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標題:

Phy 2007 mc no.44

發問:

Please explain why the answer is C.

最佳解答:

44. The answer is C. (3) only. (1)For the voltage output to be doubled, from 6V to 12V,it should be thenumber of turns in secondary coils to be doubled, while thatof theprimary coils remain unchanged. Since Vp / Vs = Np / Ns For Vs to be doubled, Ns should bedoubled, not Np. So, (1) is incorrect. (2) The change in input current does not affect thevoltage output. So (2) is incorrect. (3) For resistance remains unchanged. For the outputvoltage doubled, the output power should be 4 times as before. For 6V, P = (6V)2 / R = 36V2 / R Now, 6V is altered to 12V, P’ = (12V)2 / R = 4(36V2 / R) = 4P And the result follows~~~

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