromagnetic wave < size of a circuit).

Where did you read that?

I don't remember the exact name of the book... I just know that it was about electromagnetism.

 

Wavelength has nothing to do with voltage, or current.

It is the space at which the Sine/Square wave occupies before it repeats itself.

The faster it's frequency the smaller the wavelength, Vice-versa.

 

It's all about the difference in signal phase (delay) between two points caused by the finite speed of c (speed of light) in the universe. The more the signal phase difference the more we see changing magnetic fields across space and induced emf in circuit loops from electromagnetic induction and radiation. This can violate the special case of Kirchhoff loop laws in circuits.

For a 10MHz 30M wavelength signal at c the 90 degree shift 1/4 wavelength is about 7.5M long. That's an electrically long phase shift (like an antenna) while an electrically short delay across 30cm circuit board would be a much smaller 1/100 wavelength phase shift at 10MHz.

https://en.wikipedia.org/wiki/Electrical_length#Phase_length

 

It's all about the difference in signal phase (delay) between two points caused by the finite speed of c (speed of light) in the universe. The more the signal phase difference the more we see changing magnetic fields across space and induced emf in circuit loops from electromagnetic induction and radiation. This can violate the special case of Kirchhoff loop laws in circuits.

For a 10MHz 30M wavelength signal at c the 90 degree shift 1/4 wavelength is about 7.5M long. That's an electrically long phase shift (like an antenna) while an electrically short delay across 30cm circuit board would be a much smaller 1/100 wavelength phase shift at 10MHz.

https://en.wikipedia.org/wiki/Electrical_length#Phase_length

Very well explained, thank you very much! It's clear now.

 

I read that if size of a circuit is bigger then wavelength, Kirchhoff laws can't be applied on that circuit. So, my question is: which wavelength? I know that wavelength is wave parameter, but voltage and current are not actualy waves, and that is confusing me a little.

You have that backwards -- if the size of circuit is SMALL compared to the wavelengths of the electromagnetic waves in question, we can't use our basic Kirchhoff's laws.

[EDIT: Brain fart -- Of course this is not backwards. We need the circuit to be small compared to the wavelength in order to be able to claim that the voltage/current at all points of the nodes are the same at any moment in time.]

One of the reasons is that KVL and KCL as we normally use them assume that the voltage on a conductor is the same everywhere along the wire and that the current in that conductor is likewise everywhere the same.

But if the conductor is longer than a reasonably small fraction of a wavelength, then the voltage and current vary at difference places on the conductor even at the same moment in time. Thus we can't apply KVL and KCL using the assumptions that they don't.

 

You have that backwards -- if the size of circuit is SMALL compared to the wavelengths of the electromagnetic waves in question, we can't use our basic Kirchhoff's laws.

I don't understand this part. If f=50hz => wavelength = c/f = 6000000 m, then size of circuit is small compared to the wavelength, and I can use KVL and KCL on this frequency.

Everything else is clear. Thank you!

 

You are correct. Brain fart.

We need the circuit to be small in order to use KVL/KCL. I don't know what I was thinking.

 

I know that wavelength is wave parameter, but voltage and current are not actually waves,

Yes, they are. For example, a small audio amplifier powered by 9 Vdc has voltages and currents that are the audio signal, which vary from 20 Hz to 20 kHz. So the shortest wavelength of interest would be that of a 20 kHz sinewave. If a circuit has a small microcontroller with a 12 MHz clock frequency, the highest harmonic frequency of interest will be over 100 MHz.

ak

 

I read that if size of a circuit is bigger then wavelength, Kirchhoff laws can't be applied on that circuit. So, my question is: which wavelength? I know that wavelength is wave parameter, but voltage and current are not actualy waves, and that is confusing me a little.

Voltage and Current ARE waves!

 

The way I tend to remember it, being a ham, 30MHz is in the 10m band.

Hence 300MHz would be in the 1m band.
If your circuit has dimensions much less than 1m, then 300MHz is not a problem.
If we are talking ¼ wavelength, then the circuit must be much less than 25cm @ 300MHz.