物理 1999 年 DSE、CE 選擇題（第 3 頁）

21.

Battery with resistor R, ammeter A in series and a voltmeter V connected across R and the ammeter

A student uses the above circuit to find the resistance of a resistor $R$ (i.e. resistance $=\dfrac{\text{voltmeter reading}}{\text{ammeter reading}}$ ). Which of the following statements is/are correct ?

(1)The ammeter reading records the actual current passing through

R

.

(2)The voltmeter reading records the actual voltage across

R

.

(3)The value of the resistance of

R

obtained is smaller than its actual value.

22.

Cell delivering current I to three parallel resistors of 60 ohm, 60 ohm and 30 ohm

In the above circuit, the current delivered by the cell is $I$ . What is the current passing through the $30\ \Omega$ resistor ?

23.

Ideal transformer with ammeter in primary, voltmeter and bulb P across the secondary, and bulb Q in series with switch S

Two bulbs $P$ and $Q$ are connected to an ideal transformer as shown. Which of the following statements is/are true when switch $S$ is closed ?

(1)The brightness of bulb

P

decreases.

(2)The reading of the ammeter increases.

(3)The reading of the voltmeter remains unchanged.

24.

Which of the following is/are the advantage(s) of using high voltages for long distance transmission of electricity ?

(1)The transmission speed can be increased.

(2)Some heavy industries operate at high voltages.

(3)The energy loss in the transmission cable can be reduced.

25.

A '6 V, 6 W' light bulb is to be operated at its rated value. Three circuits are set up as shown below.

Circuit (1): 12 V battery with 6 ohm resistor in parallel with the bulb; circuit (2): 12 V battery with two 6 ohm resistors and the bulb; circuit (3): 12 V a.c. supply with a transformer of 200 turns primary and 100 turns secondary

The power supplies all have negligible internal resistance. In which of the above circuits is the bulb working at its rated value ?

26.

The background count rate recorded by a Geiger-Muller counter is $80$ counts per minute. When a radioactive source is placed closely in front of the Geiger-Muller tube, the count rate recorded is $560$ counts per minute. After $6$ hours, the count rate drops to $140$ counts per minute. Find the half-life of the source.

27.

Which of the following statements about $\alpha$ particles is incorrect ?

28.

Grid of mass number A (233 to 237) against atomic number Z (90 to 93) with points R, Q, S, P and Np

The above diagram shows the mass number $A$ and atomic number $Z$ of a few nuclides. The isotope of neptunium (Np) shown decays by emitting an $\alpha$ particle and then a $\beta$ particle. Which of the following represents the resulting nuclide ?

29.

Which of the following applications of radioactivity makes use of the fact that a radioactive nuclide has a constant half-life ?