科目：物理 Physics (9702)
年份：2020 春季
试卷：Paper 42
总页数：24
章节：12-13: Circular Motion & Gravitational Fields, 14-16: Temperature, Ideal Gases & Thermodynamics, 17: Oscillations, 18: Electric fields, 19: Capacitance, 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) TESS is a satellite of mass 360 kg in a circular orbit about the Earth as shown in Fig. 1.1. <div style="text-align: center;">  </div> Fig. 1.1 (not to scale) The radius of the Earth is $6.4 \times 10^6$ m and the mass of the Earth, considered to be a point mass at its centre
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(ii) Calculate the change in gravitational potential energy between TESS in orbit and TESS on a launch pad on the surface of the Earth.
change in gravitational potential energy = ………………………………………… J [3]
(iii) Use the information in (b)(i) to calculate the ratio:
$$
\frac{\text{gravitational field strength on surface of Earth}}{\text{gravitational field strength at location of TESS in orbit}}
$$
ratio = ………………………………………… 2
© UCLES 2020 9702/42/F/M/20 [Turn over6
2 A large container of volume $85 \, \text{m}^3$ is filled with $110 \, \text{kg}$ of an ideal gas. The pressure of the gas is $1.0 \times 10^5 \, \text{Pa}$ at temperature $T$ .
The mass of $1.0 \, \text{mol}$ of the gas is $32 \, \text{g}$ .
(a) Show that the temperature $T$ of the gas is approximately $300 \, \text{K}$ .
[3]
(b) The temperature of the gas is increased to $350 \, \text{K}$ at constant volume. The specific heat capacity of the gas for this change is $0.66 \, \text{J} \, \text{kg}^{-1} \, \text{K}^{-1}$ .
Calculate the energy supplied to the gas by heating.
energy = ………………………………………… J [2]
(c) Explain how movement of the gas molecules causes pressure in the container.
…………………………………………………………………………………………………………
…………………………………………………………………………………………………………
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(d) The temperature of a gas depends on the root-mean-square (r.m.s.) speed of its molecules.
Calculate the ratio:
$$
\frac{\text{r.m.s. speed of gas molecules at } 350\,\text{K}}{\text{r.m.s. speed of gas molecules at } 300\,\text{K}}
$$
ratio = ………………………………………… 2
© UCLES 2020 9702/42/F/M/20 [Turn over8
3 (a) A body undergoes simple harmonic motion.
The variation with displacement $x$ of its velocity $v$ is shown in Fig. 3.1.
Fig. 3.1
(i) State the amplitude $x_o$ of the oscillations.
$x_o = \underline{\hspace{5cm}} \text{ m [1]}$
(ii) Calculate the period $T$ of the oscillations.
$T = \underline{\hspace{5cm}} \text{ s [3]}$
(iii) On Fig. 3.1, label with a P a point where the body has maximum potential energy. [1]
© UCLES 2020 9702/42/F/M/209 (b) A bar magnet is suspended from the free end of a spring, as shown in Fig. 3.2. Fig. 3.2 One pole of the magnet is situated in a coil of wire. The coil is connected in series with a switch and a resistor. The switch is open. The magnet is displaced vertically and then released. The magnet oscillates with simple harmonic motion. (i) State Faraday’s law of electromagnetic induction. ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... ..................
10 4 (a) (i) Explain why ultrasound used in medical diagnosis is emitted in pulses. ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... [2] (ii) Explain the principles of the detection of ultrasound waves used in medical diagnosis. ............................................................................................................................... .....................................................................................................
11 (ii) Use your value in (b)(i) to explain why gel is applied to the surface of the skin during an ultrasound scan. ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... 2 © UCLES 2020 9702/42/F/M/20 [Turn
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5 (a) State two advantages of the transmission of data in digital form, rather than analogue form.
1. ...............................................................................................................................
2. ...............................................................................................................................
[2]
(b) Optic fibres are used for the transmission of data.
(i) A signal in an optic fibre is carried by an electromagnetic wave of frequency $1.36 \times 10^{14} \text{Hz}$ . The speed of the wave in the fibre is $2.07 \times 10^8 \text{m s}^{-1}$ .
For this electromagnetic wave, determine the ratio:
$$
\frac{\text{wavelength in free space}}{\text{wavelength in fibre}}
$$
ratio = ...........................................13 BLANK PAGE © UCLES 2020 9702/42/F/M/20 [Turn over
14 6 Two positively charged identical metal spheres A and B have their centres separated by a distance of 24 cm, as shown in Fig. 6.1. <div style="text-align: center;">  </div> Fig. 6.1 (not to scale) The variation with distance $x$ from the centre of A of the electric field strength $E$ due to the two spheres, along the line joining their centres, is represented in Fig. 6.2. <div style="text-align: center;">  </div> Fig. 6.2 (a) State the radius of the two spheres. radius = ______________________ cm [1] © UCLES 2020 9702/42/F/M/20
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(b) The charge on sphere A is $3.6 \times 10^{-9} \, \text{C}$ . Determine the charge $Q_B$ on sphere B.
Assume that spheres A and B can be treated as point charges at their centres.
Explain your working.
$$
Q_B = \underline{\hspace{5cm}} \, \text{C} \, [3]
$$
(c) (i) Sphere B is removed.
Use information from (b) to determine the electric potential on the surface of sphere A.
$$
\text{electric potential} = \underline{\hspace{5cm}} \, \text{V} \, [2]
$$
(ii) Calculate the capacitance of sphere A.
$$
\text{capacitance} = \underline{\hspace{5cm}} \, \text{F} \, [2]
$$
[Total: 8]
© UCLES 2020 9702/42/F/M/20 [Turn over16 7 (a) On Fig. 7.1, sketch the temperature characteristic of a negative temperature coefficient (n.t.c.) thermistor. Label the axes with quantity and unit. Fig. 7.1 [2] (b) An n.t.c. thermistor and a resistor are connected as shown in Fig. 7.2. Fig. 7.2 The temperature of the thermistor is increased. State and explain the change, if any, to the reading on the voltmeter. ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... [2] © UCLES 2020 9702/42/F/M/2
17 (c) The variation with the fractional change in length $\Delta x/x$ of the fractional change in resistance $\Delta R/R$ for a strain gauge is shown in Fig. 7.3. Fig. 7.3 The unstrained resistance of the gauge is $120\,\Omega$ . Calculate the new resistance of the gauge when it is extended to a strain of $0.020$ . resistance = ………………………………………… $\Omega$ 3 © UCLES 2020 9702/42/F/M/20 [Turn over
18 8 (a) Explain what is meant by a magnetic field. ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... [1] (b) The apparatus shown in Fig. 8.1 is used in an experiment to find the magnetic flux density $B$ between the poles of a horseshoe magnet. Assume the magnetic field is uniform between the poles of the magnet and zero elsewher
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<div style="text-align: center;">
</div>
Fig. 8.2
Calculate the magnetic flux density $B$ .
$$
B = \underline{\hspace{5cm}} \text{ T [2]}
$$
© UCLES 2020 9702/42/F/M/20 [Turn over20
(c) In a different experiment, electrons are accelerated through a potential difference and then enter a region of magnetic field. The magnetic field is into the plane of the paper and is perpendicular to the direction of travel of the electrons, as illustrated in Fig. 8.3.
<div style="display: flex; align-items: center; gap: 10px;">
<div style="text-align: center;">
<div>electron beam</div>
<div style="width: 20px; height: 20px; border-top: 1px solid black; margin-top: 5px;"></div>
</div>
<div style="width: 100px; height: 150px; background-color: #e0e0e0; border: 1px solid black; border-radius: 10px; position: relative;">
<div style="position: absolute; top: 0; left: 0; width: 100%; height: 100%; border: 1px solid black; border-radius: 50%;"></div>
</div>
<div s21
9 (a) The output of a power supply is represented by:
$$
V = 9.0 \sin 20t
$$
where $V$ is the potential difference in volts and $t$ is the time in seconds.
Determine, for the output of the supply:
(i) the root-mean-square (r.m.s.) voltage, $V_{r.m.s.}$
$$
V_{r.m.s} = \underline{\hspace{5cm}} \text{V [1]}
$$
(ii) the period $T$ .
$$
T = \underline{\hspace{5cm}} \text{s [2]}
$$
(b) The variations with time $t$ of the output potential difference $V$ from two different power supplies are shown in Fig. 9.1 and Fig. 9.2.
<div style="display: flex; justify-content: space-between; width: 100%;">
<div style="text-align: center;">
<br>
Fig. 9.1
</div>
<div style="text-align: center;">
<br>
Fig. 9.2
</div>
</div>
The22 (c) (i) The power supply in (a) is connected to a transformer. The input power to the transformer is 80 W. The secondary coil is connected to a resistor. The r.m.s. voltage across the resistor is 120 V. The r.m.s. current in the secondary coil is 0.64 A. Calculate the efficiency of the transformer. efficiency = ………………………………………… [3] (ii) State one reason why the transformer is not 100% efficient. ……………………………………………………………………………………………………… [1] ……………………………………………………………………………………………………… [Total: 8] © UCLES 2020 9702/42/F/M/20
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10 (a) By reference to the photoelectric effect, explain what is meant by work function energy.
...............................................................................................................................
...............................................................................................................................
............................................................................................................................... [2]
(b) In an experiment, electromagnetic radiation of frequency $f$ is incident on a metal surface.
The results in Fig. 10.1 show the variation with frequency $f$ of the maximum kinetic energy $E_{MAX}$ of electrons emitted from the surface.
Fig. 10.1
(i) Determine the work function e24
(ii) The work function energy in eV for some metals is given in Table 10.1.
Table 10.1
<table>
<tr>
<th>metal</th>
<th>work function/eV</th>
</tr>
<tr>
<td>tungsten</td>
<td>4.49</td>
</tr>
<tr>
<td>magnesium</td>
<td>3.68</td>
</tr>
<tr>
<td>potassium</td>
<td>2.26</td>
</tr>
</table>
Determine the metal used in the experiment. Show your working.
............................................................................................................................ [1]
(c) The intensity of the electromagnetic radiation for one particular frequency in (b) is increased. State and explain the change, if any, in:
(i) the maximum kinetic energy of the emitted electrons
...............................................................25
11 Electrons are accelerated through a potential difference of 100kV. They are then incident on a metal target, they decelerate, and X-ray photons are emitted.
(a) Calculate the maximum possible frequency of the emitted X-ray photons.
frequency = ………………………………………… Hz [2]
(b) Explain why an aluminium filter may be placed in the X-ray beam when producing an X-ray image of a patient.
………………………………………………………………………………………………………………………………
………………………………………………………………………………………………………………………………
………………………………………………………………………………………………………………………………
………………………………………………………………………………………………………………………………
…………………………………………………………………………………………………………………………… [3]
(c) The linear attenuation (absorption) coefficients $\mu$ for X-rays in bone, blood and muscle are given in Table 11.1.
Table 11.1
<table>
<tr>
<th26 (ii) In an X-ray image, white regions show greater absorption of X-rays than dark regions. State and explain the difference between the X-ray image of bone compared to that of muscle. ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... 2 © UCLES 2020 9702/42/F/M/20
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12 (a) Explain what is meant by the *binding energy* of a nucleus.
...............................................................................................................................
...............................................................................................................................
............................................................................................................................... [2]
(b) The following nuclear reaction takes place:
$$
{}^{236}_{92}U + {}^1_0n \rightarrow {}^{144}_{55}Cs + {}^{90}_xRb + y_0^1n
$$
(i) Determine the values of $x$ and $y$ .
$x = \text{............................................................................................................}$
$y = \text{............................