物理 1996 年 DSE、CE 選擇題（第 4 頁）

31.

Two long insulated wires crossing at right angles with points P, Q, R and S around the crossing

Two long insulated wires carrying equal currents are placed perpendicular to each other on a table as shown in the figure. The points $P$ , $Q$ , $R$ and $S$ are all of equal distances from the wires. At which point(s) is the resulting magnetic field pointing out of the paper ?

32.

200 V supply on a 3600-turn primary; secondary tapped with two 90-turn sections giving points P, Q and R

The primary coil of a transformer has $3600$ turns and is connected to $200$ V a.c. supply. The secondary coil has $180$ turns, which can be tapped at different points as shown above. A '40 W, 10 V' light bulb is connected to the transformer so that it works at its rated value. Which of the following statements is/are correct ?

(1)The bulb should be connected to points

P

and

Q

.

(2)The current through the bulb is

4

A.

(3)If the efficiency of the transformer is

80\%

, the current in the primary coil is

0.25

A.

33.

In the below nuclear reactions, what do $X$ , $Y$ and $Z$ represent ?

(1)

{}^{210}_{84}\mathrm{Po}\to{}^{206}_{82}\mathrm{Pb}+X

(2)

{}^{2}_{1}\mathrm{H}+{}^{3}_{1}\mathrm{H}\to{}^{4}_{2}\mathrm{He}+Y

(3)

{}^{234}_{90}\mathrm{Th}\to{}^{234}_{91}\mathrm{Pa}+Z

$X$	$Y$	$Z$
an $\alpha$ particle	a proton	a $\beta$ particle
an $\alpha$ particle	a neutron	a $\beta$ particle
an $\alpha$ particle	a neutron	$\gamma$ rays
a $\beta$ particle	a neutron	$\gamma$ rays

34.

Which of the following can be deflected by both an electric field and a magnetic field ?

(1)

\alpha

particles

(2)

\beta

particles

(3)

\gamma

rays

35.

The activity of a radioactive isotope falls to $\frac{1}{16}$ of its initial value in one hour. Find the half-life of the isotope.