Inspired by
* Erik Lindberg´s recent contribution (The oscillator - principle of nature), and
* a discussion in RG about analogy between electrical and other technical systems
I wonder if there are some non-electrical and self-sustaining effects due to positive feedback. In this context, I have asked myself since a long time: What is the physical background of the thing called "fire"?
It is a self-sustaining phenomenon - and I think, we can observe something like positive feedback. Of course, the fire burns only as long as this feedback can exist - that means: as long as there is some material that is able to burn.
Question: Is this a kind of analogy to the electrical supply voltage (necessary to let oscillators oscillate)?
As another analogy, there will be, of course, a continuous energy exchange
* from biological/chemical energy to heat, resp.
* from chemical/mechanical (dc supply) to electrical energy.(ac voltage).
What do you experts think about this positive feedback phenomenon?
Perhaps an attempt to explain my view:
The high temperatures of the flame (nothing else than high-temperature air) causes a chemical (?) effect within the wood, which creates heat again and, thus, keeps the fire alive.
Hi Lutz and welcome again! As I can see, your vacation has affected very positively your creativity:) You have asked a nice question about a great natural phenomenon; I highly appreciate it!
I only wonder if you have not emphasized the role of the positive feedback in creating oscillations in nature. Is it only necessary for the appearance of oscillations?
In a sustainable oscilation you either need a resonant system with no friction (resistance in the case of electronics) or, if friction is present, you need a source to replace the lost energy (so it's not actually self-sustainable...) A pendulum is such an oscillation.
A pendulum... but with an additional system compensating the losses... Do you know how all these 1-transistor electromechanical oscillating toys operate?
Maybe an anchor escapement can serve as a goos example of a mechanical self-sustaining system?
Pendulum, anchor escapement,.... OK - I am afraid, I didn`t make my point clear.
I don`t want to collect non-electrical oscillation phenomena but, instead, I like to discuss the mentioned "fire" example only.
That means: Is the burning of a candle light a positive feedback effect?
So, your qiestion is about the positive feedback phenomenon and its manifestations?
About the "fire" example... I am not a chemist or physicist but, relying on my human common sense, I think there is a positive feedback in this process... Maybe we can find a relation with the electrical arcing? And to see a kind of a "fire negative impedance":)?
Well, let's make an analogy with a negative resistor. The steady fire corresponds to the linear mode (a bridled positive feedback) while the explosion - to the bistable mode (avalanche-like positive feedback) of a negative resistor...
Hi, I work in aircraft flight dynamics. There are plenty of nonlinear phenomena that would qualify as self sustained oscillations in aircraft dynamics, for example, wing rock, autorotational oscillatory spin, etc...which result from nonlinear aerodynamics of aircraft at moderate to high angles of attack (aircraft orientation with respect to the relative wind).
Quote Lutz v.W.:
"What is the physical background of the thing called "fire"?"
and
"The high temperatures of the flame (nothing else than high-temperature air) causes a chemical (?) effect within the wood, which creates heat again and, thus, keeps the fire alive."
Dear Lutz, Thank you for your question.
Fire is a reaction between oxygen and the surroundings. The books made of paper in our libraries are slowly "burning" at low temperature.
Concerning steady state oscillations I have seen a chemical experiment where two clear fluids were put together and oscillation between a clear and an unclear phase in the compound were observed. Positive feed-back is found everywhere..
Dear Erik - thanks for replying.
But I like to ask again:
Is the thing called "fire" an effect caused by positive feedback - which means:
The result (high temperature) retroacts onto the "input" (material that is able to create again heat that keeps the fire alive).
I would call an explosion a proper example of positive feedback. Since fire is sustained and can be regulated by means of air flow, oxygen concentration or amount of available fuel, there must be a total feedback that is negative.
Mr. Olof Dahlberg, thank you so much for your answer , which brings my question again "on the table".
I cannot comment on the "explosion" becaus I know not much about the chemical process of such a phenomenon.
However, I don`t think that a fire is comparable to negative feedback because - as you wrote - regulation is possible. For systems with negative feedback there is no self-sustained output because we always need a signal input.
But it is possible to CONTROL a system with positive feedback with the aim to limit the output at a certain level. And that is what happens - I think - also in case of a fire (limited amount of wood or candle wax, oxygen concentration,..).
IMO an explosion is an example of the "self-reinforcing positive feedback" exploited by latches...
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