Re: 傳輸線大哉問～
既然你要用英文回，我也用英文回好了
The most fundamental concepts of KCL and KVL is: Maxwell Equation
It CAN be rigorously proved using physical theorems
The concept of V is based on the electric field, and the concept of I
is based on the charge
ALL simulator circuits uses KVL and KCL for calculation, theorem-wise
there is no difference between lumped circuit components (L, R, C, etc.)
and distributed circuits (transmission lines). The statement of KVL and KCL
only work for lumped components is completely wrong. You just don't know
how to express a transmission line with L, C components; a transmission line
expressed with a limited number of L, C components is a low pass model, and
increases in complexity as operating frequency increases. A proper approach
is to use an s-parameter expression.
You probably are not aware that there are many other circuit simulators other
than spice, such as ADS, MWO, and Designer. All of them handle distributed
components with ease. SPICE simulator is a time-domain simulator, therefore
it has problems dealing with distributed components. You can also refer to
any microwave engineering text, to see that KVL and KCL holds for any circuit.
Even for multi-mode behavior in waveguides, an equivalent circuit in terms
of s-parameters can be described; in this case, there are no DC current or
DC voltage, but KVL and KCL still holds.
KVL and KCL works because it is derived from Maxwell equations, not because
we want it to work. The fundamental meaning of KVL and KCL is conservation
of energy and conservation of charge. Any reputable textbook on
electromagnetics, fundamental circuit theory will back my statement.
※ 引述《pow (體脂肪35%)》之銘言：
: Just to share some idea...
: I think the most fundamental assumption of using KCL and KVL is...
: you are using abstract concept to describe things.
: The beauty of EE is that we can use abstract V-I so well instead of
: direct dealing with E-M...so well that we all feel
: V-I is the real quantity, and embeded them into our everyday live. E-M is
: from Physics department but we have our own way to live with it.
: This V-I concept happened to work with the "pattern recognition" technique.
: By "pattern recognition" I mean the topology. Top line is VDD. Bottom line
: is ground. Left is input. And right is output. We borrow the concept of
: topology from Math department, do some modification and apply to our EE world.
: With the two weapons in hand, we can put complex calculation away and start
: to "model" things. However, there's always limitation that our method
: won't work anymore. Berkeley SPICE was developed to help us calculate complex
: circuit, but still, they can only do V-I and topology. It's just that...
: over time we get used to "back of the envelope" calculation (and SPICE.)
: So we figure out some other way to "model" something that's not supposed to
: work with V-I (instead, they should work with E-M). Probably it's our nature,
: if the model fit data at certain range under certain environment,
: we are happy.
: So if you are claiming KCL and KVL will work as long as there's model
: for it, this arguement is kind of loose, don't you think? It works because
: we let it be that way, not because it works by itself.
: ※ 引述《cpt (post blue)》之銘言：
: : 我想做的只是提供一個我覺得合理的思考方式
: : 這當然和嚴謹的推導不同
: : epsilon0 和 mu0 是物理常數
: : 把它們設為零, 就好像把光速設為無限大一樣
: : 在適當的尺度下, 這樣的假設是合理的, 我並沒有要做過多的延伸
: : 但是把 epsilon0 或 mu0 設為零, 在根本上當然與事實不符
: : 這也就是為什麼 KCL/KVL 在根本上也是與事實不符
: : 只不過這個誤差在大部分情況之下可以忽略
: : KVL 之所以成立, 是因為我們額外去 model 了這個"電感"
: : 但事實上電感無所不在, 如果有任何一小段沒有 model 進去
: : KVL 就無法反映實際的物理現象
: : 因此, 與其說是"沒有時變磁場", 為什麼不能想成"忽略時變磁場"呢?
: : 這跟把 mu0 設成 0 不是一樣的意思嗎?
: : (這不是在反駁你, 我是真的想知道為什麼這樣不對)
--
※ 發信站: 批踢踢實業坊(ptt.cc)
◆ From: 140.112.19.162
推
02/08 23:42, , 1F
02/08 23:42, 1F
→
02/08 23:43, , 2F
02/08 23:43, 2F
→
02/08 23:43, , 3F
02/08 23:43, 3F
→
02/08 23:44, , 4F
02/08 23:44, 4F
推
02/08 23:56, , 5F
02/08 23:56, 5F
→
02/08 23:57, , 6F
02/08 23:57, 6F
推
02/09 06:56, , 7F
02/09 06:56, 7F
討論串 (同標題文章)
本文引述了以下文章的的內容：
完整討論串 (本文為第 15 之 21 篇)：