科目：物理 Physics (9702)
年份：2020 冬季
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, 19: Capacitance, 20: Magnetic fields, 21: Alternating currents, 22: Quantum physics, 24: Medical physics
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Answer all the questions in the spaces provided.
1 (a) Define gravitational potential at a point.
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(b) The Earth may be considered to be a uniform sphere of radius $6.4 \times 10^6$ m with its mass of $6.0 \times 10^{24}$ kg concentrated at its centre.
A satellite of mass $2.4 \times 10^3$ kg is launched from the Equator. It is placed in an equatorial orbit at a height of $5.6 \times 10^6$ m above the5
(c) Before the satellite in (b) is launched, its speed at the Equator due to the Earth’s rotation is $470 \, \text{m} \, \text{s}^{-1}$ .
Suggest why the energy required to launch the satellite depends on whether the satellite, in its orbit, is travelling from west to east or from east to west.
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2 (a) State what is meant by the *internal energy* of a system.
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(b) The atoms of an ideal gas occupy a container of volume $2.30 \times 10^{-3} \, \text{m}^3$ at pressure $2.60 \times 10^5 \, \text{Pa}$ and temperature $180 \, \text{K}$ , as illustrated in Fig. 2.1.
<div style="text-align: center;">
<table>
<tr>
<td>2.30 × 10⁻³ m³<br>2.60 × 10⁵ Pa<br>180 K</td>
<td>→</td>
<td>→7 (c) During the change in (b), the thermal energy supplied to the gas is 980 J. (i) Determine the work done on the gas during this change. Explain your working. work done = ………………………………………… J [3] (ii) Determine the change $\Delta U$ in internal energy of the gas. $\Delta U =$ ………………………………………… J 1 © UCLES 2020 9702/42/O/N/20 [Turn over
8 3 A simple pendulum consists of a metal sphere suspended from a fixed point by means of a thread, as illustrated in Fig. 3.1.  Fig. 3.1 (not to scale) The sphere of mass 94.0 g is displaced to one side through a horizontal distance of 12.7 cm. The centre of gravity of the sphere rises vertically by 0.90 cm. The sphere is 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*. ............................................................................................................................... ............................................................................................................................... ....................................
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(iii) Use your answer in (ii) to show that the angular frequency $\omega$ of the oscillations of the pendulum is $3.3 \, \text{rad} \, \text{s}^{-1}$ .
[2]
(c) The period $T$ of oscillation of the pendulum is given by the expression
$$
T = 2\pi \sqrt{\left(\frac{L}{g}\right)}
$$
where $g$ is the acceleration of free fall and $L$ is the length of the pendulum.
Use data from (b) to determine $L$ .
$L = \underline{\hspace{5cm}} \, \text{m} \, [3]$
[Total: 10]
© UCLES 2020
9702/42/O/N/20
[Turn over10 4 (a) State two advantages of the transmission of data in digital, rather than analogue, form. 1. ............................................................................................................................... ............................................................................................................................ 2. ............................................................................................................................... ............................................................................................................................ [2] (b) An analogue signal is to be transmitted in digital form. The transmission system may be represented in block form as in Fig. 4.1. plaintext analogue signal → [analogue-to-di
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Table 4.1
<table>
<tr>
<th>time $t$ /ms</th>
<td>0</td>
<td>0.10</td>
<td>0.20</td>
<td>0.30</td>
<td>0.40</td>
<td>0.50</td>
</tr>
<tr>
<th>analogue signal /mV</th>
<td>0</td>
<td>5.7</td>
<td>6.2</td>
<td>……………</td>
<td>……………</td>
<td>……………</td>
</tr>
<tr>
<th>digital signal</th>
<td>0000</td>
<td>0101</td>
<td>0110</td>
<td>……………</td>
<td>……………</td>
<td>……………</td>
</tr>
</table>
(i) In Table 4.1, underline the least significant bit (LSB) in the digital signal for the time of 0.20 ms. [1]
(ii) Complete Table 4.1. [3]
(c) A single bit from the output of the digital-to-analogue converter corresponds to an output analogue signal of 1.0 mV.
Assume that the conversion and transmission do n12 5 (a) (i) State what is meant by a *field of force*. ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... [2] (ii) State one similarity and one difference between the electric field due to a point charge and the gravitational field due to a point mass. similarity: ............................................................................................................................... ..................................................................
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(ii) the electric field strength at point P, a distance $x = 0.45\,\text{m}$ from the centre of the sphere.
electric field strength = ………………………………………… $\text{V}\,\text{m}^{-1}$ [2]
(c) Use information from (b) to show, on the axes of Fig. 5.2, the variation of the electric field strength $E$ with distance $x$ from the centre of the sphere for values of $x$ from $x = 0$ to $x = 0.45\,\text{m}$ .
<div style="text-align: center;">
</div>
Fig. 5.2
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© UCLES 2020 9702/42/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 Electrons in a beam are travelling at high speed in a vacuum. The electrons are incident on a metal target, causing X-ray radiation to be emitted. The variation with wavelength $\lambda$ of the intensity $I$ of the emitted X-ray radiation is shown in Fig. 7.1. Fig. 7.1 Explain why: (a) there is a continuous distribution of wavelengths ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... .................................................................
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8 (a) An ideal operational amplifier (op-amp) is said to have infinite bandwidth and infinite slew rate.
State what is meant by:
(i) *infinite bandwidth*
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(ii) *infinite slew rate.*
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(b) An amplifier circuit incorporating an op-amp is shown in Fig. 8.1.
<div style="text-align: center;">
</div>
Fig. 8.1
The resistance of resistor R is to be fixed so that, for an input potential difference $V_{IN}$ of 0.40 V, the amplifier is on the point of saturation.
Determine:
(i) the gain of the amplifier circuit
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(ii) the resistance of resistor R.
$$
\text{resistance} = \underline{\hspace{5cm}} \Omega \ [2]
$$
[Total: 6]
9 (a) A small coil is placed close to one end of a solenoid connected to a power supply. The plane of the small coil is normal to the axis of the solenoid, as illustrated in Fig. 9.1.
Fig. 9.1
© UCLES 2020 9702/42/O/N/20 [Turn over18 The power supply causes the current $I$ in the solenoid to vary with time $t$ as shown in Fig. 9.2.  Fig. 9.2 (i) State Faraday’s law of electromagnetic induction. ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... [2] (ii) On the axes of Fig. 9.3, sketch a graph to show the variation with time $t$ of the electromotive force (e.m.f.) induced in the small coil.  Fig. 9.3 [4] © UCLES 2020 9702/42/O/N/20
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(b) The small coil in (a) is now replaced by a Hall probe.
The Hall probe is positioned so that the reading for the probe is a maximum.
The current $I$ in the solenoid varies again as shown in Fig. 9.2.
On the axes of Fig. 9.4, sketch a graph to show the variation with time $t$ of the reading $V_H$ of the probe.
<div style="text-align: center;">
</div>
Fig. 9.4
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10 (a) A long straight vertical wire A carries a current in an upward direction.
The wire passes through the centre of a horizontal card, as illustrated in Fig. 10.1.
<div style="text-align: center;">
</div>
Fig. 10.1
The card is viewed from above. The card is shown from above in Fig. 10.2.
<div style="text-align: center;">
</div>
Fig. 10.2
On Fig. 10.2, draw four lines to represent the magnetic field produced by the current-carrying wire.
[3]
© UCLES 2020 9702/42/O/N/2021 (b) Two wires A and B are now placed through a card. The two wires are parallel and carrying currents in the same direction, as illustrated in Fig. 10.3. Fig. 10.3 (i) Explain why a magnetic force is exerted on each wire. ............................................................................................................................... ............................................................................................................................... ............................................................................................................................... [2] (ii) State the directions of the forces. ............................................................................................................................... .........
22 11 (a) Electromagnetic radiation is incident on a metal surface. It is observed that there is a minimum frequency of electromagnetic radiation below which emission of electrons does not occur. This observation provides evidence for a particulate nature of electromagnetic radiation. State two other observations associated with photoelectric emission that provide evidence for a particulate nature of electromagnetic radiation. 1. ....................................................................................................................... .................................................................................................................... 2. .......................................................................................................................
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(i) Use Fig. 11.1 to determine the threshold frequency $f_0$ .
$$
f_0 = \underline{\hspace{5cm}} \text{ Hz [2]}
$$
(ii) Use the gradient of the line on Fig. 11.1 to determine a value for the Planck constant $h$ . Explain your working.
$$
h = \underline{\hspace{5cm}} \text{ Js [4]}
$$
(c) The electromagnetic radiation is now incident on a metal with a larger work function energy than the metal in (b).
On Fig. 11.1, sketch the variation with $\frac{1}{\lambda}$ of $E_{\text{MAX}}$ .
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