物理 1994 年 DSE、CE 選擇題（第 4 頁）

31.

200 V a.c. supply, transformer and a light bulb marked 40 W 10 V

In the circuit shown, the rating of the light bulb is '40 W, 10 V'. The efficiency of the transformer is $80\%$ . If the bulb works at its rated value, find the current in the primary coil.

32.

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 $R$ ?

33.

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 ?

34.

Arrange $\alpha$ , $\beta$ and $\gamma$ radiation in ascending order of their ionizing powers :

35.

Which of the following symbols represents a neutron ?

36.

The activity of a radioactive source drops from $640$ Bq to $40$ Bq in $2$ hours. Find the half-life of the source.