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osnarf

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In summary, the conversation discusses the concept of opening a switch in a circuit with an inductor, which causes a spark due to the inability of the current to change instantaneously. It is mentioned that in a perfect absolute vacuum, this effect may have little impact since the dielectric effect of air is minimal. The conversation also explores the role of inductors in preventing sparks and their impact on signal processing. The idea of an absolute vacuum is questioned and defined as a volume devoid of space.

- #1

osnarf

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- #2

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osnarf said:

Very likely it would have little effect since the dielectric effect of a tiny amount of air as the circuit opened would only make it a tiny bit easier for the spark to fly across the gap than it would do it without the air there.

- #3

RedX

- 970

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osnarf said:

I thought it was the opposite, that having an inductor prevents an instantaneous change of current through the circuit which would be disastrous, since an instantaneous change in current would create an infinite voltage which would lead to sparks.

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RedX said:I thought it was the opposite, that having an inductor prevents an instantaneous change of current through the circuit which would be disastrous, since an instantaneous change in current would create an infinite voltage which would lead to sparks.

You have it right but seem to misunderstand the effect. The inductance in the circuit prevents an instantaneous change in current so what happens, as the OP described, is that a spark occurs when the circuit opens because the voltage goes WAY up in order to provide a current across the gap for a VERY brief time so that the current can go to zero quickly but NOT instantaneously. That is, the current CAN'T change instantaneously, and the voltage in the circuit is NOT going to be enough to drive a spark across a gap, so the inductive effect causes the voltage to rise dramatically very quickly so that the current can decrease very quickly. It doesn't take infinite voltage to create a gap spark.

- #5

RedX

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phinds said:You have it right but seem to misunderstand the effect. The inductance in the circuit prevents an instantaneous change in current so what happens, as the OP described, is that a spark occurs when the circuit opens because the voltage goes WAY up in order to provide a current across the gap for a VERY brief time so that the current can go to zero quickly but NOT instantaneously. That is, the current CAN'T change instantaneously, and the voltage in the circuit is NOT going to be enough to drive a spark across a gap, so the inductive effect causes the voltage to rise dramatically very quickly so that the current can decrease very quickly. It doesn't take infinite voltage to create a gap spark.

I think I see what you mean, but in an LR circuit, upon disconnection, the current goes as:

[tex]I(t)=I_0e^{-Rt/L} [/tex]

so that the voltage across the gap with high resistance R is:

[tex]V(t)=-L*I'(t)=RI_0e^{-Rt/L} [/tex]

so it seems that this effect doesn't depend on the value of L at all, just the value of the gap resistance R. The maximum voltage drop across the gap will always be [tex]RI_0 [/tex] at time zero, regardless if you have an inductor or just have the self-inductance of the wires.

- #6

256bits

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The value of resistance across the gap is thus not a constant value of R, but is increasing from 0 at the time the switch is thrown to a greater and greater resistance.

Subsequently, the voltage across the gap increases to a higher and higher value.

From your second equation V(t)=−L∗I′(t)=RI0e−Rt/L , with the R being variable and increasing, the voltage itself increases alongside the gap becoming wider if an inductance is included in the circuit..

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RedX

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256bits said:

The value of resistance across the gap is thus not a constant value of R, but is increasing from 0 at the time the switch is thrown to a greater and greater resistance.

Subsequently, the voltage across the gap increases to a higher and higher value.

From your second equation V(t)=−L∗I′(t)=RI0e−Rt/L , with the R being variable and increasing, the voltage itself increases alongside the gap becoming wider if an inductance is included in the circuit..

That certainly makes sense, and is very perceptive. If you include a time-varying gap resistance, then the equation for the voltage drop across the gap:

[tex] V(t)=I_0 R(t) e^{-(1/L) \int R(t) dt} [/tex]

If R(t) is increasing with time, then a huge inductance L will allow the factor of R(t) in front to increase with time without much drop in the exponential as time passes, so the voltage attains higher value with increasing L.

That is strange though as I thought inductors were good things and prevented sparks, but were bad for things like signal processing as they smooth out waveforms.

- #8

Gfellow

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A slight segue here. An absolute vacuum? Really? Really? Do you mean a partial vacuum? Absolute vacuums are the realm of theoretical physics. Example: Present definition of a vacuum is "http://www.thefreedictionary.com/vacuum" " Since mass/energy, space and time all seem to be intertwined, perhaps a better definition might be that an absolute vacuum is "A volume devoid of space." (The volume only being defined and described by space surrounding the absence.)

This is frontier stuff, mostly dealt with in far-fetched scientific papers and http://www.youtube.com/watch?v=2fdhyhPu6PY".

This is frontier stuff, mostly dealt with in far-fetched scientific papers and http://www.youtube.com/watch?v=2fdhyhPu6PY".

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An inductor circuit is a type of electrical circuit that contains an inductor, which is a passive electronic component that stores energy in the form of a magnetic field. It is typically made up of a coil of wire and is used in a variety of electronic devices, such as radios, televisions, and computers.

An absolute vacuum is a space that contains no matter, including air molecules. It is a theoretical concept and is impossible to achieve in reality. However, a close approximation of an absolute vacuum can be created in a laboratory setting using specialized equipment.

The switch on an inductor circuit is opened in an absolute vacuum to prevent the flow of electric current. In a vacuum, there is no air or other particles to conduct electricity, so the inductor will not be able to function. This allows for a controlled experiment to study the effects of the inductor without any interference from external factors.

Opening the switch on an inductor circuit in an absolute vacuum poses minimal risks as there is no electricity flowing. However, there is a possibility of damaging the inductor if it is exposed to extreme temperatures or pressures in the vacuum. It is important to use proper equipment and techniques to ensure the safety of the inductor and the researcher.

Opening the switch on an inductor circuit in an absolute vacuum allows for the study of the inductor's behavior without any external factors. This can provide valuable insights into the properties of inductors and their use in electronic devices. It can also help in the development of new technologies and advancements in the field of electromagnetism.

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