物理 1990 年 DSE、CE 選擇題（第 3 頁）

21.

Part of the electromagnetic spectrum: infra-red, P, Q, X-rays

The diagram shows part of the electromagnetic spectrum. Which of the following statements is/are true ?

(1)The wavelength of

P

is longer than that of

Q

.

(2)The velocity of

P

in a vacuum is smaller than that of

Q

.

(3)

Q

can be deflected by an electric field.

22.

A positively-charged metal sphere is mounted on an insulating stand. When the sphere is earthed with a finger,

23.

Copper rod BD suspended horizontally by threads AB and CD in a magnetic field into the paper, connected to a battery and switch

In the figure shown above, a copper rod is suspended horizontally by two insulating threads $AB$ and $CD$ , and the direction of the magnetic field is into the paper. What happens when the switch of the circuit is closed ?

24.

6 V battery with 4 ohm resistor, then 3 ohm and 1 ohm resistors in series (point X between them) in parallel with a 4 ohm resistor, point Y at the junction

In the circuit shown, the potential difference between $X$ and $Y$ is

25.

Cell with bulbs L1, L2 and L3 and a switch S in the L2 branch

In the circuit shown, $L_1$ , $L_2$ and $L_3$ are identical light bulbs. Which of the following statements is/are correct when the switch $S$ is closed ?

(1)The brightness of

L_1

decreases.

(2)

L_2

and

L_3

are of the same brightness.

(3)

L_1

is brighter than

L_2

.

26.

An electric heater takes a time $T$ to boil a kettle of water when connected to a $200$ V source. What will be the time required to boil the same kettle of water when the heater is connected to a $100$ V source ? (You may assume that the resistance of the heater remains unchanged.)

27.

Figure 1: magnet moving towards a coil connected to a galvanometer; Figure 2: magnet pushed from the other side

In Figure 1, the galvanometer deflects to the left with a reading of $10\ \mu$ A. The north pole of the same magnet is now pushed towards the coil from the other side at a faster rate as shown in Figure 2. The deflection on the galvanometer will be

28.

200 V a.c. supply, transformer, two lamps each marked 10 V 40 W in parallel

In the circuit shown, the efficiency of the transformer is $80\%$ . If the two lamps are to work at their rated values, what is the current in the primary coil and what kind of transformer is being used ?

	Current in primary coil	Transformer
	$0.2$ A	$20:1$ step down
	$0.4$ A	$20:1$ step down
	$0.5$ A	$20:1$ step down
	$0.4$ A	$10:1$ step down

29.

a.c. source across a 5 kilo-ohm resistor with a CRO connected to the Y-input; Y-gain dial set at 5 V/cm

In the figure above, the peak current through the resistor is

30.

In an experiment to measure the half-life of a radioactive isotope in a place where the background count rate is $20$ counts per minute, the following results are recorded :

Time / minute	0	2	4	6	8	10	12
Total count rate / counts per minute	116	96	80	69	58	50	44

The half-life is about