物理交流電 DSE、CE 選擇題（第 4 頁）

31.

Transformer with 200 V a.c. source in the primary circuit and resistor R in the secondary circuit carrying current I

In the transformer shown, the ratio of the number of turns on the primary coil to that on the secondary coil is $10:1$ . If the power input is $50$ W, find the current in the secondary coil, assuming that the transformer has a $80\%$ efficiency.

32.

What is/are the advantage(s) of using high voltage in power transmission over long distances ?

(1)Current in the transmission cables can be reduced.

(2)Less energy is wasted as heat.

(3)Higher voltage can be used in household circuit.

33.

Which of the following statements concerning direct current (d.c.) and alternating current (a.c.) is/are correct ?

(1)The magnitude of voltage in d.c. is constant while that in a.c. varies.

(2)The direction of current in d.c. does not change while that of a.c. reverses periodically.

(3)Both d.c. and a.c. can have heating effect on a resistor.

34.

Iron core with coil A on top and coils B and C on the sides

Three coils $A$ , $B$ and $C$ are wrapped around an iron core as shown. Coil $A$ has $N$ turns while coil $B$ and $C$ both have $\frac{1}{2}N$ turns. If an input voltage $V$ is applied across $A$ , what will be the voltage acting across coil $C$ ?