物理 1997 年 DSE、CE 選擇題（第 3 頁）

21.

The following are the steps to charge an isolated metal sphere by induction, but they are in the wrong order.

(1)Removing the finger away from the metal sphere.

(2)Touching the metal sphere with a finger.

(3)Bringing a positively charged rod near the metal sphere.

(4)Removing the positively charged rod away from the metal sphere.

Which of the following shows the correct order ?

22.

The kilowatt-hour is a unit of

23.

All the resistors in the below networks are identical.

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

Arrange the networks in ascending order of equivalent resistances :

24.

Which of the following statements about the earth wire in an electric iron is/are correct ?

(1)The earth wire should be connected to the metal body of the iron.

(2)If the iron is working properly, no current pass through the earth wire.

(3)In case the neutral wire is broken, the earth wire provides a spare wire for the return path to the mains socket.

25.

Battery with switch S, ammeter A, heater Y in series, and heater X with a voltmeter V across it

In the circuit shown, $X$ and $Y$ are identical heaters. The ammeter has negligible resistance while the voltmeter has very high resistance. When switch $S$ is closed, the ammeter records a reading but the voltmeter reading is zero. Which of the following provides a possible explanation ?

26.

Which of the following devices converts mechanical energy into electrical energy when it works ?

27.

Metal ring released above a vertical bar magnet with N at top and S at bottom; ring positions X (top) and Y (bottom)

A metal ring is released and falls vertically around a magnet as shown in the above diagram. Which of the following diagrams correctly describes the directions of the induced current, if any, in the ring at positions $X$ and $Y$ ?

Four diagrams A to D showing induced current directions in the ring at positions X and Y

28.

a.c. source across a variable resistor PQ; terminal Q and sliding contact R connected to the Y-input of a CRO

In the above circuit, the terminal $Q$ and the sliding contact $R$ of the variable resistor are connected to the $Y$ -input of a CRO. If $R$ is moved towards $P$ , how would the amplitude and period of the trace displayed on the CRO be affected ?

	Amplitude of the trace	Period of the trace
	increases	remains unchanged
	increases	increases
	decreases	remains unchanged
	decreases	decreases

29.

Simple generator with rectangular coil between N and S poles and a commutator, connected to a load

The diagram shows an electricity generator connected to a load. Which of the following can increase the voltage produced by the generator ?

(1)Rotating the coil at a greater speed

(2)Reducing the resistance of the load

(3)Replacing the coil with one of larger area

30.

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