2007年phy mc第44題

voltage input=220V
voltage output=6V&12V

the above figure shows a portable transformer .It outputs different voltage by varying the turns ratio between the primary and the secondary coils.Assume there is no power loss in the transformer and the resistance of the load remains unchange, when the output changes from 6V to 12V , which of the following statements describling the transformer is/are correct?

1)the number of turns of primary coil should be doubled and the number of turns of the secondary coil remains unchanged
2)the input current should be doubled
3)the output power should be 4 times as before

A.(1)ONLY
B.(2)ONLY
C.(3)ONLY
D.(1),(2),(3)





答案係C
點解係C既?
transformer唔係一定input power = output power既咩?(如果no power loss)
但佢e家又會話咩係之前4倍._.