物理 2006 年 DSE、CE 選擇題（第 4 頁）

31.

Flash lamps used by professional photographers can find object distances by using infrared waves so as to adjust the flash output. Which of the following is/are the reason(s) of using infrared waves instead of ultrasonic waves in such flash lamps ?

(1)Speed of infrared waves is much faster than that of ultrasonic waves making the time for finding object distances shorter.

(2)Objects to be photographed will usually emit infrared waves.

(3)The sound produced by ultrasonic waves makes photographers feel annoyed.

32.

The following data show the frequencies and sound intensity levels of some musical notes produced by a piano.

Note	Frequency / Hz	Intensity level / dB
C	256	64
D	288	68
E	320	65
F	341	63

Which of the following statements is/are correct ?

(1)The note F has the lowest pitch.

(2)The note D has the greatest loudness.

(3)The note C played on a guitar will sound differently from the same note produced by the piano.

33.

Label: Lithium cell 3.6 V, Capacity 800 mA h, Time (stand-by mode) about 3 days

The figure shows a label on a lithium cell of a mobile phone. The capacity $800$ mA h indicates the quantity of electric charges that the cell will discharge in $3$ days when the mobile phone is in stand-by mode. Estimate the average power of the cell assuming the voltage remains constant during the discharge.

34.

Home-made sound device: a coil above a plastic cup whose base has a fixed magnet, with signal input

The diagram shows a home-made device which can produce sound. In the device, a magnet is fixed to the base of a plastic cup. When a signal passes through the coil, the base vibrates to produce a sound. Which of the following methods can make the sound louder ?

(1)using a stronger magnet

(2)inserting a copper rod into the coil

(3)increasing the number of turns in the coil

35.

Ideal transformer with ammeter in the primary, bulb X in parallel with bulb Y and switch S

In the above circuit, the transformer is ideal (efficiency $= 100\%$ ). At the beginning, the switch $S$ is closed and the light bulbs $X$ and $Y$ are operated at their rated values. What would happen to the brightness of the bulb $X$ and the ammeter reading if $S$ is now opened ?

	Brightness of $X$	Ammeter reading
	increases	increases
	increases	decreases
	remains unchanged	decreases
	remains unchanged	remains unchanged

36.

The photograph shows an adaptor for charging up the battery inside a mobile phone. Which of the following statements about the adaptor are correct ?

(1)It is used to step down the voltage from the mains to a value for the mobile phone.

(2)It can convert the voltage from a.c. to d.c.

(3)It is black so that it can radiate heat more efficiently than those in other colours.

Photograph of a mobile phone charger adaptor with cable and phone

37.

An electric appliance draws a current $2$ A when it is operating at $220$ V. Which of the following is the best description of the current and the voltage of each wire of the electric appliance when it is connected to a $220$ V mains supply ?

Live wire: Current	Live wire: Voltage	Neutral wire: Current	Neutral wire: Voltage	Earth wire: Current	Earth wire: Voltage
$2$ A	$220$ V	$1$ A	$220$ V	$1$ A	$0$
$2$ A	$220$ V	$2$ A	$220$ V	$0$	$0$
$2$ A	$220$ V	$2$ A	$0$	$0$	$0$
$2$ A	$220$ V	$0$	$0$	$0$	$0$

38.

Two light bulbs are marked ' $220$ V, $50$ W' and ' $220$ V, $100$ W' respectively. If they are connected in series to a $220$ V mains supply, what is the current drawn from the mains supply ?

39.

In the circuit shown, the resistors represent the heating elements in a rice cooker. The resistances of the elements are $5R$ and $R$ respectively. The rice-cooker can be operated in two modes, namely, cooking and keeping warm. The power consumed by the cooker in the cooking mode is $600$ W when $S$ is closed. What is the power consumed by the rice-cooker in the mode of keeping warm when $S$ is open ?

Circuit with switch S in parallel with a 5R resistor, in series with a resistor R and the mains

40.

A radioisotope $X$ has a half-life of $2$ days while another radioisotope $Y$ has a half-life of $1$ day. Initially there are $N$ undecayed atoms of $X$ and $8N$ undecayed atoms of $Y$ . After how many days will $X$ and $Y$ have the same number of undecayed atoms ?