electricity (f.2 science)

resistance is an intrinsic property of a particular size of a particular INDIVIDUAL item (resistor) i.e. it is a constant and is independent of how you positon it (no matter in a seires or parallel circuit)
resistance of an item describes the reluctance of an object against the electron drift in that item.
connected with a circuit, electrons drifts towards positive terminal. however, this action is hindered by the fixed atoms when the drifting electrons is collided with the atoms. this contributes to the reluctance of the movement of the electrons.
resistance of individual resistor is thus regardless of how you connected it but depends on various substance and the dimensions ( cross sectional area and length) of the resistor.

however, EQUIVALENT resistance differs when you connected two or above number of resistors together.
when you connected two resistors in series, the equivalent resistance is GREATER than both of the individual one. this can be regarded as more obstacle are placed on the same runway, electrons need more time to travel and overcome the larger obstacle (resistance)
when you connected two resistors in parallel, the equivalent resistance is SMALLER than both of the individual one. this can be regarded as two pathways are both provided for electrons to travel so that electrons are less crowed in one obstacle (resistor) and the electrons can move faster.

2007-04-25 09:53:12 補充：
when the magnitude of current is determined, EQUIVALENT resistance is considered (not individual)assume light bulb has a large resistance, R, and connecting wires has a smaller resistance, r.

2007-04-25 09:53:41 補充：
in path 1: the equivalent resistance equal to the sum of individual resistances of the bulb and the connecting wires and should be R r (wire and the bulb can be considered in series)

2007-04-25 09:54:06 補充：
in path 2: the equivalent resistance equal to the individual resistance of the wire, r (as there is no bulb)comparing the EQUIVALENT resistance of both pathways, path 2 has a smaller resistance than that in path 1. thus, larger current will pass through path 2.

2007-04-25 09:54:21 補充：
moreover, as connecting wires are made of copper, which has very small resistance, thus path 2 actually has a much much smaller resistance than that in path 1. thus the current in path 2 is so large that heat the circuit fiercely (short circuit)

2007-04-26 22:31:43 補充：
to add ,all items has its own fixed value of resistance, no matter they are in state of individuals or connected to a circuit by all means, parallel or in series.a light builb has a resistance of 100ohm, when it is disconnected from the circuit, it still has a resistance of 100ohm.

2007-04-26 22:32:06 補充：
the resistance of a resistor can also be regarded as the ratio of potential difference across the resistor, V, to current passing through it.all in all, resistance depends on different objects (even composed of same materials) but not how the objects are arranged

假設串聯電路內有三條電阻器, 分別是10歐姆, 20歐姆和30歐姆.
其實這三枚串聯一起的電阻對電源來說只是等於一枚60歐姆的電阻. 因串聯電阻的等效阻值是等於所有電阻值的和. 因此串聯電路內亦都只得一個電流.