真空中瞬間斷開電感，會怎樣？

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在實際中，流有電流的電感儲能LI^2/2，瞬間斷開電感迴路，則由於能量不能突變，會有反電動勢產生，且電動勢很高，擊穿空氣，拉出電弧。這個還是能夠理解的。
假如在真空中進行上述實驗，又會怎麼樣呢？

寫了這個回答不知道會不會被中美兩國網管同時盯上……

結論：真空中瞬間斷開流有電流的電感，會產生一個非常大的感應電動勢，這個電動勢的電壓值是如此的高以至於會以一個強電磁脈衝的形式擴散到空間中。而這就是電磁脈衝彈的基本原理，即炸開一個通有大電流的線圈。

這類問題屬於電氣工程的極端條件研究方向，比如電感突然斷開，比如兩個不同電壓的電容瞬間搭接等等。分析這類問題的工具還是麥克斯韋方程組的四個方程，但是由於時間極短，對時間求導的方程也基本都成了衝激函數，所以物理描述也並不容易。

一般電路中，電感中儲存的電磁能為LI^2/2。由於能量不能突變，電感電流不能突變，所以通常如果想斷開電路，即會在斷開的觸頭處形成電弧。因為畢竟斷開是一個連續的物理過程，導體間隙是從0到一個有限值。電現象都是光速，遠快於物理移動速度，所以在觸頭剛剛脫離的瞬間，電弧就產生了。有空氣的空氣被擊穿，即氣體放電。沒有空氣的真空也會出現場致電子發射，這時候就成了真空管了。電子會被強大的電場推送著繼續按照原來的方向運動。這就是電網中的斷路器需要乾的事情：滅弧。

那麼對於題主這種極端情況下的電路，電路卻被炸開了，幾十個毫秒之內，導電通路被徹底炸斷，這時候電流表示很無奈啊，電路徹底沒了，你讓我電流還怎麼流。於是此時電路模型便不再適用，需要用波場模型來描述了。電路分析永遠只是麥克斯韋方程組的一個特例，一旦電路模型失效，就得用原裝的四大方程來分析。

由於電流瞬間消失，電流對時間的偏導數將成為一個衝激函數δ(x,y,z,t)，此時的反電動勢將特別高。接下來就是用一系列衝擊函數來模擬電壓函數，你會發現方程求解出來是一個衝激行波函數，即電磁能會以一個強磁脈衝的方式輻射到空間中。這個電磁脈衝是如此的強大，以至於所到之處，凡是導體，都會感應出一個強烈的反電動勢抵禦它。如果這個脈衝功率夠大，可以使得一切晶元從內部擊穿，所有電子設備都會隨之癱瘓。

友情提示，這種實驗需要在有防護的環境內操作，請各位同學不要作死，不要在家做喲。做一次家電換一遍不說，說不定還會被你們所在地區的國安部門盯上喲，畢竟異常電磁信號太容易捕捉了。

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被編輯推薦了，那更得嚴謹一些。再次強調，請不要自己嘗試。電磁脈衝對於心臟起搏器病人有致命傷害，請不要自行實驗。

真空當中如果電壓夠高的話也會有陰極射線，也就有電流，相當於擊穿了。

如果電壓不夠的話其實這個事也很簡單，斷開的地方形成電容，於是變成一個LC振蕩迴路，因為這個電容非常小，所以振蕩頻率很高，最終能量會變成電磁波輻射出去。

其實你想要知道的，就是能量去哪了。

斷開前你可以認為是一個充能過程，固定的電流在電感中產生一個磁場，能量就存在這。「瞬間」斷開後，其實就變成了一個線圈+線圈兩端電容的LC resonator。

你說的電弧只是一種特殊的情況，感應電勢高到把空氣電容擊穿了。因為這在一個斷開的動態過程中是在所難免的，開關的兩個觸點在斷開的一瞬間距離足夠近，不管是什麼介電常數的介質，都有那麼一段時間，觸點距離近到會擊穿這個電容。很大一部分能量被擊穿電流消耗掉，剩下的是線圈內阻和電磁輻射。

真空中情況比較複雜，因為沒有介電質共電場電離，所以電場的能量完全作用於觸點材料，專業點叫靶材，這時候就要看電勢有多高，低於靶材的晶格勢能，斷開就是斷開了，LC震蕩，能量完全被線圈的內阻發熱和電磁輻射消耗掉。如果感應電勢高於靶材的勢能，會把靶材的電子拉出來，也就是陰極發射，變成電子槍，跟老式CRT顯示器裡面那個真空電子槍一回事。能量還是以電流的形式在線圈內阻上消耗掉一部分，剩下的電磁輻射。

這裡面比較詭異的其實就是電容，其實任意兩個金屬端子擺在那，你說他是電容他就是電容。線圈兩端，或者說你的開關兩端也一樣。搞清楚這個就明白了。

要知道，中壓配電中(一般是路邊電線杆最頂上的三根線)，使用最普遍的一種開關，叫做真空斷路器。真空斷路器的連接觸頭即完全密封在高度真空的瓷殼內。其真空度比燈泡高几個數量級。

當真空斷路器斷開大型電動機上時，就是題主說的這種情況。大型電機結構上都是線圈，相當於很大的電感。此時，有兩個現象:
一，斷開時並不是沒有電弧，還可能很大。但是比空氣好的是，電弧完全來自觸頭材料的電子發射和正離子激發，沒有空氣電離，使得電弧容易斷開。一般交流電的話，電流過零點就會斷開。這就是使用真空斷路器技術的主要原因。當然為了促進斷弧，觸頭等也有形狀磁路等設計，不然過零點之後還可能電弧復燃，造成斷開失敗。
在斷開過程中，電弧產生的電子發射和高溫會造成觸頭損耗，使表面燒蝕，材料微粒飛散沉積到外殼內壁上，使得壽命減少。
第二個現象是，斷開電動機時，一般不可能嚴格電流過零點，都是在幾安培甚至幾十安培就會因能量太小而電弧熄滅，完全開斷。此時因為電動機電感的存在，在電路中會產生很高的電磁脈衝，表現就是一個瞬間高電壓脈衝，對於外面正常工作額定一萬伏特的高壓線路，這個過電壓可能高達10萬伏特以上，可以直接破壞電動機絕緣。所以，電力系統中一般會配置過電壓吸收器來抑制電感斷開瞬間的電壓尖峰。
希望對題主有幫助。

這個問題快半年了，可能是我不會用搜索引擎，剛開始都沒查到什麼答案或討論。剛剛找到一篇國外的討論。複製過來。直接複製的網頁，懶得排版了。

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英語不好，先說下自己的理解：真空既然理想，那就不像在空氣中那樣有拉弧現象，沒有空氣拉個啥弧？電流瞬間切斷，一團磁能要瞬間泄放，要泄放就要有迴路，或者沒迴路也有泄放的形式，比如不以熱能釋放，以輻射釋放。斷開開關的觸點之間，泄放能量前如果還是沒啥變化，那電壓就一直高，直到把觸點表面的電子激出去。能把電子激出去，這個能量就已經跑了，跑了一部分後，沒有那麼多能量的時候，沒有能力激出電子了，那就發生在觸頭內部，至於發生了什麼，我也不知道。

以上全是個人瞎扯，請真正明白的人給詳細解說一下。謝謝。

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他國英語討論回答如下：

What will happen if we place an inductor (ideal) connected to a dc power supply (ideal) and open the switch after steady state in a vacuum? And why will it happen?

4 Answers

Loring Chien, Spacecraft depend on physics

Updated Nov 19 2015

The interruption of the current will cause cause a voltage breakdown between the electrodes of the switch according to V = L dI/dt if dI is large (e.g. change of X amps in zero time is large) then the voltage can be infinite theoretically, since you have an ideal source and an ideal inductor, it will be infinite.

There will be a breakdown in the switch, even though there"s a vacuum, the metals will vaporize eventually and allow an arc or electrical current transfer to form between electrodes (switch contacts) even in a vacuum. It may not be visible but it will occur. Field electron emission

I found this discussion in Physics Forum discussion on voltage Arc in Vacuum

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More volts are needed than in air. They make "vacuum capacitors" for use in high power transmitters and they operate at pretty high voltage. But they will arc across, given high enough volts. The electric field around the plates would be high enough to ionise any significant number of air molecules and to start a discharge. The field at the metal surface needs to be higher to release electrons and to start an arc. A pointed electrode can easily produce enough field around it to start an arc in a vacuum with only a few kV applied to a nearby plate.

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Sure, you can create field emission electron flow on the surface of the electrode in total vacuum.

Field electron emission

Reference Arc in a vaccum?

So you have a large voltage kick, even though its in a vacuum it will arc over and vaporize parts of the switch contacts. This gradual damage to the contacts will sooner or later cause the switch to malfunction.915 Views · 2 Upvotes · Answer requested by Kael Thas

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Mayur Balwani, I am an Electrical Engineering student!

Answered Oct 18 2015

It"ll blow up the switch. Inductor, in its steady state, has stored energy in the form of magnetic field.

This phenomenon of over voltage generation due to the opening of the switch is called the "Inductive kick".

Now, the question is why high voltage is generated ?

Answer is the formula v = L di/dt.

Now, L is constant and di is the rate is change of current. Now suppose the L is 1mH and I at the steady state is 1A. Now, if we assume that you somehow succeeded to open the switch and make the current in inductor zero in the time span of 1millisecond, then theoritically, the voltage of 1kV would have been generated by inductor across the switch during the transition from rated current to zero.

Mechanically, this can be explained by the train analogy. Suppose a massive train ( equivalent to the Inductor) is running at some velocity and you suddenly go against that mass in the motion with some inertia and try to stop it by colliding against it. What will happen ? What force will you experience ?

Now, the situation is same when you try to turn off the switch. You simply open circuit it, but that interference is affecting in the orders of the magnitude that any one can"t imagine. ( We simply say what will that tiny inductor do !!).

By the way, in ballasts, this principle was used to ignite the fluorescent lamps. Inductor was charged first and the switch was open circuited suddenly and as the switch was connected in parallel to the lamp, high voltage broke down the gas molecules and ignited the bulb !

Same kicking principle, but in a controlled sort of manner is used in a flyback SMPS. You can refer to any good power electronics book to understand that.

I hope this was helpful.

Good luck!

Mayur.

Edits according to your new twisted question:

Since you"ve assumed everything ideal, I would like to tell you that ideally, vacuum is a perfect insulator.

But practically, breakdowns are observed even in a very high vacuum which, scientists have tried to explain by the Particle exchange mechanism, Field emission theory, etc.

If you assume ideal vacuum, then nothing should happen, inductor will generate very very high voltage, but again, as vacuum is ideal insulator, it will... But where would the magnetic field go ?

How would it exist even if there is no current in the open circuit !!!

Ehhh... That screws the mind and thinking. I can"t think more on it.

But you really choked the inductor to the death !!!

See, these models are approximations using the lump theory, not the absolute ones.

If you want answers, you have to do the absolute modelings using quantum mechanics and mathematics, and put the constrains and see what"s happening. That"s not possible to do!

I think what happens in the practical world is important. If you apply the constrains and do ideal assumptions, you"ll never know what will happen because we don"t know the model of the system exactly. In this case, nothing should happen to the vacuum, but then what will happen to the inductor and electrons? To know that, you"ll need to go inside the absolute modeling of the inductor and see what happens inside when electron flow is stopped suddenly, is the whole system goes into the state of chaos ? Or the entropy increaces so much that inductor melts ! ( Please don"t say there is an ideal conductor which has an infinite current carrying capacity!)747 Views · 7 Upvotes

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You better do that with remote control! The ideal DC supply shoul have internal resistance, if not the case there is no ¨steady state¨ as the current will keep increassing for ever in an ideal inductor. Opening the switch after a while force us to allow parasitic capacitance among coil turns in the model, if not the case the voltage will rise to infinite instantly and the non stopped current will soon starve electrons from positive end.

Allowed some capacitance the kinetic energy of the inductor current will be converted in potencial for the voltage generated accross capacitance, then the current reverted and will oscillate producing RF that will dissipate the energy.

EMP：從原理到精通

電感是通低頻阻高頻，通直流阻交流的特點，有阻礙電流發生變化的特性。
斷電瞬間，電流降低，電感想不讓電流降低，於是只有通過提高電壓來維持電流
而斷開了提高電壓也不能維持電流不變，於是電壓達到很高。
實際原理就是電流下降，磁通量發生變化，於是感應發電。

如果電感是串聯，停電時說明電源那端已經斷開，仍然會在斷開處產生高壓
如果電感是並聯，則有幾種可能，可能維持用電器工作一段時間，也可能電感提供的電流過小，沒什麼影響，也可能瞬間產生高壓損毀用電器

會變成lc振蕩電路，剩餘能量變成電磁波發射出去。

擊穿空氣也就是說某二極（正負極或者是正極與零勢面）的電勢差足夠高足以擊穿那麼我們可以理解為以某二極為極板的電容被擊穿了
同理在真空中如果電勢差足夠大也可以發生與空氣中類似喝多事情