物理 1991 年 DSE、CE 選擇題（第 3 頁）

21.

Plane water waves travelling to the right towards an obstacle

A series of water waves, generated in water of uniform depth, is travelling towards an obstacle as shown above. Which of the following diagrams best shows the wave pattern after passing the obstacle ?

Four wave patterns A to D after the obstacle

22.

Which of the following graphs correctly shows the variation of speed $v$ against frequency $f$ of the electromagnetic waves in vacuum ?

Four graphs A to D of speed v against frequency f: A straight line through origin, B curve levelling off, C curve rising, D horizontal line

23.

Which of the following is an application of ultrasonic in everyday life ?

24.

Oscilloscope traces of musical notes X and Y

The diagrams above show the traces of two musical notes $X$ and $Y$ on an oscilloscope. Which of the following statements is/are true ?

(1)

X

has a higher pitch than

Y

.

(2)The loudness of

X

is greater than that of

Y

.

(3)

X

and

Y

are longitudinal waves.

25.

For safety, the correct way of connecting the fuse and switch to electrical appliances should be

26.

Negatively charged object C near two insulated metal spheres X and Y in contact

In the figure shown, $X$ and $Y$ are two insulated metal spheres in contact with each other. A negatively charged object $C$ is brought near $X$ . $X$ is touched momentarily with a finger, and then $X$ and $Y$ are separated. The charges on $X$ and $Y$ are

	Sphere $X$	Sphere $Y$
	positive	positive
	positive	negative
	negative	negative
	positive	zero

27.

Battery with resistors P and Q in series and resistor R with switch S in parallel with Q

In the above circuit, $P$ , $Q$ and $R$ are identical resistors. Which of the following is true when switch $S$ is closed ?

28.

The following diagrams show three networks consisting of different resistors.

Network (1): 1 ohm, 1 kilohm and 1 megohm in parallel; (2): the three in series; (3): 1 megohm in parallel with 1 ohm, in series with 1 kilohm

Arrange the above networks in descending order of equivalent resistances :

29.

Two long parallel wires AB and CD each connected to a battery

Two long parallel wires $AB$ and $CD$ are connected to batteries as shown in the figure. The force acting on $CD$ is

30.

3 V battery, switch S, voltmeter V1 across the battery and switch, 10 ohm and 20 ohm resistors in series with voltmeter V2 across the 20 ohm resistor

In the circuit shown, what are the readings of voltmeters $V_1$ and $V_2$ if switch $S$ is closed ?

	Voltmeter $V_1$	Voltmeter $V_2$
	$0$ V	$2$ V
	$0$ V	$3$ V
	$1$ V	$2$ V
	$3$ V	$0$ V