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發表於 13-4-2008 20:31:18
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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 [S]【YU】 於 13-4-2008 20:44 編輯 ] |
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