物理電磁感應 DSE、CE 選擇題（第 3 頁）

21.

Which of the following statements about direct current (d.c.) and alternating current (a.c.) is/are correct ?

(1)All d.c. sources produce constant voltages.

(2)The direction of current in an a.c. circuit changes with time.

(3)Both d.c. and a.c. produce a heating effect in a resistor.

22.

Which of the following correctly shows the major energy change in the device when it is working ?

Device	From	To
A microphone	electrical	sound
A loudspeaker	sound	electrical
A dynamo	electrical	electrical
A motor	electrical	mechanical

23.

Magnet between the ends of a soft iron core with a coil connected to a resistor R, terminals P and Q

A magnet is initially placed between the ends of a soft iron core as shown above. The magnet is then quickly rotated clockwise through one complete revolution. Which of the following statements correctly describes the induced current flowing through the resistor $R$ ?

24.

CRO screen showing a vertical line trace, grid squares of 1 cm

The diagram shows the trace of a signal on a CRO with the time base switched off. The $Y$ -gain is set at $1\ \mathrm{V\,cm^{-1}}$ . Which of the following statements correctly describes the input signal ?

25.

Two coils wound on a soft-iron core; one connected to a cell and switch S, the other to a resistor R

Two coils of conducting wires are wrapped on a soft iron core as shown in the above figure. Switch $S$ is closed and after a while re-opened. Which of the following statements is/are true ?

(1)At the moment when

S

is closed, a current flows through

R

from

P

to

Q

.

(2)When

S

remains closed, there is no current flowing through

R

.

(3)At the moment when

S

is re-opened, a current flows through

R

from

P

to

Q

.

26.

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

The above diagram shows a simple generator. Which of the following graphs below shows the time variation of the voltage produced by the generator ?

Voltage-time graphs: A sine wave, B full-wave rectified humps, C half-wave humps, D sawtooth

27.

Figure 1: magnet moving towards a coil connected to a galvanometer; Figure 2: magnet pushed from the other side

In Figure 1, the galvanometer deflects to the left with a reading of $10\ \mu$ A. The north pole of the same magnet is now pushed towards the coil from the other side at a faster rate as shown in Figure 2. The deflection on the galvanometer will be

28.

a.c. source across a 5 kilo-ohm resistor with a CRO connected to the Y-input; Y-gain dial set at 5 V/cm

In the figure above, the peak current through the resistor is

29.

Three figures of a magnet falling through a copper ring viewed by an eye from above: magnet above the ring (1), inside the ring (2), below the ring (3)

A magnet is allowed to fall through a copper ring. What is the direction of the induced current (if any) on the ring observed by the eye when the magnet is in the position as shown in Figure (1), (2) and (3) ?

Figure (1)	Figure (2)	Figure (3)
Clockwise	No current	Anticlockwise
Anticlockwise	No current	Clockwise
Anticlockwise	No current	Anticlockwise
Anticlockwise	Anticlockwise	Clockwise

30.

CRO display of a sine wave over two and a half cycles; grid squares of 1 cm

The figure shows the display on a CRO with the time base set at $1\ \mathrm{ms\,cm^{-1}}$ and $Y$ -gain at $0.5\ \mathrm{V\,cm^{-1}}$ . The peak voltage and frequency of the a.c. voltage applied across the $Y$ -plates are

	Peak voltage	Frequency
	$1.5$ V	$500$ Hz
	$1.5$ V	$250$ Hz
	$3.0$ V	$50$ Hz
	$3.0$ V	$250$ Hz