物理 1993 年 DSE、CE 選擇題（第 3 頁）

21.

Which of the following statements about ultrasonics is INCORRECT ?

22.

Battery with ammeter A3, two resistors in series with ammeter A1, and a resistor with ammeter A2 in a parallel branch

In the circuit shown above, all resistors are identical and the ammeters are of negligible resistance. If the readings of ammeter $A_1$ is $2$ A, find the readings of ammeters $A_2$ and $A_3$ .

	$A_2$	$A_3$
	$2$ A	$2$ A
	$2$ A	$4$ A
	$4$ A	$2$ A
	$4$ A	$6$ A

23.

Which of the following circuits can be used to measure the resistance of the light bulb ?

Four circuits A to D with a battery, light bulb, ammeter and voltmeter connected in different ways

24.

An uncharged light conducting sphere is suspended by an insulating thread. The metal dome of an operating Van de Graaff generator is brought near the sphere. Which of the following statements best describes the motion of the sphere ?

25.

Battery with bulb L1 in series with bulbs L2 and L3 in parallel

In the circuit, $L_1$ , $L_2$ and $L_3$ are identical light bulbs. Which of the following statements is/are true ?

(1)

L_2

and

L_3

have the same brightness.

(2)

L_1

is brighter than

L_2

.

(3)The power dissipated in

L_1

is equal to the sum of powers dissipated in

L_2

and

L_3

.

26.

Which of the following is NOT a correct unit for the corresponding physical quantity ?

	Physical quantity	Unit
	charge	coulomb
	current	ampere
	resistance	ohm
	voltage	joule

27.

Two coils wound on a soft-iron core; one connected to a cell and switch S, the other to a resistor R

Two coils of conducting wires are wrapped on a soft iron core as shown in the above figure. Switch $S$ is closed and after a while re-opened. Which of the following statements is/are true ?

(1)At the moment when

S

is closed, a current flows through

R

from

P

to

Q

.

(2)When

S

remains closed, there is no current flowing through

R

.

(3)At the moment when

S

is re-opened, a current flows through

R

from

P

to

Q

.

28.

A motor lifts a load of $40$ N vertically upwards at a steady speed of $0.2\ \mathrm{m\,s^{-1}}$ . The voltage applied to the motor is $12$ V and the current drawn is $2$ A. Find the efficiency of the motor.

29.

Simple generator with rectangular coil between N and S poles and a commutator, connected to output terminals

The above diagram shows a simple generator. Which of the following graphs below shows the time variation of the voltage produced by the generator ?

Voltage-time graphs: A sine wave, B full-wave rectified humps, C half-wave humps, D sawtooth

30.

Which of the following signs is used to indicate radioactive material ?

Four signs: A skull and crossbones, B radiation trefoil, C letter R, D a lamp with rays