1. 2008 Science (Physics) Section A Questions, Answers and Comments

2. If you are using topical TYS (2002-2010 - version) Qn in 2008 Paper Topical TYS (Section A) 1Topic 4(6) – Mass, weight density (Q4) 2Topic 13(5) - Electromagnetic Spectrum (Q3) 3Topic 7(11) – Energy, Work, Power (Q6) 4Topic 15(4) – Static Electricity (Q1) 5Topic 12C(5) – Light - Lens (Q2) 6Topic 10(7) – Thermal Properties of Matter (Q2) 7Topic 1(6) – Measurements – Vectors (Q1) 8Topic 18(8) – Practical Electricity (Q4) 9Topic 17(13) – DC Circuit (Q4)

3. What did the examiner say?

4. Mass, Weight and Density Easy chapter to score Must know definition of mass, weight, gravitational field and gravitational field strength Must be able to compare mass and weight Formula : w = mg (note: m is in kg while g, the gravitational field strength depends on the location) Formula : density = (note: be careful with formula of volume and mass)

5. ANS: Mass = density x volume = 5.5 x 20 = 110 g ANS: mass = 110 g (mass does not change with location)

6. ANS: w = mg = x 1.6 = 0.176 N Mass must be in kg.

7. Electromagnetic Spectrum EM waves – are all transverse waves; – travel with speed of 3 x 10 8 m/s in vacuum Application

8. (a) Infrared (b) Microwave (satellite TV is not the same as TV communication which uses radio wave. Radio wave is easily absorbed and cannot make it to the satellite) (c) Gamma rays

9. Kinematics Define speed and velocity Formula (if object is decelerating, then v will be smaller than u and a will be negative) Average speed = total distance travelled total time taken (total time would also include the rest time in the journey)

10. Kinematics acceleration of free-fall for a body near the earth is 10 m/s 2. (if air resistance is ignored, all objects will fall at the same acceleration of 10 m/s 2 which means that their velocity will increase by 10 m/s every second)

11. Kinematics Speed-Time Graph Area under speed – time graph = distance travelled by object

12. Kinematics

14. KE = ½ mv 2 = ½ (300) (40) 2 = 240 000 J *must calculate acceleration first before force can be calculated (since F = ma) a = v - u t = (0 – 40) / 8 = - 5 m/s 2 F = ma = 300 x 5 = 1500 N

16. The dust particles will gain negative charge from the metal grid and get attracted to the positive metal plate. This is because unlike charges attract.

17. What else you need to know How to draw electric field of a point charge Define electric field

18. Focal length = 1.8 cm – 1.9 cm

19. refractive index of glass = speed of light in air speed of light in glass 1.5= 3 x 10 8 / v v = 3 x 10 8 / 1.5 = 2 x 10 8 m/s

21. This is because when the solid in the ice pack melts, it will absorb heat from the surrounding food and cools it down. For solid that does not melt, less heat is absorbed from the food.

22. What else you need to know? Which processes below absorb energy and which one releases energy to the environment? Boiling melting evaporation freezing condensation

23. Vector quantity has direction but scalar quantity does not have direction.

24. Magnitude = 5 N 5 N 3 N 4 N 5 N 3 N 4 N

25. What else must you take note of for vector questions? Bring long ruler and protractor for Physics paper Use their diagram if it is provided in the qn (e.g. 2009 QA3, 2010 QB14) The angle have been measured for you, just continue on the lines drawn. Your line can be longer or shorter than theirs.

27. When current flows in the solenoid, it gets magnetised. When the current is larger than its rated value, the magnetic field is strong enough to attract the armature. The springy metal will break contact with the iron armature. The circuit remains open until the reset button is pressed to push the springy metal to touch the armature again. What’s wrong with this answer? When current flows in the solenoid, it becomes magnetised and attracts the armature…….

28. This will ensure that when the circuit breaker blows, the appliance will be isolated from high potential / will not be ‘live’ / will not be connected to high potential and that will protect the user from electrocution.

29. P = V I 24 = 12 x I I = 24/12 = 2 A

30. V = R I 12 = 3 x I I = 12/3 = 4 A R E = V I = 12 (2 + 4 ) = 2 Ω 4 A 2 A R (bulb) = V/I = 12/2 = 6 Ω 1/R E = 1/R 1 + 1/R 2 = 1/6 + 1/3 = 3/6 R E = (3/6) -1 = 2 Ω

31. Go on to try other year’s paper!