CCC Yenching College

CCC Yenching College 2001-2002 Mock Examination S.7 Physics Paper II Suggested Answer

Section A: Multiple Choice 1 D 11 C 21 D 31 E 41 E 2 E 12 E 22 C 32 E 42 D 3 A 13 D 23 E 33 E 43 C 4 E 14 A 24 A 34 C 44 A 5 C 15 C 25 E 35 D 45 C

6 D 16 D 26 B 36 E 7 D 17 A 27 D 37 C 8 B 18 E 28 C 38 D 9 B 19 B 29 D 39 B 10 A 20 E 30 C 40 C

Section B 1. (a) (i) Law 1: Every body continues to be in a state of rest or to move with uniform velocity unless a resultant force acts on it. ½ Law 2: The rate of change of momentum of a body is proportional to the resultant force and occurs in the direction of the force. ½ Law 3: If body A exerts a force on body B, then body B exerts an equal but opposite force on body A. ½ (ii) Every particle of matter attracts every other particle with a force m m whose magnitude is given by F  G 1 2 . r 2 ½

S.7 Physics Mock Exam (01-02) / Paper II / Suggested Answer / P.1 (b) weight Mass Gravitational attraction by the Measure of inertia earth Depends on the strength of the Constant all over the world gravitational field and the mass Vector Scalar ½ @ Unit is N Unit is kg (c) (i) When the spacecraft and the man inside the spacecraft are moving around the earth, the gravitational force acting on them provides the centripetal force required. Since there is no normal 1 reaction force acting on the astronaut by the spacecraft, the astronaut feels weightlessness. 1 (ii) When the spaceship is at a distance of 100 times of the earth’s

g o radius, g ~ (where go = gravitational field at earth’s surface 100 1 = 10 N kg-1). Therefore g = 0.001 N kg -1. He will feel nearly 1 weightlessness. 1 (iii) Assume that air resistance is negligible. The man falls with an acceleration due to gravity (g = 10 N kg-1). Resultant force on the man is therefore equal to his own weight ( W = mg ). Since 1 there is no normal reaction force acting on the man, he fill feel weightlessness. 1 (d) The moment of inertia of a body depends on 1. the mass of the body 1 2. the way the mass is distributed 1 3. the axis of rotation 1 The mass in linear motion represents the factor determining the linear acceleration associated with a particular force. 1 The moment of inertia represents the factor determining the angular acceleration associated with a particular torque. 1 Both are factors representing the “inertia” of the system, which give resistance to the respective motion.

2. (a) (i) A frequency of 2 kHz means that the diaphragm is vibrating 2 103 times per second. 1 (ii) When the diaphragm is pushed forward, it compresses the surrounding air. The compressed air travels a short distance

S.7 Physics Mock Exam (01-02) / Paper II / Suggested Answer / P.2 before colliding with other air molecules and passing its momentum on and the process is repeated. In this way, compression wave travels through the air. 1 Likewise when the diaphragm is pulled back, the surrounding air molecules are given extra space, the expansion created moves away from the diaphragm. The process is repeated so that the net effect is the movement of an expansion wave away from the source. 1 Hence the combination of a compression wave and rarefaction leads to the propagation of a longitudinal progressive wave. 1 (b) (i) The length of the vertical line increases to a maximum and then 1 decreases to a spot. This is repeated as long as the microphone is 1 moving. The antinode of the stationary wave results in maximum length 1 of the vertical line trace whereas the node produces the dot trace. 1 (ii) As the microphone is shifted, the distance moved between two ½ successive antinodes is measured. This distance is equal to  / 2 ½ where is the wavelength of the stationary wave. The frequency, f, of the sound wave is read from the signal generator. ½ The velocity, c of the sound wave is calculated from c  f . ½ (c) (i)

1 Monochromatic light is reflected by the reflector G so that it falls normally on the air film formed between the convex lens and the glass plate. 1 Interference occurs between light reflected from the lower surface ABC of the lens and the upper surface DBE of the plate. 1 Since the thickness of the air wedge is difference at different point, alternate bright and dark rings can be observed. 1

S.7 Physics Mock Exam (01-02) / Paper II / Suggested Answer / P.3 (ii) Since the phase difference between the light is  , the total path  difference is 2t  where t is the thickness of the air wedge at 2 the point of interference. 1  Constructive interference occurs when 2t   m . 2 ½  1 Destructive interference occurs when 2t   (m  ) . 2 2 ½

3. (a) (i) Electromotive force is the electrical energy given to one coulomb of charges when they pass through the terminals of the supply. 1 Potential difference between two points is the amount of energy converted from electrical energy to other forms when one coulomb of charges passing through those points. 1 (ii) Drift velocity is the net displacement per second of the charge carriers in the presence of electric field. 1 Speed of electrical signal is the speed at which signal is transmitted, which is equal to the speed of light. 1 (b) In Figure 3.1, the voltmeter is connected only across R. So the ammeter measures the current through R and the current through the voltmeter. In practice, the resistance of voltmeter causes one 1 to read an overestimate of the current through R assuming zero current through the voltmeter. Hence the calculated resistance R = V/I will be too small. 1 In Figure 3.2, the voltmeter is connected across both the ammeter and R. The ammeter current is the same as the current through R, but the voltage drop across the ammeter is non-zero in practice. Thus the voltmeter gives an overestimated reading of 1 the voltage across R and the calculated resistance R will be large. 1 (c) (i) The slider connecting the point between resistor R and S to the uniform wire AB is used to find the balance point. At a balance 1 R resis tan ce _ of _ wire _ from _ A _ to _ C point, say C,  . S resis tan ce _ of _ wire _ from _ C _ to _ B 1 Since the wire is uniform, its resistance is proportional to the

S.7 Physics Mock Exam (01-02) / Paper II / Suggested Answer / P.4 R l 1 corresponding length.   1 S l2 l 1 Thus unknown resistance R  1  S at the balance point. l2 (ii) Having the balance point C near to the centre would eliminate errors inherent in having a balance point nearer to one end of the 1 wire. (iii) A resistance value which is either very low or very high would result in a balance point that is very near to one end of the 1

uniform wire. Hence if either l1 and l2 is small, the resistance corresponding to it will be very low comparable to the end 1 connection wires. The particular measurement would then not be accurate. 1

4. (a) (i) Faraday’s law of electromagnetic induction states that a changing magnetic flux through a circuit induces an e.m.f. whose magnitudes is proportional to the rate of change of flux- linkage. 2 (ii) Lenz’s law states that the direction of an induced e.m.f. is such that the resulting induced current tends to oppose the change in magnetic flux. 2 (b) (i) When the rod moves with velocity v perpendicular to the magnetic field, a magnetic force acts on the free electrons existing within the metal rod. This downward force pushes the electrons to one end of the rod. Hence one end of the rod 1 becomes negatively charged whereas the opposite end becomes positively charged. Thus an e.m.f. is induced between the ends 1 of the rod. (ii) Suppose an e.m.f. E is induced which causes a current I. For a steady current to exist, the rod must move with uniform velocity. Magnetic force acting to the left = External force acting to the right, F = BIl. ½ Work done by force F in time t  BIlvt . ½ This is equal to electrical energy produced which is IEt . ½ Thus IEt  BIlvt  E = blv ½

S.7 Physics Mock Exam (01-02) / Paper II / Suggested Answer / P.5 3

1 Place a metre rule at about the middle position of the long wire and perpendicular to it. ½ Adjust the rheostat R and a.c. source until a suitable alternating current is measured by the ammeter. ½ The search coil is put at a certain distance r from the wire and is carefully oriented until maximum voltage V in the coil is read by the CRO. 1 Alter the distance and the relation between V and r can be studied. 1 As V is proportional to the amplitude of the magnetic field B produced by the wire, V  B . The variation of B with r can be investigated quantitatively. 1

5. (a) Macroscopic definition of an ideal gas: If the pressure of a fixed mass of gas is inversely proportional to its volume at constant temperature, then it is an ideal gas. 1½ Microscopic definition of an ideal gas: A gas which satisfies the following assumptions is called an

S.7 Physics Mock Exam (01-02) / Paper II / Suggested Answer / P.6 ideal gas: 1. the intermolecular forces are negligible except during ½ collisions 2. the volume of the molecules can be neglected compared with ½ the volume occupied by the gas 3. the duration of a collision is negligible as compared with the ½ time between collisions 4. the collisions between molecules and with the walls are ½ perfectly elastic ½ 5. the molecules moving randomly within the container (b) At high density, the molecules become closer. The actual volume of the molecules b becomes more important compared with the measured volume occupied by the gas. 1 At high density, the molecules become closer. The weak intermolecular attractive forces between molecules become more significant. 1 (c)

Consider a molecule moving with speed c, and let u, v, w be its speed along x-, y- and z- axis respectively.

For motion along x-axis. Impulsive force on S = change in momentum / time 1

2mu mu 2 = 1  1 2l / u1 l

Total impulsive force on S, 2 2 2 mu1 mu2 mu N F =  ⋯ 1 l l l Total pressure on S = F/A m = (u 2  u 2 ⋯ u 2 ) l3 1 2 N 1

S.7 Physics Mock Exam (01-02) / Paper II / Suggested Answer / P.7 Let u 2 = mean square speed of the molecules along x-axis,

u 2  u 2 ⋯ u 2  Nu 2 1 2 N 1 Nm P  u 2 V

2 2 c Nmc Nm 2 Since u 2  v 2  w2  , then P  , PV  c 3 3V 3 1 (d) U  change in internal energy of the system. 1 Q = heat supplied to the gas 1 W = work done on the system 1 (e)

S.7 Physics Mock Exam (01-02) / Paper II / Suggested Answer / P.8