9702_s20_qp_42
A paper of Physics, 9702
Questions:
12
Year:
2020
Paper:
4
Variant:
2
Login to start this paper & get access to powerful tools
1
Define gravitational potential at a point. An isolated solid sphere of radius r may be assumed to have its mass M concentrated at its centre. The magnitude of the gravitational potential at the surface of the sphere is φ. On , show the variation of the gravitational potential with distance d from the centre of the sphere for values of d from d = r to d = 4r. –1.0 –0.5 gravitational potential +0.5 r 2r 3r 4r +1.0 φ φ φ φ d The sphere in is a planet with radius r of 6.4 × 106 m and mass M of 6.0 × 1024 kg. The planet has no atmosphere. A rock of mass 3.4 × 103 kg moves directly towards the planet. Its distance from the centre of the planet changes from 4r to 3r. Calculate the change in gravitational potential energy of the rock. change = J Explain whether the rock’s speed increases, decreases or stays the same.
13. Gravitational fields  →  13.4. Gravitational potential
5. Work, energy and power  →  5.2. Gravitational potential energy and kinetic energy
Open
2
A square box of volume V contains N molecules of an ideal gas. Each molecule has mass m. Using the kinetic theory of ideal gases, it can be shown that, if all the molecules are moving with speed v at right angles to one face of the box, the pressure p exerted on the face of the box is given by the expression pV = Nmv 2. (equation 1) This expression leads to the formula p = 1 3ρ 〈c 2〉 (equation 2) for the pressure p of an ideal gas, where ρ is the density of the gas and 〈c 2〉 is the mean‑square speed of the molecules. Explain how each of the following terms in equation 2 is derived from equation 1: ρ : : 〈c 2〉: An ideal gas has volume, pressure and temperature as shown in . volume 6.0 × 10–3 m3 pressure 3.0 × 105 Pa temperature 17 °C The mass of the gas is 20.7 g. Calculate the mass of one molecule of the gas. mass = g
15. Ideal gases  →  15.3. Kinetic theory of gases
15. Ideal gases  →  15.2. Equation of state
Open
4
A dish is made from a section of a hollow glass sphere. The dish, fixed to a horizontal table, contains a small solid ball of mass 45 g, as shown in . C ball mass 45 g surface of dish x The horizontal displacement of the ball from the centre C of the dish is x. Initially, the ball is held at rest with distance x = 3.0 cm. The ball is then released. The variation with time t of the horizontal displacement x of the ball from point C is shown in . –4 –2 –3 –1 x / cm t / s The motion of the ball in the dish is simple harmonic with its acceleration a given by the expression a = –(g R) x where g is the acceleration of free fall and R is a constant that depends on the dimensions of the dish and the ball. Use to show that the angular frequency ω of oscillation of the ball in the dish is 2.9 rad s–1. Use the information in to: determine R R = m calculate the speed of the ball as it passes over the centre C of the dish. speed = m s–1 Some moisture collects on the surface of the dish so that the motion of the ball becomes lightly damped. On the axes of , draw a line to show the lightly damped motion of the ball for the first 5.0 s after the release of the ball.
17. Oscillations  →  17.1. Simple harmonic oscillations
2. Kinematics  →  2.1. Equations of motion
Open
6
Telephone signals may be transmitted either by means of an optic fibre or by means of a wire pair. State three advantages of the use of an optic fibre rather than a wire pair. 1. 2. 3. It is proposed to transmit a signal over a distance of 4.5 × 103 km by means of an optic fibre. The input signal has a power of 9.8 mW. The minimum signal that can be detected at the output has a power of 6.3 × 10–17 W. For this signal power, the signal‑to‑noise ratio is 21 dB. Calculate: the power of the background noise power = W the maximum attenuation per unit length of the optic fibre that allows for uninterrupted transmission of the signal. attenuation per unit length = dB km–1
8. Superposition  →  8.3. Interference
Open
7
A metal sphere of radius R is isolated in space. Point P is a distance x from the centre of the sphere, as illustrated in . x R P The variation with distance x of the electric field strength E due to the charge on the sphere is shown in . E / 105 V m–1 x / cm State what is meant by electric field strength. Use to determine the radius R of the sphere. Explain your working. R = cm Use to determine the charge Q on the sphere. Q = C An α‑particle is situated a distance 8.0 cm from the centre of the sphere. Calculate the acceleration of the α‑particle. acceleration = m s–2
18. Electric fields  →  18.3. Electric force between point charges
18. Electric fields  →  18.4. Electric field of a point charge
18. Electric fields  →  18.5. Electric potential
Open
8
An ideal operational amplifier (op‑amp) is connected to a load resistor. The op‑amp is assumed to have infinite bandwidth and zero output resistance. State: what is meant by infinite bandwidth the effect, if any, on the output voltage of increasing the load resistance. A student designs the circuit shown in in order to indicate changes in temperature of the thermistor T. – – + + 100 kΩ 150 kΩ 40 Ω 110 Ω 1.5 V Q P T V Explain why point P is known as a virtual earth. Calculate the potential at point Q. potential = V At a temperature of 13 °C, the resistance of the thermistor T is 230 kΩ. Show that the potential difference measured with the voltmeter is 0.88 V. The resistance of the thermistor T in decreases as its temperature rises. Explain the effect of this change in temperature on the potential difference measured with the voltmeter.
9. Electricity  →  9.3. Resistance and resistivity
10. D.C. circuits  →  10.3. Potential dividers
10. D.C. circuits  →  10.1. Practical circuits
Open
9
An electron is travelling at speed v in a straight line in a vacuum. It enters a uniform magnetic field of flux density 8.0 × 10–4 T. Initially, the electron is travelling at right angles to the magnetic field, as illustrated in . path of electron region of uniform magnetic field The path of the electron in the magnetic field is an arc of a circle of radius 6.4 cm. State and explain the direction of the magnetic field. Show that the speed v of the electron is 9.0 × 106 m s–1. A uniform electric field is now applied in the same region as the magnetic field. The electron passes undeviated through the region of the two fields, as illustrated in . path of electron region of uniform electric and magnetic fields On , mark with an arrow the direction of the uniform electric field. Use data from to calculate the magnitude of the electric field strength. field strength = N C–1 The electron in is now replaced by an α‑particle travelling at the same speed v along the same initial path as the electron. Describe and explain the shape of the path in the region of the magnetic and electric fields.
20. Magnetic fields  →  20.3. Force on a moving charge
18. Electric fields  →  18.2. Uniform electric fields
Open
10
State Faraday’s law of electromagnetic induction. A simple iron‑cored transformer is illustrated in . output laminated soft-iron core secondary coil primary coil input State one function of a transformer. A sinusoidal alternating current in the primary coil gives rise to a varying magnetic flux linking the secondary coil. Use Faraday’s law to explain why the output from the transformer is an electromotive force (e.m.f.) that is alternating. State why the soft‑iron core of the transformer is laminated.
20. Magnetic fields  →  20.5. Electromagnetic induction
Open
11
The uppermost energy bands in a solid are known as the valence band (VB), the forbidden band (FB) and the conduction band (CB). A copper wire is at room temperature. Use band theory to explain why the resistance of the copper wire increases as its temperature increases. The structure of a copper crystal is to be examined using electron diffraction. Electrons, having been accelerated from rest through a potential difference V, are incident on the crystal. The de Broglie wavelength λ of the electrons is 2.6 × 10–11 m. Calculate the accelerating potential difference V. V = V
9. Electricity  →  9.3. Resistance and resistivity
24. Medical physics  →  24.2. Production and use of X-rays
Open
12
State what is meant by the mass defect of a nucleus. Some masses are shown in Table 12.1. Table 12.1 mass / u proton 1 1p neutron 1 0n helium‑4 (4 2He) nucleus 1.007 276 1.008 665 4.001 506 Show that: the energy equivalence of 1.00 u is 934 MeV the binding energy per nucleon of a helium-4 nucleus is 7.09 MeV. Isotopes of hydrogen have binding energies per nucleon of less than 3 MeV. Suggest why a nucleus of helium‑4 does not spontaneously break down to become nuclei of hydrogen. To avoid the issue of disclosure of answer‑related information to candidates, all copyright acknowledgements are reproduced online in the Cambridge Assessment International Education Copyright Acknowledgements Booklet. This is produced for each series of examinations and is freely available to download at www.cambridgeinternational.org after the live examination series.
23. Nuclear physics  →  23.1. Mass defect and nuclear binding energy
Open