物理 2001 年 DSE、CE 選擇題（第 3 頁）

21.

Water waves of wavelength $\lambda$ are diffracted as they pass through a gap of width $d$ as shown in the figure above. Which of the following changes would produce the most significant diffraction effect ?

$Plane water waves passing through a gap of width d in a barrier and diffracting$

	$\lambda$	$d$
	remains unchanged	halved
	remains unchanged	doubled
	halved	halved
	halved	doubled

22.

Which of the following relations is incorrect ?

23.

Two charged spheres A and B hanging from a point on a ceiling by two identical nylon threads

Two charged spheres $A$ and $B$ hanging at a point on a ceiling by two identical nylon threads. They remain at rest as shown above. Which of the following statements must be correct ?

(1)Both spheres carry positive charges.

(2)The force acting on

B

by

A

is larger than the force acting on

A

by

B

.

(3)The mass of

A

is larger than

B

.

24.

If the ammeter in the circuit reads $3$ A, find the voltage of the battery.

Battery with 2 ohm and 1 ohm resistors in parallel, in series with a 3 ohm resistor and an ammeter

25.

Which of the following diagrams shows the correct connection of the fuse and switch of an electric kettle to the mains supply ? ( $L$ : live, $N$ : neutral, $E$ : earth)

Four diagrams A to D showing different positions of a fuse and switch in the live, neutral and earth wires of an electric kettle

26.

Two electric heaters $X$ and $Y$ are of ratings ' $110$ V, $40$ W' and ' $110$ V, $80$ W' respectively. Which of the below deductions about the two heaters is/are correct ?

(1)The operating resistance of

X

is twice that of

Y

.

(2)

X

will consume a power of

80

W when it is connected to a

220

V mains supply.

(3)Both heaters work at their rated values when they are connected in series to a

220

V mains supply.

27.

12 V a.c. supply connected to a transformer whose secondary coil is tapped at equal intervals

A $12$ V a.c. supply is connected to a transformer with turns ratio $1:1$ . The secondary coil is tapped at equal intervals as shown above. Two '6 V, 0.5 W' light bulbs are connected to the secondary coil of the transformer. In which of the following cases will the two bulbs work at their rated values ?

Three ways (1), (2), (3) of connecting the two bulbs to the tapped secondary coil

28.

Bar magnet released above a vertical solenoid connected to the Y-input of a CRO

A long solenoid is placed in a vertical position and its two ends are connected to the $Y$ -input of a CRO (with the time base switched on). A bar magnet is released above the solenoid so that it falls through the solenoid. Which of the following figures best represents the trace shown on the CRO ?

Four CRO traces A to D: A positive then negative spike, B two positive spikes, C a wide dip, D a flat-topped pulse then a dip

29.

Electromagnet: a coil on a soft iron core connected to a 12 V battery and a variable resistor

The figure shows an electromagnet which is used to pick up iron objects. Which of the following can increase the strength of the electromagnet ?

(1)increasing the number of turns in the coil

(2)reducing the resistance of the variable resistor

(3)replacing the battery with a

12

V a.c. power supply

30.

The figure shows a model power line. An a.c. supply and two ideal transformers are used to operate a lamp of rating '6 V, 24 W'. The total resistance of the cables $AB$ and $CD$ is $10\ \Omega$ . If the lamp operates at its rated value, find the power loss in the cables.

a.c. supply, 1:20 step-up transformer, cables AB and CD, 20:1 step-down transformer and a lamp rated 6 V 24 W