物理 1998 年 DSE、CE 選擇題（第 3 頁）

21.

Plane water waves in a shallow region meeting the boundary with a deep region at an angle

The diagram above shows a series of plane water waves travelling from a shallow region to a deep region of water. Which of the following diagrams best shows the wave pattern in the deep region ?

$Four refraction diagrams A to D$

22.

A series of plane water waves travel towards an obstacle in a ripple tank. When the waves pass the obstacle, they bend around the corners of the obstacle. Which of the following statements is/are correct ?

(1)The phenomenon is called diffraction.

(2)The speed of the waves remains unchanged as they bend round the corners.

(3)The degree of bending of the waves depends on the size of the obstacle relative to the wavelength of the waves.

23.

Stationary wave on a string of length 1 m with particles P, Q and R marked

A vibrator generates a stationary wave on a string. The above diagram shows the string at the instant of maximum displacement. Which of the following statements is incorrect ?

24.

Which of the following statements about ultrasonic waves is correct ?

25.

Three insulated uncharged metal spheres X, Y, Z in contact with a negatively charged rod near X

Three insulated uncharged metal spheres $X$ , $Y$ and $Z$ are placed in contact as shown. A negatively charged rod is brought near $X$ and sphere $Y$ is then earthed momentarily. If the charged rod is still near $X$ , which of the following describes the charges on $X$ , $Y$ and $Z$ ?

Sphere $X$	Sphere $Y$	Sphere $Z$
positive	zero	zero
positive	zero	negative
positive	positive	positive
zero	zero	zero

26.

6 V battery with resistor R in series with an ammeter and 6 ohm resistor in parallel with a 12 ohm resistor

In the above circuit, the reading of the ammeter is $0.4$ A. Find the resistance of the resistor $R$ .

27.

Circuit with a 12 V supply, a 10 ohm resistor and a motor M in series

A motor is connected in series with a $10\ \Omega$ resistor and a $12$ V power supply as shown. If the current in the circuit is $0.5$ A, find the power consumed by the motor.

28.

Cross-sections of wires X and Y (current out of the paper) and Z (current into the paper), with a direction key: upwards, downwards, left, right

The above diagram shows the cross-section of three parallel straight wires $X$ , $Y$ and $Z$ . $X$ and $Y$ carry currents flowing out of the paper and $Z$ carries a current flowing into the paper. What is the direction of the resultant force acting on $Y$ ?

29.

Electrical appliance connected to a power supply through connecting wires of total resistance R

An electric appliance is connected to a power supply of voltage $V$ by long connecting wires of total resistance $R$ as shown in the circuit. It is found that the current passing through the appliance is only $\frac{1}{2}I_0$ , where $I_0$ is the current required for the appliance to work at its rated value. Which of the following changes could increase the current through the appliance to $I_0$ ?

	Voltage of power supply	Total resistance of connecting wires
	increases to $2V$	remains unchanged
	increases to $2V$	reduces to $R/2$
	remains unchanged	increases to $2R$
	remains unchanged	reduces to $R/2$

30.

Metal rod on a loop containing a galvanometer, magnetic field into the paper, induced current shown anticlockwise; direction key: upwards, downwards, left, right

In the above diagram, a metal rod is placed inside a magnetic field pointing into the paper. In which direction should the rod be moved in order to produce an induced current as shown in the diagram ?