標題:
One physics MC about refraction, one MC about transformer, please help?
發問:
For q 16, why is (3) wrong? For q 32, please explain the answer once, thanks a lot 16 ans A, 32 ans B
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最佳解答:
16. How come that the refracted ray be split into two rays after passing the interface? From Snell's law, a given angle of incident only gives one definite angle of refraction. There won't be two refracted rays. 32. A transformer works on electromagnetic induction. The voltage (since a CRO measures voltages) induced in the secondary coil follows Faraday's Law of Electromagnetic induction, which states that the induced emf depends on the RATE OF CHANGE of magnetic flux through the coil . The magnetic flux through the secondary coil is, in turn, proportional to the current, and hence voltage, in the primary coil. That said, the induced emf in the secondary coil is proportional to the RATE OF CHANGE of voltage in the primary coil. Given the above understanding, it is not difficult to see that the RATE of change of voltage in the primary coil becomes the highest at the two minimum points of the given wave form. Hence, the induced emf at the secondary coil will show a maximum voltage. Because the turns-ratio of the transformer is 1:2, the induced emf at the secondary coil will be twice as that at the primary coil. When the primary voltage rises from zero to a maximum, the rate of change of the voltage gradually decreases (i.e. the slope of the curve decreases). The induced emf at secondary is thus decreasing accordingly. Likewise, at the two wave crests, the RATE of change of voltage is zero (i.e. the slope of the curve is zero). This gives rise to zero induced emf at the secondary coil. After passing the first crest, the primary voltage decreases. The rate of change of primary voltage now becomes increasing (the slope of the curve increases, but in the -ve manner). This leads to an increase of induced secondary voltage, but in the opposite direction to that before the wave crest. The same induction process repeats itself after the minimum point. The result of the voltage wave form will be that shown in option B.
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