物理 2011 年 DSE、CE 選擇題（第 4 頁）

31.

Figure (a) shows a series of particles ( $E$ - $O$ ) at their equilibrium positions. Figure (b) shows the positions of the particles at a certain instant when a longitudinal wave travelling to the right passes through the particles.

Figure (a): particles E to O at equilibrium. Figure (b): positions at a certain instant, wave travelling right, scale 5 cm

Which of the following statements is incorrect ?

32.

Circular wavefronts through two narrow gaps S1 and S2 in a barrier, with points P and Q marked

The figure above shows the wavefronts formed in a ripple tank. Solid lines represent crests. $S_1$ and $S_2$ are two narrow gaps allowing water waves to pass through and interfere. Which of the following statements is correct ?

33.

Telecommunication companies nowadays use optical fibres to transmit data. What are the advantages of using optical fibres over copper wires in transmitting data ?

(1)Less data loss in the transmission.

(2)Data can be transmitted at a higher rate.

(3)For the same data transmission rate, optical fibres take up less space.

34.

A ray of light enters a transparent rectangular block from air and emerges. Which of the following ray diagrams is/are impossible ? The dotted lines represent normal to the surfaces.

Three ray diagrams (1), (2), (3) of a ray entering and leaving a rectangular block, with angles 40, 45 and 50 degrees marked

35.

Battery, ammeter and bulb X in series with bulb Y and a voltmeter across the combination

In the circuit shown, both bulbs $X$ and $Y$ light up normally. Both the ammeter and the voltmeter readings are non-zero. Which of the following will cause the ammeter reading to drop to zero while the voltmeter reading is still non-zero ? Assume meters are ideal.

36.

In the figure below, the arrow shows the direction of the magnetic field at $Z$ due to a current-carrying solenoid.

Solenoid XY with a soft iron core, terminals P and Q, and an arrow at Z pointing to the right

The direction of current through the solenoid and the magnetic north pole of the solenoids are

	Direction of current	Magnetic north pole
	from $P$ to $Q$	$X$
	from $P$ to $Q$	$Y$
	from $Q$ to $P$	$X$
	from $Q$ to $P$	$Y$

37.

220 V mains supply, ideal transformer, lamps L1 and L2 in series on the secondary with a switch S across L1

The figure above shows an ideal transformer. The primary coil of the transformer is connected to the $220$ V a.c. mains and two lamps $L_1$ and $L_2$ are connected in series to the secondary coil. Initially, $S$ is closed. Which of the following statements is correct when $S$ is opened ?

38.

Electrical power is transmitted from a power station to local substations by an alternating current and high voltage. It is because

(1)the voltage of an alternating current can be stepped up or down using transformers easily.

(2)by using high voltages, electrical power can be transmitted faster along the cables.

(3)by using high voltages, there is less power loss in the transmission cables.

39.

Metal rod PQ moving to the right with speed v on metal rails in a uniform magnetic field into the paper, resistor connected across RS

A metal rod $PQ$ is moving with a uniform speed $v$ on a metal rail placed in a uniform magnetic field as shown in the figure above. A resistor is connected across $RS$ . Which of the following descriptions is/are correct ?

(1)Kinetic energy of the rod is converted into electrical energy.

(2)Current flows through the resistor from

S

to

R

.

(3)The induced current will be reversed if the rod moves in the opposite direction.

40.

Radioactive source in a lead box between parallel plates X (+5000 V) and Y (0 V), with a GM tube moved along a dotted line through P, Q and R

The figure shows a radioactive source placed near two parallel metal plates $X$ and $Y$ that are connected to a power supply. When a GM tube is moved along the dotted line ( $- \cdot - \cdot -$ ), the count rate shows a significant increase at $P$ and $Q$ respectively. Which of the following statements is correct when a magnetic field pointing out of paper is applied between $X$ and $Y$ ?