Re: 傳輸線大哉問～
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：
: ※ 引述《sexyman (現在新歌沒好聽的)》之銘言：
: : 這個板友，如果你是念電機系，我想你的電磁學從大學修過課後
: : 就從來就沒有用過了，因為你的推導與觀念完全是錯誤的
: 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
: : 第一點是你一直強調要假設 epsilon0 與 mu0 為零的觀念為錯誤
: : epsilon0 與mu0 是一個物理常數，是不容許你假設為零的
: : 如果你這一點原則都無法柄持，你也不必浪費你的時間去做以下的推導
: : 因為你的推導根本就是在一個錯誤的假設上
: : 第二點是你的證明用錯誤的方法去簡化不必簡化的東西
: : 比方說，由 curl(E) 的公式得到 KVL，不是因為 mu0 = 0
: : 而是因為沒有時變磁場而造成的；如果有時變磁場的話
: : KVL 仍然成立，那個時變磁場所造成的積分量叫做「電感」
: 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)
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