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問phy mc

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題目: http://img519.imageshack.us/img519/1408/phy200031a.png 答案係A 點解(1)對，(2)錯?

最佳解答:

In the circuit, R1 // R2 (R1 parallel R2), which are both in series with R3. When the switch is closed, it by-passes R3, you could then regard R3 be removed from the circuit, hence just leaving R1 // R2 connected to the battery. Because of the removal of R3, the total resistance of the circuit decreases. By Ohm's Law, the current delivered by the battery thus increases, giving larger current to flow through R1 and R2. Statement (1) is correct. Current through R1, I1 = V/R1, where V is the voltage across R1 and current through R2, I2 = V/R2 hence, I1/I2 = R2/R2 The ratio of currents through R1 and R2 only depends on the ratio of the resistances, and is independent of the voltage across the resistors. Therefore, after R3 is removed by closing the switch, the voltage across R1 abd R2 increases, but this would not cause any change to the ratio of the currents. Hence, statement (2) is wrong Statement (3) is clearly wrong, as R3 is already by-passed by closing the switch.