物理 2005 年 DSE、CE 選擇題（第 3 頁）

21.

Flat circular current-carrying coil in the plane of the page, current entering and leaving at the bottom

Which of the following diagrams shows the magnetic field pattern formed around a flat circular current-carrying coil, in the plane as shown above ?

Four magnetic field line patterns A to D around the coil cross-section

22.

Simple motor with magnet, coil, rheostat and battery box

The figure shows a simple motor. Which of the following changes can increase the turning effect of the coil ?

(1)using a stronger magnet

(2)reducing the resistance of the rheostat

(3)using a coil with a smaller number of turns

23.

Source in a lead castle in a magnetic field pointing into the paper, with Geiger-Muller tubes at positions P and Q

A radioactive source is placed in front of a uniform magnetic field pointing into the paper as shown above. If a high count rate is recorded at positions $P$ and $Q$ , what kinds of radiation have been detected ?

	$P$	$Q$
	$\gamma$	$\alpha$
	$\gamma$	$\beta$
	$\beta$	$\alpha$
	$\beta$	$\gamma$

24.

A thorium nucleus ( ${}^{234}_{90}\mathrm{Th}$ ) decays by emitting a $\beta$ particle to form a daughter nucleus $X$ . Which of the following equations represents this decay ?

25.

Which of the following nuclear reactions is a nuclear fusion ?

26.

Questions 28 and 29 : Two blocks $X$ and $Y$ are connected by a light string passing over a smooth pulley as shown below. The mass of $X$ is greater than that of $Y$ . The blocks are released from rest.

Block X on a smooth horizontal surface connected by a string over a pulley to hanging block Y

Which of the following pairs of forces is an action and reaction pair ?

27.

Kelvin is standing on a balance inside a lift. The table shows the readings of the balance in three situations.

Motion of the lift	Reading of the balance
moving upwards with a uniform speed	$R_1$
moving downwards with a uniform speed	$R_2$
moving upwards with an acceleration	$R_3$

Which of the following relationships is correct ?

28.

A car $P$ of mass $1000$ kg moves with a speed of $20\ \mathrm{m\,s^{-1}}$ and makes a head-on collision with a car $Q$ of mass $1500$ kg, which was moving with a speed of $10\ \mathrm{m\,s^{-1}}$ in the opposite direction before the collision. If the two cars stick together after the collision, find their common velocity immediately after the collision.

29.

Fish hung from a light string stretched between two poles, the string making 70 degrees with each pole

A fish is hung on a light string as shown above. If the tension in the string is $10$ N, find the total weight of the fish and the hook.

30.

If equal masses of boiling water and melting ice cubes are mixed, which of the following best describes the state of the mixture ? (Specific heat capacity of water $= 4200\ \mathrm{J\,kg^{-1}\,{}^\circ C^{-1}}$ ; Specific latent heat of fusion of ice $= 3.34\times10^5\ \mathrm{J\,kg^{-1}}$ )