物理 1982 年 DSE、CE 選擇題（第 3 頁）

21.

Transformer with 200 V a.c. source in the primary circuit and resistor R in the secondary circuit carrying current I

In the transformer shown, the ratio of the number of turns on the primary coil to that on the secondary coil is $10:1$ . If the power input is $50$ W, find the current in the secondary coil, assuming that the transformer has a $80\%$ efficiency.

22.

Three metal spheres X, Y, Z in contact, Y earthed, and a negatively charged rod near Z

Three metal spheres $X$ , $Y$ and $Z$ are placed in contact as shown. A negatively charged rod is brought near $Z$ . The sphere $Y$ is then earthed while the charged rod is still nearby. Which of the following statements is/are true ?

(1)

X

is negatively charged.

(2)

Y

is neutral.

(3)

Z

is positively charged.

23.

A radioactive substance has a half-life of $10$ minutes. Which of the following statements is/are correct ?

(1)All the atoms of the radioactive substance will split into

4

equal parts in

5

minutes.

(2)All the atoms of the radioactive substance will decay completely in

20

minutes.

(3)All the atoms of the radioactive substance will decay within

10

minutes.