物理 1980 年 DSE、CE 選擇題（第 3 頁）

21.

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.

22.

An electromagnetic wave has a frequency of the order of $10^{16}$ Hz. What should be the type of the electromagnetic wave ? Given that the speed of light in vacuum is $3\times10^{8}\ \mathrm{m\,s^{-1}}$ .

23.

In a $\beta$ decay, element $X$ , having a half-life of $3$ days, decays into a stable element $Y$ . If the initial mass of $X$ is $4$ g, what will be the masses of $X$ and $Y$ after $6$ days ?

	Mass of $X$	Mass of $Y$
	$0$ g	$4$ g
	$1$ g	$3$ g
	$2$ g	$2$ g
	$3$ g	$1$ g

24.

Cell, two resistors R, an ammeter and a switch K: one R in series with the ammeter, the other R with switch K in a parallel branch

Two identical resistors $R$ , an ammeter, a switch $K$ and a cell are connected as shown. When $K$ is open, the ammeter reading is $1$ A. When $K$ is closed, the reading will

25.

The two isotopes ${}^{35}_{17}\mathrm{Cl}$ and ${}^{37}_{17}\mathrm{Cl}$ of chlorine have different

(1)numbers of protons

(2)number of neutrons

(3)chemical properties

26.

A radioactive nuclide ${}^{A}_{Z}X$ undergoes radioactive decay inside a cloud chamber. The radiations emitted are subjected to a magnetic field and the resulting tracks are as shown in the figure. What are the atomic number and the mass number of the remaining nuclide ?

Cloud chamber with a magnetic field acting out of the paper; a radioactive nuclide emits a track that curves

	Atomic Number	Mass Number
	$Z-2$	$A-4$
	$Z+1$	$A-4$
	$Z+1$	$A$
	$Z-1$	$A-4$

27.

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.

28.

2 V cell with ammeter A and switch K in series, voltmeter V in parallel with that branch, and a 100 ohm resistor in series

In the circuit shown, $V$ is a voltmeter of high internal resistance and $A$ is an ammeter of low internal resistance. What is the voltmeter reading when (a) switch $K$ is open, and (b) switch $K$ is closed ?

	$K$ open	$K$ closed
	$0$ V	$0$ V
	$0$ V	$2$ V
	$1$ V	$1$ V
	$2$ V	$0$ V

29.

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 $C$ ?