PHY09SOLUTION

估計curve:
overal:
A: 82%
B: 72%
C: 60%
D: 50%
E: 38%

Paper II

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我估應該係咁

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Paper I (SECTION B在#14)

(1)(a) Less pollution is produced due to less fuels burnt.  (1)
(b) By Newton's second law, the acceleration is directly proportional to pushing force but inversely to the mass of the object.  (1)
   i.e. a = F/m
   As the mass of the hybrid car is smaller than the standard one  (1)
   less pushing force is needed for hybrid car to achieve the similar acceleration.  (1)
(c) For hybrid car, kinetic energy of the car is converted into chemical energy in the battery. (1)
For standard car, kinetic energy of the car is converted into internal energy of the tyres and the floor and sound energy. (1)

(2)(a) Kinetic energy gained = Gravitational energy lost = mgh (1)
                                 = 60 x 10 x (15-11.5)
                                 = 6900J
(b) Work done by cushion = Kinetic energy lost + potential energy lost (1)
      Fs = 6900 + mgh (1)
      F = (6900 + 60 x 10 x 3) /(3.5-0.5)
         = 2900N
(c) When thicker cushion is used and still he is 0.5m above the ground when stopped,
      total displacement the force is applying upon is longer. (1)
      At the same time, the distance from the top of building to the cushion is shorter,
      implying the kinetic energy at the surface of cushion is smaller. (1)
      By F = Kinetic energy / stopping distance, the force acting on the man must be smaller
      which makes him safer. (1)
      Thus, the thick cushion is better for rescuing the man.

(3)(a) At R, all ice cubes melt. (1)
(b)(i) Even though there is heat loss to the surrounding,
         the temperature of the water is maintained at 0 degree by the ice such that there is no temperature difference between ice and water.  (1)
         As from Q to R, the ice cubes are melting and absorbing the latent heat of fushion from the water.
         No energy is used for gaining the temperature of the ice cubes.  (1)
      (ii) From S to T, the temperature of the water is the same as the surrounding, and no heating occurs. (1)

(4)(a)(i) Energy needed = mL + mc dT (1)
                              = (20/1000) x 2.26 x 10^6 + 20/1000 x 4200 x (100-28) (1)
                              = 51248J (1)
      (ii) Energy lost by plate = energy gained by steam
         C dT = 51248
         dT = 51248/637 = 80.5 (1)
         Final T = 200 - 80.5 = 119.5 C
(b) Same energy is needed for producing additional 20g steam.
      Temperature decrease of iron plate should be 80.5. (1)
      If so, the final temperature is 39, which is lower than the temperature of steam. (1)
      Assume that the temperature of iron plate is not gained by the current immediately,
      it is impossible to achieve the state that the steam is produced at which the temperature of the iron plate is above 100. (1)

(5)(a) The lens is concave  (1)
      as it diverges the light ray.  (1)
(b)(i)冇
         (1 mark for ray x backward, 1 mark for ray passing centre, 1 mark for image)
      (ii) 0.6
(c) 冇(1)
(d) 冇(1)
(6) First, cut the thin nichrome wire and thick nichrome wire so that they have the same length. (1)
   Then, clip, by two crocodile clips,  both end of one of the wires.  (1)
   Read the ammeter reading and find the current passing through the circuit. (1)
   Repeat to another wire.
   Higher reading implies lower resistance of the wire and relationship can be found. (1)

* communication = 1 mark

(7)(a) Half life undergo: 15.2/3.4 = 4 times  (1)
      Mass = 1 x 10^-5 x (1/2)^4
            = 6.25 x 10^-7 g
(b) Alpha radiation
(c) Pb => Bi + e- (2)
(d) Mass change: 222-206 = 16
      As mass change is only due to alpha radiation (decrease by 4 each time) (1)
      Total alpha particle = 16/4 = 4 (1)
      Charge decrease due to 4 alpha radiation = 4x2 = 8 (1)
      As beta radiation increase charge by 1,
      Total beta particle = 8 - (86-82) = 4 (1)

(8)(a) Out of paper (1)
(b)(i)冇
   (ii) Fx is greater than Fy. (1)
         The magnetic field from wire Y is stronger than that from wire X. (1)

   (iii) Repelsion still occurs.  (1)
         Even if the direction of current is reversed continuously by a.c., the direction of the magnetic force is reversed too.
         By Fleming's left hand rule, the direction of force remains unchanged.  (1)

(9)(a)(i) (2 marks for points, 1 mark for scale, 1 mark for stragiht line)
(ii) Slope = (2.5281-0.6241)/(1.6-0.4) = 1.59 (1)
(iii) s = ut + 1/2 at^2 where u=0
      1/a = 1/2 (t^2/s) = 0.793 (1)
      a = 1.26 ms^-2 (1)
(b) 冇圖
      (1 mark for naming, 1 mark for direction)
(c) Resultant of weight and reaction force = mgsin25 = 0.752
      0.752 - friction = ma = 0.178 x 1.26 (1)
      friction = 0.528N (1)

(10)(a)(i) 0.09/2.25 = 0.04m (1)
      (ii) Number of cycle of oscillation from t = 3 to 7s : 5 times  (1)
            Frequency = 5/4 = 1.25Hz (1)
      (iii) Wave speed = f人 = 1.25 x 0.04 = 0.05 (1)
            It takes 3 second to start motion.  (1)
            Distance = vt = 0.05 x 3
                        = 0.15m (1)
   (b)(i) Q (1)
      (ii) S (1)
   (c)冇圖
      (1 mark for shape, 1 mark for particle S)

(11)(a) Ultrasound is the sound wave with frequency high than 20kHz (1)
   (b) Time for arrival of tsunami: (1800x1000)/250 = 7200s = 2 hours (1)
         Time for signal from X to Warning Centre:  6000/1500 + (36000+40000)x1000/(3x10^8) = 4.25 seconds (1)
         2 hours - 4.25 seconds > 1 hour
         It can receive the signal before 1 hour.  (1)
   (c) Sound cannot be transmitted in vacuum. (1)
   (d) Radio wave is more appropriate. (1)
         It is much faster than ultrasound / It can be diffracted in greater extent in case of blockage of mountains. (1)

(12)(a) Turn ratio = Voltage ratio
         turns = 500 x (100/10)
               = 5000 turns  (1)
   (b)(i) Voltage = IR (1)
                     = 0.1 x 80
                     = 8V (1)
      (ii) Voltage = across MN - MP - NQ
                     = 100 - 8 - 8
                     = 84V (1)
      (iii) Power supplied = VI = 100 x 0.1 = 10W (1)
            Final power = 10 x 84/100
                           = 8.4W    (1)
(c) Reduce the resistence of the long transmission cable. (1)
      Increase the number of turns of secondary coils in transformer X.    (1)
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