科目：物理 Physics (9702)
年份：2020 冬季
试卷：Paper 41
总页数：20
章节：12-13: Circular Motion & Gravitational Fields, 14-16: Temperature, Ideal Gases & Thermodynamics, 17: Oscillations, 18: Electric fields, 19: Capacitance, 20: Magnetic fields, 22: Quantum physics, 24: Medical physics
4 BLANK PAGE © UCLES 2020 9702/41/O/N/20
5 Answer all the questions in the spaces provided. 1 (a) (i) State what is meant by a field of force. ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... [2] (ii) Define gravitational field strength. ............................................................................................................................... ............................................................................................................................... [1]
6 2 (a) The first law of thermodynamics may be expressed as $$ \Delta U = (+q) + (+w) $$ where $\Delta U$ is the increase in internal energy of the system. State the meaning of: +q ............................................................................................................... ............................................................................................................ +w. ............................................................................................................. ............................................................................................................... [2] (b) The variation with pressure $p$ of the volume $V$ of a fixed mass of an ideal gas is shown in Fig. 2.1.  Fig. 2.1 The gas undergoe
7 (i) Show that the magnitude of the work done during the change A to B is 390 J. 1 (ii) State and explain the total change, if any, in the internal energy of the gas during one complete cycle. ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... [2] (c) During the change A to B, 1370 J of thermal energy is transferred to the gas. During the change B to C, no thermal energy enters or leaves the gas. The work done on the gas during this change is 550 J. Use
8 3 A pendulum consists of a metal sphere P suspended from a fixed point by means of a thread, as illustrated in Fig. 3.1. Fig. 3.1 The centre of gravity of sphere P is a distance $L$ from the fixed point. The sphere is pulled to one side and then released so that it oscillates. The sphere may be assumed to oscillate with simple harmonic motion. (a) State what is meant by *simple harmonic motion*. ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... [2] (b
9
Use Fig. 3.2 to determine the frequency $f$ of the oscillations of sphere P.
$$
f = \underline{\hspace{5cm}} \text{ Hz [3]}
$$
(c) The period $T$ of the oscillations of sphere P is given by the expression
$$
T = 2\pi \sqrt{\left(\frac{L}{g}\right)}
$$
where $g$ is the acceleration of free fall.
Use your answer in (b) to determine the length $L$ .
$$
L = \underline{\hspace{5cm}} \text{ m [2]}
$$
(d) Another pendulum consists of a sphere Q suspended by a thread. Spheres P and Q are identical. The thread attached to sphere Q is longer than the thread attached to sphere P.
Sphere Q is displaced and then released. The oscillations of sphere Q have the same amplitude as the oscillations of sphere P.
On Fig. 3.2, sketch the variation of the velocity $v$ with displacement $x$ for spher10 BLANK PAGE © UCLES 2020 9702/41/O/N/20
11 4 (a) Explain the principles of the *generation* of ultrasound waves for use in medical diagnosis. ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... ...................................................
12
5 (a) Define *electric potential* at a point.
...............................................................................................................................
...............................................................................................................................
............................................................................................................................... [2]
(b) Two point charges A and B are separated by a distance of 12.0 cm in a vacuum, as illustrated in Fig. 5.1.
<div style="text-align: center;">
</div>
Fig. 5.1
The charge of A is $+2.0 \times 10^{-9} \, \text{C}$ .
A point P lies on the line joining charges A and B. Its distance from charge A is x.
The variation with distance13
Use Fig. 5.2 to determine:
(i) the charge of B
charge = ………………………………………… C [3]
(ii) the change in electric potential when point P moves from the position where $x = 9.0$ cm to the position where $x = 3.0$ cm.
change = ………………………………………… V [1]
(c) An $\alpha$ -particle moves along the line joining point charges A and B in Fig. 5.1.
The $\alpha$ -particle moves from the position where $x = 9.0$ cm and just reaches the position where $x = 3.0$ cm.
Use your answer in (b)(ii) to calculate the speed $v$ of the $\alpha$ -particle at the position where $x = 9.0$ cm.
$v = \text{……………………………………… m s}^{-1}$ [3]
[Total: 9]
© UCLES 2020 9702/41/O/N/20 [Turn over14 6 (a) (i) Define the capacitance of a parallel plate capacitor. ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... [2] (ii) State three functions of capacitors in electrical circuits. 1. ............................................................................................................................... 2. ............................................................................................................................... 3. .......
15
7 An ideal operational amplifier (op-amp) is to be used in a comparator circuit. Part of the comparator circuit is shown in Fig. 7.1.
Fig. 7.1
Three resistors, each of resistance $1000\,\Omega$ , and a negative temperature coefficient thermistor are available to complete the circuit.
The circuit is to be designed so that, at low temperatures, the output $V_{OUT}$ is $-5.0\,\text{V}$ and at higher temperatures, the output $V_{OUT}$ is to be $+5.0\,\text{V}$ .
(a) On Fig. 7.1, draw the input circuit to the inverting and non-inverting inputs of the op-amp. [4]
(b) State a suitable value for the thermistor resistance when the thermistor is at:
(i) low temperature where $V_{OUT}$ is $-5.0\,\text{V}$
.....................................................................................16
8 A slice of a conducting material has its face QRLK normal to a uniform magnetic field of flux density $B$, as illustrated in Fig. 8.1.
<div style="text-align: center;">
</div>
Fig. 8.1
Electrons enter the slice travelling perpendicular to face PQKJ.
(a) For the free electrons moving in the slice:
(i) state the direction of the force on an electron due to movement of the electron in the magnetic field
...............................................................................................................................
............................................................................................................................... [1]
(ii) identify the faces, using the letters on Fig. 8.1, between which a potential difference is 17
(c) The number of free electrons per unit volume in the slice of material is $1.3 \times 10^{29} \, \text{m}^{-3}$ .
The thickness $PQ$ of the slice is $0.10 \, \text{mm}$ .
The magnetic flux density $\vec{B}$ is $4.6 \times 10^{-3} \, \text{T}$ .
Calculate the potential difference across the slice for a current of $6.3 \times 10^{-4} \, \text{A}$ .
potential difference = ………………………………………… V [2]
(d) The slice in (c) is a metal.
By reference to your answer in (c), suggest why Hall probes are usually made using semiconductors rather than metals.
…………………………………………………………………………………………………………
…………………………………………………………………………………………………………
……………………………………………………………………………………………………… 2
© UCLES 2020 9702/41/O/N/20 [Turn ove18 9 (a) Define magnetic flux. ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... [2] (b) A simple transformer consists of two coils of wire wound on a soft-iron core, as illustrated in Fig. 9.1.  Fig. 9.1 There is a sinusoidal current in the primary coil. Explain: (i) how this current gives rise to an induced electromotive force (e.m.f.) in the secondary coil ........................................................................
19 (c) Explain why the soft-iron core in (b) is laminated. ....................................................................................................................... ....................................................................................................................... ....................................................................................................................... 2 © UCLES 2020 9702/41/O/N/20 [Turn over
20 10 (a) Outline briefly the principles of computed tomography (CT scanning). ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... ..........................................................................
21
(b) One section of a model designed to illustrate CT scanning is divided into four voxels. The pixel numbers K, L, M and N of the voxels are shown in Fig. 10.1.
<div style="text-align: center;">
</div>
Fig. 10.1
The section is viewed, in turn, from four different directions $D_1$, $D_2$, $D_3$ and $D_4$, as shown in Fig. 10.1.
The detector readings for each direction are noted and these are summed to give the values shown in Fig. 10.2.
<table>
<tr>
<td>42</td>
<td>45</td>
</tr>
<tr>
<td>51</td>
<td>30</td>
</tr>
</table>
Fig. 10.2
The background reading is 24.
Determine the pixel numbers K, L, M and N shown in Fig. 10.1.
$$
K = \text{....................................................}
$$
$$
L = \text{..........................22 11 A photon of wavelength 540 nm collides with an isolated stationary electron, as illustrated in Fig. 11.1.  Fig. 11.1 The photon is deflected elastically by the electron. The wavelength of the deflected photon is 544 nm. (a) (i) State what is meant by a *photon*. ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... [2] (ii) On Fig. 11.1, draw an arrow to indicate the approximate direction of motion of the deflected electron. [1]
23 (b) Calculate: (i) the momentum of the deflected photon momentum = ………………………………………… Ns [2] (ii) the energy transferred to the deflected electron. energy = ………………………………………… J [2] (c) Another photon of wavelength 540 nm collides with an isolated stationary electron. Explain why it is not possible for the deflected photon to have a wavelength less than 540 nm. ………………………………………………………………………………………………………… ………………………………………………………………………………………………………… ……………………………………………………………………………………………………… 2 © UCLES 2020 9702/41/O/N/20 [Turn over