原帖由 Zend 於 13-4-2008 18:34 發表 http://www.nakuz.com/bbs/images/common/back.gif
我最驚係你知多左更加影響concept
關
係我果條link入面
current density (vector quantity) 係同E-field 有關係
而係A-level, E=-dV/dr (你可以理解E 係V-r graph 的slope 的-ve 數)
佢將佢合埋, 從數學上bu ...
但聽說你就係果個CE去讀AL野最後變了神的人[]
原帖由 piyopiyo 於 13-4-2008 11:37 發表 http://www.nakuz.com/bbs/images/common/back.gif
但聽說你就係果個CE去讀AL野最後變了神的人[]
-.- 沒有這回事...
Electric problem are different from mechanic problem.
We use V instead of F in electric, we consider motive force for current, not for charge.
Current = flow rate
V = IR , electromotive force drives constant current
F = ma, mechanic force causes acceleration
Special property in electric cannot be explained by mechanic rule (Newton law)
所以CE最好唔好知咁多.
下面既都係由我學倒既推論出黎.
E-field of resistor is defined as dV/dx ,where V is the pd across resistor, x is the length
to maintain constant current, constant F is needed to balanced to force by E-field , F=qE = qV/x
The only source of F can only be provided by the battery.
But we cannot treat this as a mechanic problem.
Let's consider
If there are 2 resistors with same resistance , but different length, then the battery have to provide 2 different force to balance the 2 different forces of the E-fields.
In electric, we learn that we can combine many resistor into one single resistor, so by V=IR to calculate current. But we cannot do it in mechanic, because there are differences.
Anyway, think in this way make thing simpler.
Electrical potential energy is converted into other form of energy in the E-field, instead of ke.
pd of resistor = different ratio of emf
larger emf => larger pd => larger E-field = V/d => larger resistive force = qV/d
larger emf => larger EPE(the work done of the force to maintain constant current flow)
Current is larger , but constant.
take it easy => EPE is dissipated to maintain current flow constantly, instead of ke.
[ 本帖最後由 【YU】 於 13-4-2008 20:44 編輯 ]
but this still cannot solve the problem.
Why the current should be a constant in a series electrical circuit?