1989 年

物理 · 共 23 題 · 點擊選項即時核對答案

只看錯題本頁已答 0 / 3

21.

A radioactive source has a half-life of $22$ years. After $66$ years, what fraction of the source remains undecayed ?

22.

Which of the following correctly shows the major change of energy in the devices ?

	Device	From	To
(1)	an electric motor	electrical	mechanical
(2)	a loudspeaker	sound	mechanical
(3)	a transformer	electrical	electrical

23.

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