物理 1990 年 DSE、CE 選擇題（第 2 頁）

11.

A convex lens is used as a magnifying glass to read small printing in a book. Which of the following statements is/are true ?

(1)The image distance is greater than the object distance.

(2)The image of the printing is real.

(3)The image of the printing is erect.

12.

Some gas is sealed inside a container of fixed volume. If the gas is heated, which of the following statements is/are true ?

(1)The pressure of the gas increases.

(2)The kinetic energy of the gas molecules increases.

(3)The density of the gas increases.

13.

The pressure of a fixed mass of gas at $25^\circ$ C is $2\times10^5$ Pa. How would its pressure be if the gas were reduced to half its volume and its temperature were increased to $95^\circ$ C ?

14.

The apparatus shown is used to find the specific latent heat of vaporization of water. Which of the following is NOT correct ?

Beaker of water with immersion heater connected to joulemeter and power supply, standing on an electronic balance

15.

Graph of temperature against time for a solid heated by a 20 W heater: rising, then a flat melting section, then rising again

A $20$ W heater is used to melt a solid. A graph of temperature $\theta$ against time $t$ is plotted as shown above. If a heater of $40$ W is used, which graph (using the same scale) would be obtained ?

16.

Straight barrier PQ with a pulse AB travelling towards it at 30 degrees to the barrier

A straight pulse $AB$ is travelling towards a straight barrier $PQ$ in a ripple tank as shown above. Which of the following figures best shows the reflected pulse ?

Four figures A to D showing a reflected pulse at different angles

17.

Two loudspeakers L1 and L2, 3 m apart, each 4 m from the line XY; Q and P are marked on the line XY

In the figure shown, the two loudspeakers $L_1$ and $L_2$ emit sound waves of wavelength $2$ m. The waves emitted are exactly opposite in phase. Which of the following graphs best represents the variation of sound intensity $I$ along the line $XY$ ?

Four graphs A to D of sound intensity I against position along XY

18.

In a ripple tank experiment, a series of plane water waves passes through a narrow slit. Which of the following properties of the waves will remain unchanged ?

(1)speed

(2)direction of travel

(3)frequency

19.

Which of the following statements about sound is/are correct ?

(1)Sound cannot travel through water.

(2)Loudness increases with the amplitude of the sound wave.

(3)Pitch increases with the wavelength of the sound wave.

20.

A bat emits sound waves of frequency $30$ kHz and receives a reflected signal from an obstacle after $0.15$ s. The speed of the sound in air is $340\ \mathrm{m\,s^{-1}}$ . How far is the obstacle away from the bat ?