物理 1986 年 DSE、CE 選擇題（第 3 頁）

21.

A transformer can be used to

(1)step-up the voltage of a given a.c. supply.

(2)increase the power of a given a.c. supply.

(3)step-up the voltage of a given battery.

22.

Arrange the following circuits in ascending order of equivalent resistances :

Three resistor networks (1), (2) and (3)

23.

Circuit with ammeters A1, A2 and A3, two resistors, a constant voltage source and a switch S

In the above circuit, $A_1$ , $A_2$ and $A_3$ are ammeters connected to a constant voltage source. What will happen to the readings of the ammeters if the switch $S$ is closed ?

$A_1$	$A_2$	$A_3$
decreases	increases	becomes zero
increases	decreases	becomes zero
no change	increases	becomes zero
no change	decreases	increases

24.

An electric heater of rating " $2000$ W, $200$ V" and an electric cooker of rating " $500$ W, $200$ V" are connected in parallel to a $200$ V a.c. source. The total power of the two appliances is

25.

Two close parallel wires X and Y connected to a d.c. source and a resistor

Two close, long, parallel, straight metal wires $X$ and $Y$ form part of the circuit shown above. $X$ and $Y$

26.

The efficiency of a transformer is $50\%$ and the input and output voltages are $200$ V and $12$ V respectively. If the power output is $30$ W, the input current is

27.

The speeds of X-rays, $\gamma$ rays and $\beta$ rays in air are denoted by $v_X$ , $v_\gamma$ and $v_\beta$ respectively. Which of the following is true ?

28.

$P$ and $Q$ are bulbs of rating " $40$ W, $200$ V" while $R$ and $S$ are of rating " $60$ W, $200$ V". Which of the following circuits gives the maximum brightness ? (Assume all circuits are connected to the same voltage.)

Four circuits A to D each containing bulbs P, Q, R and S: A all four in parallel, B P and Q in parallel with R and S in series, C P and Q in series in parallel with R and S in series, D P and Q in series with R and S in parallel

29.

Which of the following equations represent(s) possible nuclear reactions ?

(1)

{}^{10}_{5}\mathrm{B}+\text{neutron}\to{}^{6}_{3}\mathrm{Li}+\alpha\text{ particle}

(2)

{}^{210}_{83}\mathrm{Bi}\to{}^{210}_{84}\mathrm{Po}+\beta\text{ particle}

(3)

{}^{14}_{7}\mathrm{N}+\alpha\text{ particle}\to{}^{17}_{8}\mathrm{O}+\text{proton}