物理 1992 年 DSE、CE 選擇題（第 3 頁）

21.

Plane waves in a shallow region approaching a barrier with a small slit, deep region beyond

The figure above shows a series of plane water waves travelling in a shallow region of water. The waves pass through a small slit to a deep region of water. Which of the following diagrams shows the wave pattern in the deep region ?

Four wave patterns A to D beyond the slit

22.

Given that the wavelengths of visible light range from $4\times10^{-7}$ m to $7\times10^{-7}$ m, which of the following combinations of wavelengths for infrared, red and violet lights is possible ?

Infrared	Red light	Violet light
$1\times10^{-4}$ m	$7\times10^{-7}$ m	$4\times10^{-7}$ m
$1\times10^{-7}$ m	$7\times10^{-7}$ m	$4\times10^{-7}$ m
$1\times10^{-4}$ m	$4\times10^{-7}$ m	$7\times10^{-7}$ m
$1\times10^{-7}$ m	$4\times10^{-7}$ m	$7\times10^{-7}$ m

23.

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

24.

The potential difference between two points $X$ and $Y$ in a circuit is $10$ V. Which of the following must be true ?

25.

Positively charged rod near an insulated metal sphere

A positively charged rod is brought near an insulated uncharged metal sphere as shown in the figure. If the sphere is earthed with a finger,

26.

Battery with ammeters A1, A2, A3, resistors and a switch S in parallel with one resistor

In the circuit above, what happens to the readings of the three ammeters if switch $S$ is closed ?

$A_1$	$A_2$	$A_3$
increases	increases	increases
remains unchanged	becomes zero	increases
increases	increases	becomes zero
decreases	increases	becomes zero

27.

6 V battery in series with a 10 ohm resistor and a 5 ohm resistor

In the circuit shown, what is the power dissipated in the $10\ \Omega$ resistor ?

28.

A small compass is placed near a long current carrying wire. In which of the following diagrams is/are the compass needle pointing in the correct direction ?

(1)Compass above the wire

(2)Compass below the wire

(3)Current flowing out of paper

Three diagrams of a compass near a current-carrying wire: above the wire, below the wire, and with current out of the paper

29.

200 V a.c. supply, transformer and four light bulbs in parallel on the secondary coil

In the circuit shown, the rating of each light bulb is '20 W, 10 V'. The current in the primary coil is $0.5$ A. If all the bulbs work at their rated values, find the turns ratio and the efficiency of the transformer.

	Turns ratio	Efficiency
	$5:1$ step down	$80\%$
	$20:1$ step down	$20\%$
	$20:1$ step down	$40\%$
	$20:1$ step down	$80\%$

30.

Which of the following can increase the efficiency of a transformer ?

(1)Increasing the number of turns of the secondary coil.

(2)Using a laminated iron core.

(3)Using a thicker copper wires to make the coils.