物理 2009 年 DSE、CE 選擇題（第 3 頁）

21.

Two parallel straight wires carrying equal upward currents, passing through a plane

The figure above shows two parallel straight wires carrying equal currents. Which of the following diagrams correctly shows the resultant magnetic field lines ?

Four magnetic field line patterns A to D for two wires with currents out of the plane

22.

A current carrying wire, perpendicular to the plane of the paper, is located at $P$ . $P$ is in the midway between two identical bar magnets with unknown polarities $X$ and $Y$ as shown in the figure. The current in the wire is flowing into the paper. The magnetic force $F$ acting on the wire is downward. Which of the following statements are correct ?

(1)

X

is a north pole and

Y

is a south pole.

(2)

F

is reversed if the current direction is reversed.

(3)

F

is larger if stronger bar magnets are used.

Wire at P between bar magnets X and Y, current into the paper, force F downward

23.

Electric kettle with wires X, Y (through a switch and fuse) and Z

Which of the following is the correct connection of $X$ , $Y$ and $Z$ to the mains ?

$X$	$Y$	$Z$
live	earth	neutral
live	neutral	earth
neutral	earth	live
neutral	live	earth

24.

${}^{2}_{1}\mathrm{H}+{}^{3}_{1}\mathrm{H}\to{}^{4}_{2}\mathrm{He}+\mathrm{n}$ Which of the following descriptions about the nuclear reaction above is correct ?

25.

$X\xrightarrow{\alpha}Y\xrightarrow{\beta}Z$

The above shows part of a decay series. Which of the following deductions is/are correct ?

(1)

X

and

Z

are isotopes of the same element.

(2)

X

has two more neutrons than

Z

.

(3)

Z

has one more proton than

Y

.

26.

The half-life of a radioactive sample is $15$ hours. The initial count rate recorded is $1000$ counts per minute. After $15$ hours, the count rate recorded becomes $528$ counts per minute. What is the background count rate ? (Measured in counts per minute.)

27.

Which of the following conditions is/are necessary to sustain the chain reaction in the nuclear fission of uranium-235 ?

(1)Each fission produces a large amount of energy.

(2)At least one neutron is released in each fission.

(3)Each fission produces two smaller nuclei.

28.

A diver jumps up vertically in the air from a high platform and falls into water. The $v$ - $t$ graph below shows the variation of the velocity of the diver against time from the point he jumps ( $P$ ) until he is at the lowest point ( $Q$ ) in the water.

Diver path from platform point P up and down to point Q in water (not to scale), and velocity-time graph: 3 m/s at t = 0, zero at 0.3 s, -13 m/s at 1.6 s, back to zero at 2 s

Which of the following is correct ?

	Total distance travelled from $P$ to $Q$	Height of the platform above water surface
	$8.9$ m	$8$ m
	$10.6$ m	$10$ m
	$11.5$ m	$8$ m
	$11.5$ m	$10.6$ m

29.

A 50 N block hung from two threads: S1 at 30 degrees to the horizontal and S2 horizontal

A $50$ N block is suspended by two threads as shown in the figure above. Thread $S_1$ makes an angle $30^\circ$ with the horizontal and thread $S_2$ is horizontal. What is the tension in $S_2$ ?

30.

Block sliding to the right towards a vertical wall on a horizontal smooth surface

A block slides along a horizontal smooth surface as shown in the figure above. It collides with a vertical wall and rebounds along its original path. Which of the graphs below best shows the variation of its momentum $p$ with time $t$ ?

Four graphs A to D of momentum p against time t