物理 1984 年 DSE、CE 選擇題（第 2 頁）

11.

Narrow parallel beam entering a box containing two lenses X and Y and leaving as a wider parallel beam

As shown in the diagram, a narrow parallel beam of light is converted to a wider parallel beam by placing two lenses $X$ and $Y$ in the positions shown. Which of the combinations below when correctly chosen and installed could produce the effect required ?

	Lens $X$	Lens $Y$
(1)	convex	concave
(2)	concave	concave
(3)	concave	convex

12.

Which of the following will produce a diminished image of an object $O$ ? ( $f$ is the focal length)

Four diagrams A to D of an object O at different distances in front of a convex lens, with distances f and f marked

13.

Aeroplane above a pond surface with points P, Q, R and S at different heights and a diver below

As shown in the diagram, the apparent position of the aeroplane seen by the diver at the bottom of the small pond is at

14.

In a ripple tank experiment, a series of plane water waves are sent through a narrow slit. Which of the following will have changed when the water waves emerge from the slits ?

(1)wave speed

(2)wave pattern

(3)frequency

15.

Two sources A and B emitting waves towards the mid-point P

Two sound waves of the same frequency are emitted from 2 sources $A$ and $B$ as shown in the diagram. What kind of interference will occur at the mid-point $P$ when the 2 waves generated are (1) in phase; (2) anti-phase ?

	In phase	Anti-phase
	constructive	constructive
	constructive	destructive
	destructive	constructive
	cannot be determined	cannot be determined

16.

Two uncharged metal spheres in contact hanging by dry cotton threads with a positively charged rod X nearby

Two uncharged metal spheres in contact are suspended by dry cotton threads. When a positively charged rod $X$ is brought near them as shown above, the distribution of charges on the spheres will be

Four diagrams A to D of charge distributions on two touching spheres

17.

Which of the following devices is designed to convert mechanical energy into electrical energy ?

18.

Transformer with 200 V across the primary coil and 10 V across the secondary coil carrying 3 A

As shown in the diagram, a $200$ V mains supply is stepped down to $10$ V by a transformer. If the output current is $3$ A and the efficiency of the transformer is $75\%$ , what is the current in the primary coil ?

19.

Two heaters of rating ' $1$ kW, $200$ V' and ' $2$ kW, $200$ V' respectively are connected in series to a $200$ V supply. What is the total power consumed by the heaters ?

20.

$P$ and $Q$ are two long parallel straight wires carrying currents as shown. What is the direction of the force on $Q$ ?

Two long parallel wires P and Q with currents upward in P and downward in Q