物理電磁感應 DSE、CE 選擇題（第 2 頁）

11.

Generator: coil rotating between two magnet poles with slip rings, brushes and a load PQ

The diagram shows a generator connected to a load. Which of the following statements is/are correct ?

(1)The generator produces an alternating current through the load.

(2)At the instant shown, a current is flowing through the load from

P

to

Q

.

(3)The current produced reaches a maximum when the coil is vertical.

12.

Photographs of a sine-wave trace on a CRO and of the voltage gain and time-base settings

The photographs show the trace of a signal on a CRO and some settings of the CRO. The voltage gain is set at $0.5$ V per cm and the time-base is set at $10$ ms per cm. Find the frequency and peak voltage of the input signal.

	Frequency / Hz	Peak Voltage / V
	$10$	$2$
	$10$	$4$
	$20$	$2$
	$20$	$4$

13.

Bar magnet released above a vertical solenoid connected to the Y-input of a CRO

A long solenoid is placed in a vertical position and its two ends are connected to the $Y$ -input of a CRO (with the time base switched on). A bar magnet is released above the solenoid so that it falls through the solenoid. Which of the following figures best represents the trace shown on the CRO ?

Four CRO traces A to D: A positive then negative spike, B two positive spikes, C a wide dip, D a flat-topped pulse then a dip

14.

A bar magnet is placed near a solenoid. Which of the following correctly shows the direction of the induced current in the solenoid ? (Note : The arrow " $\rightarrow$ " indicates the direction of motion of the magnet or solenoid.)

Four diagrams A to D of a bar magnet and a solenoid connected to a galvanometer, with motion arrows and induced current directions

15.

Metal rod on a loop containing a galvanometer, magnetic field into the paper, induced current shown anticlockwise; direction key: upwards, downwards, left, right

In the above diagram, a metal rod is placed inside a magnetic field pointing into the paper. In which direction should the rod be moved in order to produce an induced current as shown in the diagram ?

16.

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

17.

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

18.

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

19.

Which of the following devices is not an application of electromagnetic induction ?

20.

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

The diagram shows the display on a CRO with the time base at $10\ \mathrm{ms\,cm^{-1}}$ and $Y$ -gain at $0.5\ \mathrm{V\,cm^{-1}}$ . Find the peak voltage and frequency of the signal applied across the $Y$ -plates.

	Peak voltage / V	Frequency / Hz
	$1$	$16.7$
	$1$	$25$
	$1$	$50$
	$2$	$25$