科目：物理 Physics (9702)
年份：2017 春季
试卷：Paper 42
总页数：20
章节：12-13: Circular Motion & Gravitational Fields, 14-16: Temperature, Ideal Gases & Thermodynamics, 17: Oscillations, 18: Electric fields, 20: Magnetic fields, 21: Alternating currents, 22: Quantum physics, 23: Nuclear physics, 24: Medical physics
4 Answer all the questions in the spaces provided. 1 (a) Define gravitational potential at a point. ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... [2] (b) A rocket is launched from the surface of a planet and moves along a radial path, as shown in Fig. 1.1. <div style="text-align: center;">  </div> Fig. 1.1 The planet may be considered to be an isolated sphere of radius $R$ with all of its mass $M$ concentrated at its centre. P
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(ii) The rocket motor is switched off at point A. During the journey from A to B, the rocket has a constant mass of $4.7 \times 10^4 \, \text{kg}$ and its kinetic energy changes from $1.70 \, \text{TJ}$ to $0.88 \, \text{TJ}$ .
For the planet, the product $GM$ is $4.0 \times 10^{14} \, \text{N} \, \text{m}^2 \, \text{kg}^{-1}$ . It may be assumed that resistive forces to the motion of the rocket are negligible.
Use the expression in (b)(i) to determine the distance from A to B.
distance = ………………………………………… m 3
© UCLES 2017 9702/42/F/M/17 [Turn over6 2 (a) The first law of thermodynamics can be represented by the expression $$ \Delta U = q + w. $$ State what is meant by the symbols in the expression. + $\Delta U$ ............................................................................................................... + $q$ ....................................................................................................................... + $w$ ....................................................................................................................... [2] (b) A fixed mass of an ideal gas undergoes a cycle ABCA of changes, as shown in Fig. 2.1.  Fig. 2.1 (i) During the change from A to B, the energy supplied to the gas by heating is 442 J. Use the first law of thermodynamics to show
7 (ii) During the change from B to C, the internal energy of the gas decreases by 313 J. By considering molecular energy, state and explain qualitatively the change, if any, in the temperature of the gas. ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... [3] (iii) For the change from C to A, use the data in (b)(i) and (b)(ii) to
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3 A uniform beam is clamped at one end. A metal block of mass $m$ is fixed to the other end of the beam causing it to bend, as shown in Fig. 3.1.
Fig. 3.1
The block is given a small vertical displacement and then released so that it oscillates with simple harmonic motion.
The acceleration $a$ of the block is given by the expression
$$
a = -\frac{k}{m}x
$$
where $k$ is a constant for the beam and $x$ is the vertical displacement of the block from its equilibrium position.
(a) Explain how it can be deduced from the expression that the block moves with simple harmonic motion.
...............................................................................................................................
..........................................................9 (c) The initial amplitude of the oscillation of the block is 3.0 cm. Use the expression in (b) to determine the maximum kinetic energy of the oscillations. maximum kinetic energy = ………………………………………… J [3] (d) Over a certain interval of time, the maximum kinetic energy of the oscillations in (c) is reduced by 50%. It may be assumed that there is negligible change in the angular frequency of the oscillations. Determine the amplitude of oscillation. amplitude = ………………………………………… m [2] (e) Permanent magnets are now positioned so that the metal block oscillates between the poles, as shown in Fig.3.2. <!-- Image: Fig.3.2 --> Fig.3.2 The block is made to oscillate with the same initial amplitude as in (c). Use energy conservation to explain why the energy of the oscillations decreases m
10 4 Explain the main principles of the *generation* of ultrasound waves for medical use. ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... ...............................................................
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5 (a) State three advantages of an optic fibre compared to a metal wire for the transmission of a signal.
1. ...............................................................................................................................
2. ...............................................................................................................................
3. ...............................................................................................................................
[3]
(b) An optic fibre of length 57 km is connected between a transmitter and a receiver, as shown in Fig. 5.1.
<table>
<tr>
<td>transmitter</td>
<td>57 km</td>
<td>receiver</td>
</tr>
<tr>
<td>signal power<br> $15 \times 10^{-3} \, \text{W}$ </td>
<td>optic fibre<12
6 (a) State one similarity and one difference between the electric field lines and the gravitational field lines around an isolated positively charged metal sphere.
similarity .......................................................................................................................
difference ...................................................................................................................
[2]
(b) A positive point charge $+Q$ is positioned at a fixed point $X$ and an identical positive point charge is positioned at a fixed point $Y$ , as shown in Fig. 6.1.
$$
+Q \xrightarrow{X} \xrightarrow{A} \xrightarrow{B} \xrightarrow{Y} +Q
$$
$\begin{array}{c}
\text{2.5 cm} \\
\hline
\text{10.0 cm} \\
\hline
\text{2.5 cm}
\end{array}$
Fig. 6.1
The charges are 13 (ii) On Fig. 6.2, sketch the variation of the electric field strength $E$ with distance $d$ from A to B, along the line AB. <div style="text-align: center;">  </div> Fig. 6.2 [2] (iii) A small positive charge is placed at A. The electric field causes this charge to move from rest along the line AB. Describe the acceleration of the charge as it moves from A to B. ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... ...............
14 7 (a) Describe, with a labelled diagram, the structure of a metal-wire strain gauge. ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... [3] (b) In a strain gauge, the increase in resistance $\Delta R$ depends on the increase in length $\Delta L$ . The variation of $\Delta R$ with $\Delta L$ is shown in Fig. 7.1. $\Delta R/\Omeg
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(i) The strain gauge is initially unstrained with resistance $300.0\,\Omega$ .
Use data from Fig. 7.1 to calculate the increase in length $\Delta L$ of the strain gauge that gives rise to a potential of $+2.00\,\text{V}$ at point A in Fig. 7.2.
$$
\Delta L = \underline{\hspace{5cm}}\,\text{m} \quad [3]
$$
(ii) The strain gauge undergoes a further increase in length beyond the value in (b)(i).
State and explain which one of the light-emitting diodes, X or Y, will be emitting light.
...............................................................................................................................
...............................................................................................................................
...............................................16 8 (a) State what is meant by a magnetic field. ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... [2] (b) A particle of charge $+q$ and mass $m$ is travelling in a vacuum with speed $v$ . The particle enters, at a right angle, a uniform magnetic field of flux density $B$ , as shown in Fig.8.1. <div style="text-align: center;">  <br/> <em>Fig.8.1</em> </div> The particle leaves the field after following a semi-circular path of
17 (iv) Use the expression in (b)(iii) to show that the time $T_F$ spent in the field by the particle is independent of its speed $v$. 2 © UCLES 2017 9702/42/F/M/17 [Turn over
18 9 An ideal transformer is shown in Fig. 9.1. Fig. 9.1 (a) Explain (i) why the core is made of iron, ............................................................................................................................... ............................................................................................................................... [1] (ii) why an electromotive force (e.m.f.) is not induced at the output when a constant direct voltage is at the input. ............................................................................................................................... ............................................................................................................................... [2] (b) An alternating voltage of peak value 150V is
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Use data from Fig. 9.2 to
(i) calculate the number of turns of the secondary coil,
$$
\text{number} = \text{...................................................} [2]
$$
(ii) state one time when the magnetic flux linking the secondary coil is a maximum.
$$
\text{time} = \text{...................................................} \text{ms} [1]
$$
(c) A resistor is connected between the output terminals of the secondary coil. The mean power dissipated in the resistor is 1.2 W. It may be assumed that the varying voltage across the resistor is equal to the varying e.m.f. $E$ shown in Fig. 9.2.
(i) Calculate the resistance of the resistor.
$$
\text{resistance} = \text{...................................................} \Omega [2]
$$
(ii) On Fig. 9.3, sketch the variation with time $t$20 10 (a) State what is meant by a photon. ............................................................................................................................... ............................................................................................................................... [2] (b) Light in a beam has a continuous spectrum that lies within the visible region. The photons of light have energies ranging from 1.60 eV to 2.60 eV. The beam passes through some hydrogen gas. It then passes through a diffraction grating and an absorption spectrum is observed. (i) All of the light absorbed by the hydrogen is re-emitted. Explain why dark lines are still observed in the absorption spectrum. ................................................................................
21 11 Use band theory to explain why, unlike a copper wire, the resistance of an intrinsic semiconductor decreases with an increase of temperature. ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... .....
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12 (a) Define the binding energy of a nucleus.
...............................................................................................................................
...............................................................................................................................
............................................................................................................................... [2]
(b) A stationary nucleus of uranium-238 ( $^{238}_{92}\text{U}$ ) decays to form a nucleus of thorium-234 ( $^{234}_{90}\text{Th}$ ). An $\alpha$ -particle and a gamma-ray photon are emitted. The equation representing the decay is
$$
^{238}_{92}\text{U} \rightarrow ^{234}_{90}\text{Th} + \frac{4}{2}\text{He} + \frac{0}{0}\gamma
$$
The masses of the nucl23 (iii) State and explain whether the energy of the gamma-ray photon is equal to the energy released in the reaction. ............................................................................................................................... ............................................................................................................................... ...............................................................................................................................2 © UCLES 2017 9702/42/F/M/17