9702_s19_qp_42
A paper of Physics, 9702
Questions:
12
Year:
2019
Paper:
4
Variant:
2
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1
Two point masses are separated by a distance x in a vacuum. State an expression for the force F between the two masses M and m. State the name of any other symbol used. A small sphere S is attached to one end of a rod, as shown in . 8.0 cm thread rod small sphere S view from side (not to scale) The rod hangs from a vertical thread and is horizontal. The distance from the centre of sphere S to the thread is 8.0 cm. A large sphere L is placed near to sphere S, as shown in . 8.0 cm 6.0 cm 1.2 mm thread initial position of rod final position of rod small sphere S large sphere L θ view from above (not to scale) There is a force of attraction between spheres S and L, causing sphere S to move through a distance of 1.2 mm. The line joining the centres of S and L is normal to the rod. Show that the angle θ through which the rod rotates is 1.5 × 10–2 rad. The rotation of the rod causes the thread to twist. The torque T (in N m) required to twist the thread through an angle β (in rad) is given by T = 9.3 × 10–10 × β. Calculate the torque in the thread when sphere L is positioned as shown in . torque = N m The distance between the centres of spheres S and L is 6.0 cm. The mass of sphere S is 7.5 g and the mass of sphere L is 1.3 kg. By equating the torque in to the moment about the thread produced by gravitational attraction between the spheres, calculate a value for the gravitational constant. gravitational constant = N m2 kg–2 Suggest why the total force between the spheres may not be equal to the force calculated using Newton’s law of gravitation.
13. Gravitational fields  →  13.2. Gravitational force between point masses
3. Dynamics  →  3.1. Momentum and Newton’s laws of motion
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2
The first law of thermodynamics may be expressed in the form ΔU = q + w. State, for a system, what is meant by: 1. +q 2. +w. State what is represented by a negative value of ΔU. An ideal gas, sealed in a container, undergoes the cycle of changes shown in . 8.7 × 10–4 m3 1.6 × 105 Pa 110 K 8.7 × 10–4 m3 6.6 × 105 Pa 450 K 2.4 × 10–3 m3 1.6 × 105 Pa 300 K 1.0 2.0 3.0 4.0 5.0 6.0 7.0 1.00 0.75 1.25 1.50 1.75 2.00 2.25 2.50 B C A volume / 10–3 m3 pressure / 105 Pa At point A, the gas has volume 2.4 × 10–3 m3, pressure 1.6 × 105 Pa and temperature 300 K. The gas is compressed suddenly so that no thermal energy enters or leaves the gas during the compression. The amount of work done is 480 J so that, at point B, the gas has volume 8.7 × 10–4 m3, pressure 6.6 × 105 Pa and temperature 450 K. The gas is now cooled at constant volume so that, between points B and C, 1100 J of thermal energy is transferred. At point C, the gas has pressure 1.6 × 105 Pa and temperature 110 K. Finally, the gas is returned to point A. State and explain the total change in internal energy of the gas for one complete cycle ABCA. Calculate the external work done on the gas during the expansion from point C to point A. work done = J Complete for the changes from: 1. point A to point B 2. point B to point C 3. point C to point A. change +q / J +w / J ΔU / J A B B C C A
16. Thermodynamics  →  16.2. The first law of thermodynamics
15. Ideal gases  →  15.2. Equation of state
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3
A spring is hung vertically from a fixed point. A mass M is hung from the other end of the spring, as illustrated in . L mass M spring The mass is displaced downwards and then released. The subsequent motion of the mass is simple harmonic. The variation with time t of the length L of the spring is shown in . 1.0 0.2 0.4 0.6 0.8 t / s L / cm State: one time at which the mass is moving with maximum speed time = s one time at which the spring has maximum elastic potential energy. time = s Use data from to determine, for the motion of the mass: the angular frequency ω ω = rad s–1 the maximum speed maximum speed = m s–1 the magnitude of the maximum acceleration. maximum acceleration = m s–2 The mass M is now suspended from two springs, each identical to that in , as shown in . mass M Suggest and explain the change, if any, in the period of oscillation of the mass. A numerical answer is not required.
17. Oscillations  →  17.1. Simple harmonic oscillations
6. Deformation of solids  →  6.1. Stress and strain
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4
State what is meant by the specific acoustic impedance of a medium. A parallel beam of ultrasound of intensity I0 is incident on the boundary between two media A and B, as illustrated in . medium A specific acoustic impedance ZA incident intensity I0 medium B specific acoustic impedance ZB transmitted intensity IT The two media A and B have specific acoustic impedances ZA and ZB respectively. The intensity of the beam transmitted through the boundary is IT. State how the ratio intensity IT of transmitted beam intensity I0 of incident beam depends on the relative magnitudes of ZA and ZB. The linear absorption coefficient μ of medium B is 23 m–1. Calculate the thickness of medium B required to reduce the intensity of the ultrasound beam to 34% of its initial intensity in medium B. thickness = m
24. Medical physics  →  24.1. Production and use of ultrasound
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5
For a signal transmitted along an optic fibre, state what is meant by: attenuation noise. The initial section of the transmission line for a signal from a telephone exchange is illustrated in . 52 km exchange amplifier gain 115 dB At the exchange, the input signal to the transmission line has a power of 2.5 × 10–3 W. After the signal has travelled a distance of 52 km along the transmission line, the power of the signal is 7.8 × 10–16 W. The signal is then amplified. Calculate the attenuation per unit length, in dB km–1, in the transmission line. attenuation per unit length = dB km–1 The gain of the amplifier is 115 dB. Calculate the power of the signal at the output of the amplifier. power = W
7. Waves  →  7.5. Polarisation
8. Superposition  →  8.3. Interference
24. Medical physics  →  24.1. Production and use of ultrasound
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6
State what is meant by electric potential at a point. Two parallel metal plates A and B are held a distance d apart in a vacuum, as illustrated in . x d P plate A plate B +V0 0 V Plate A is earthed and plate B is at a potential of +V0. Point P is situated in the centre region between the plates at a distance x from plate B. The potential at point P is V. On , show the variation with x of the potential V for values of x from x = 0 to x = d. distance x potential V d +V0 Two isolated solid metal spheres M and N, each of radius R, are situated in a vacuum. Their centres are a distance D apart, as illustrated in . P D y R R sphere M charge +Q sphere N charge +Q Each sphere has charge +Q. Point P lies on the line joining the centres of the two spheres, and is a distance y from the centre of sphere M. On , show the variation with distance y of the electric potential at point P, for values of y from y = 0 to y = D. potential R D y (D – R) – +
18. Electric fields  →  18.5. Electric potential
18. Electric fields  →  18.3. Electric force between point charges
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7
Use band theory to explain why the resistance of an intrinsic semiconductor decreases as its temperature rises. The variation with temperature t of the resistance R of a thermistor is shown in . 1.0 1.5 2.0 2.5 3.0 3.5 t / °C R / kΩ The thermistor is connected into the circuit shown in . B A 9.00 V 12.0 kΩ R The battery has electromotive force (e.m.f.) 9.00 V and negligible internal resistance. When the temperature of the thermistor is 25 °C, the potential difference between the terminals A and B is 1.00 V. The temperature of the thermistor changes from 25 °C to 10 °C. Determine, to two significant figures, the change in potential difference between A and B. change = V The temperature of the thermistor in changes from 25 °C to 10 °C at a constant rate. State two reasons why the potential difference between A and B does not change at a constant rate. 1. 2.
9. Electricity  →  9.3. Resistance and resistivity
10. D.C. circuits  →  10.1. Practical circuits
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8
An electron is travelling in a vacuum at a speed of 3.4 × 107 m s–1. The electron enters a region of uniform magnetic field of flux density 3.2 mT, as illustrated in . 30° region of uniform magnetic flux density 3.2 mT electron speed 3.4 × 107 m s–1 The initial direction of the electron is at an angle of 30° to the direction of the magnetic field. When the electron enters the magnetic field, the component of its velocity vN normal to the direction of the magnetic field causes the electron to begin to follow a circular path. Calculate: vN vN = m s–1 the radius of this circular path. radius = m State the magnitude of the force, if any, on the electron in the magnetic field due to the component of its velocity along the direction of the field. Use information from and to describe the resultant path of the electron in the magnetic field.
20. Magnetic fields  →  20.3. Force on a moving charge
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9
Part of a circuit incorporating an operational amplifier (op-amp) is shown in . 4.5 V device +5 V –5 V – + A relay is connected to the output of the op-amp circuit so that a lamp may be switched on or off. Complete to show the relay connected into the circuit. State and explain whether the output of the op-amp is positive or negative for the lamp to be switched on. State the device in that could be used so that the circuit indicates a change in: the bending of a rod the level of daylight to switch on a street light.
10. D.C. circuits  →  10.1. Practical circuits
20. Magnetic fields  →  20.5. Electromagnetic induction
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10
State Faraday’s law of electromagnetic induction. An ideal transformer is illustrated in . soft-iron core secondary coil 450 turns primary coil 2700 turns load resistor E Explain why, when there is an alternating current in the primary coil, there is a current in the load resistor. The primary coil in has 2700 turns. The secondary coil has 450 turns. The e.m.f. E applied across the primary coil is given by the expression E = 220 sin(100πt ) where E is measured in volts and t is the time in seconds. Calculate the root-mean-square (r.m.s.) e.m.f. induced in the secondary coil. r.m.s. e.m.f. = V
20. Magnetic fields  →  20.5. Electromagnetic induction
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12
State what is meant by the binding energy of a nucleus. Some masses are shown in . mass / u proton (1 1p) neutron (1 0n) lanthanum-141 (141 57La) nucleus 1.007 1.009 140.911 Calculate the binding energy of a nucleus of lanthanum-141. binding energy = J The nuclide lanthanum-141 (141 57La) has a half-life of 3.9 hours. Initially, a radioactive source contains only lanthanum-141. The initial activity of the source is A0. Calculate the time for the activity of the lanthanum-141 to be reduced to 0.40A0. time = hours Suggest why the total activity of the radioactive source measured at the time calculated in may be greater than 0.40A0.
23. Nuclear physics  →  23.1. Mass defect and nuclear binding energy
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