物理 2007 年 DSE、CE 選擇題（第 4 頁）

31.

Which of the following statements about evaporation are correct ?

(1)Evaporation occurs only on the surface of the liquid.

(2)The rate of evaporation is higher when the temperature is higher.

(3)After evaporation, the average kinetic energy of the remaining liquid molecules will increase.

32.

John wants to estimate the speed of a cloud in the following experiment. The cloud is moving horizontally at a height of $3000$ m above the ground. He looks at the image of the cloud in a mirror of $0.6$ m wide placed on the horizontal ground $1.5$ m below his eye level. He finds that the image of the cloud takes $20$ s to move across the mirror.

Cloud at 3000 m, observer 1.5 m above a 0.6 m wide mirror on the ground, and the image of the cloud (not drawn to scale)

What is the approximate speed of the cloud ?

33.

Which of the following descriptions about ultrasonic waves must be correct ?

34.

Two identical loudspeakers $X$ and $Y$ are connected in parallel to a signal generator. A microphone connected to a CRO detects a maximum when it is $0.2$ m from $X$ and $0.4$ m from $Y$ . It detects a minimum when it is $0.9$ m from $X$ and $0.4$ m from $Y$ . What is the possible wavelength of the sound wave ?

35.

A longitudinal wave is travelling from left to right in a medium. Figure (a) shows the equilibrium positions of particles $A$ to $J$ in the medium. At time $t=0$ , the positions of the particles are shown in Figure (b). Which of the following correctly shows the displacement-time graph of particle $I$ ? (Note : displacement to the right is taken to be positive.)

Figure (a): equilibrium positions of particles A to J. Figure (b): positions at t = 0 for a wave travelling to the right

36.

Noise barriers built along highways are used to block the noise generated by road traffic. Which of the following statements correctly explain how the noise barriers can block the noise ?

(1)The noise from vehicles is reflected.

(2)The noise is absorbed by the noise barriers.

(3)The noise is diffracted at the top edge of the noise barriers.

37.

Metal rod PQ moving to the right on a metal rail in a uniform magnetic field into the paper, with a galvanometer G in the circuit

Under an external force, a metal rod $PQ$ is moving with a uniform speed $v$ on a metal rail placed in a uniform magnetic field as shown above. Which of the following descriptions is/are correct ?

(1)Current flows through the galvanometer from

Q

to

R

.

(2)The metal rod will accelerate to right if the direction of the magnetic field is reversed.

(3)The pointer of the galvanometer will deflect to opposite direction if the direction of the magnetic field is reversed.

38.

Pictorial circuit with battery, ammeter, voltmeter, rheostat and a resistance wire AB with a crocodile clip

A student sets up the above circuit to study the effect of the length of a resistance wire on its resistance. As the crocodile clip is moved from $A$ to $B$ along the resistance wire, which of the following statements is/are correct ?

(1)The reading of the ammeter increases.

(2)The reading of the voltmeter increases.

(3)The voltmeter reading is directly proportional to the reading of the ammeter.

39.

Simple d.c. generator: coil between S and N poles with a split-ring commutator connected to a load, terminals X and Y

The above figure shows a simple structure of a d.c. generator. Which of the following statements is/are correct ?

(1)The current delivered to the load is constant.

(2)The current generated in the coil is alternating, but the current delivered to the load is unidirectional.

(3)The current flows through the load from

X

to

Y

.

40.

Bar magnet above a closed-loop copper coil placed on an electronic balance

An electronic balance reads $X$ when a closed-loop copper coil is placed on it as shown above. A bar magnet drops from certain height vertically above. Just before the magnet reaches the coil, the reading of the electronic balance is