物理 1984 年 DSE、CE 選擇題（第 3 頁）

21.

Which of the following has the greatest current when it is operated at $200$ V ?

22.

The diagrams below show three possible arrangements of four identical resistors $R$ .

Arrangements (a), (b) and (c) of four identical resistors R between X and Y

List the arrangements in order of increasing equivalent resistance, as measured between $X$ and $Y$ .

23.

Two circuits: bulbs P and Q with a switch in series with Q, and bulbs R and S with a switch in series with S

Four identical bulbs $P$ , $Q$ , $R$ and $S$ are connected as shown in the two circuit diagrams above. What happens to the brightness of $P$ and $R$ if the switches of both circuits are closed ?

	Brightness of $P$	Brightness of $R$
	decreases	remains constant
	remains constant	decreases
	increases	remains constant
	decreases	decreases

24.

An ancient piece of wood was tested for its age by carbon-14 dating method. The normal emission rate from $2$ g of carbon from a living plant is $20$ counts per minute. If the rate from $2$ g of carbon from the wood is $5$ counts per minute, and the half life of carbon-14 is $5700$ years, what is the approximate age of the wood in years ? (Background radiation may be neglected.)

25.

Electric fan with switch S and three wires X, Y and Z leading to terminals

In the figure shown, $S$ is a switch to turn on and off the electric fan. $X$ , $Y$ and $Z$ are wires to be connected to the three terminals of a plug. How should $X$ , $Y$ and $Z$ be connected to the three pins (E, L and N) of a given plug ?

Earth	Live	Neutral
$X$	$Y$	$Z$
$Y$	$X$	$Z$
$Z$	$Y$	$X$
$X$	$Z$	$Y$

26.

$S$ is a radioactive source which emits radiation as it decays. If all the radiation emitted is bent by a magnetic field in the direction shown, then the radiation consists of

Source S in a magnetic field pointing into the paper; the radiation path bends downward from the original direction