變壓器一問....................

Ans: B
(1) is wrong.
d.c. current cannot be adopted in transformer as d.c. current in the primary current produce constant magnetic field (i.e. no magnetic flux change) inducing zero e.m.f. in the secondary coil. energy cannot be transfer, let alone its step down process.

(2) is correct.
Vs/Vp=Ns/Np => Vs=110/55=2V
resistance of a light bulb=V^2/W=16/40=0.4ohm
equivalent resistance, R=0.4/2=0.2ohm
is=Vs/R=2/0.2=10A

(3) is wrong
Vs/Vp=Ip/Is => Ip=10/55=0.182A

答案: B
1. 變壓器是不可工作於直流電壓的, 如果用220V直流電源取代110V交流電, 副線圈是沒有任何電壓輸出, 燈泡亦不能運作.
2. 次級電壓只有2V(110/55), 而燈泡的內阻值是0.4歐姆(R=V*V/P), 因此每個燈泡只得5A(I=V/R)電流通過, 兩個燈泡共10A.
3. 變壓器的圈數比和電流比是反比例, 所以原線圈電流等於0.1818A(10/55), 不是5A.