物理 1995 年 DSE、CE 選擇題（第 3 頁）

21.

When a sound wave travels from air into water, its wavelength is increased by five times. If the speed of sound in air is $330\ \mathrm{m\,s^{-1}}$ , find the speed of the sound wave in water.

22.

Which of the following cannot travel through a vacuum ?

23.

Cell and 10 ohm resistor between P and Q, with 5 ohm resistor and ammeter A1 and 10 ohm resistor and ammeter A2 in parallel branches

In the above circuit, the reading of ammeter $A_1$ is $0.6$ A. Find the p.d. between points $P$ and $Q$ .

24.

Which of the following values is equivalent to one kilowatt hour ?

25.

Positively charged rod near two insulated uncharged metal spheres X and Y in contact

Two insulated uncharged metal spheres $X$ and $Y$ are in contact with each other. A positively charged rod is brought near $X$ as shown above. Which of the following diagrams correctly shows the distribution of charges on the spheres ?

Four diagrams A to D of charge distributions on the two touching spheres

26.

Four strings of light bulbs connected in parallel to an a.c. supply, each string with bulbs in series

A Christmas tree is illuminated with four strings of light bulbs. Each string has ten identical light bulbs connected in series as shown in the figure. If one light bulb suddenly burns out, which of the following will happen ?

27.

Which of the following involve(s) the application of electromagnets ?

(1)An electric bell

(2)A telephone receiver

(3)A moving-coil loudspeaker

28.

Battery with ammeter A2, bulb and ammeter A1 in one branch, bulb and switch S in another branch, and a voltmeter across the supply

In the circuit above, the ammeters have negligible resistance. Which of the following statements are true if switch $S$ is closed ?

(1)The reading of ammeter

A_1

decreases.

(2)The reading of ammeter

A_2

increases.

(3)The reading of the voltmeter remains unchanged.

29.

Electromagnet: a coil on a soft iron core connected to a battery and a variable resistor

The above diagram shows a simple electromagnet. Which of the following can increase the strength of the electromagnet ?

(1)Decreasing the resistance of the variable resistor.

(2)Replacing the soft iron core with one made of steel.

(3)Replacing the battery with a

50

Hz a.c. source.

30.

Three-pin plug with pins P, Q, R and a fuse, and a cable with wires X (Brown), Y (Blue) and Z (Green / yellow)

The diagram above shows a three-pin plug and the wires connected to it. To which of the pins should each of the wires $X$ , $Y$ and $Z$ be connected ?

$P$	$Q$	$R$
$Z$	$Y$	$X$
$Y$	$X$	$Z$
$Y$	$Z$	$X$
$Z$	$X$	$Y$