2015 Examination: Chief Assessor’s Report.

1. 2015 Examination: Chief Assessor’s Report. Andrew R. Hansen Ringwood Secondary College

2. Overall issues / recommendations • Use black or blue pen as stipulated in instructions. (Q15) • Show working for questions worth more than 1 mark, as per instructions. • Do not rely on prepared statements from A3 sheet. • Check for ridiculous answers.

3. Areas of concern • the vector nature of momentum • centripetal force • energy conversion and conservation in springs • more complex projectile motion • apparent weightlessness • explaining direction of induced current using Lenz’s law • operation of transformers and how they work in a power transmission system • understanding of series and parallel circuits • explaining aspects of the photoelectric effect • applying the concept of path difference in interference patterns • explaining electron and X-ray diffraction patterns • electron energy level diagrams and associated emissions and absorptions • how the wave nature of matter can explain the electron energy levels

4. Question 1a: Correct response

5. Question 1a: Common error Students must understand the vector nature of momentum.

6. Question 3b: Correct response

7. Question 3b: Common error Students must understand forces in vertical circular motion.

8. Question 5b: Correct response

9. Question 5b: Common error Common errors involve breaking the flight up into multiple phases (launch to top, top to init height, final drop) which led to errors. Difficult for the assessors to follow.

10. Question 6c: Correct response

11. Question 6c: Common error Students must understand that there are three energy’s that need to be accounted for.

12. Question 6d: Correct response At x=0 there is no spring force and a=g (-10). At the mid point the spring force = the gravitational force and a=0. At the bottom the spring provides an upwards force giving a=10.

13. Question 6d: Common error While many students were able to identify C, they were unable to explain why in a coherent way. Those that selected responses other than C gave supporting arguments that were not realistic. Springs make up 20% of the marks in motion and students need a more thorough understanding of the underlying Physics.

14. Question 7c: Correct response Yes. The astronauts experience apparent weightlessness as they are still in a gravitational field, and therefore have weight, but they experience no normal reaction force.

15. Question 7c: Common error A number of students said “no they would not feel weightless” and then went on to explain that they would feel “apparently weightless”. There were also a number of students who believe that weightlessness (of any kind) can only exist in a region without gravity.

16. Question 8a: Correct response

17. Question 8a: Common error There were a number of poorly designed circuits, some suggesting that students had never studied electronics at all. The most problematic was the “short circuiting” by trying to create a circuit, as shown.

18. Question 10b: Correct response

19. Question 10b: Correct response

20. Question 10b: Common error R’s are mixed up. Students need to know which resistor is which in the voltage divider equation and how to use this formula.

21. Question 11b: Correct response The amplifier is an inverting amplifier and the waveform demonstrates clipping which is where the output of the amplifier exceeds the maximum supply voltage. (or similar)

22. Question 11b: Common error The most common error was to focus on the clipping and fail to comment on the inversion.

23. Question 12c: Correct response

24. Question 12c: Common errors • The most common errors were: • omission of n. • failure to convert cm to m • incorrect value for t

25. Question 12d: Correct response • Replace the commutator with slip rings. • The slip rings maintain a constant connection with the loop. • As the loop rotates an AC current will be generated in the loop which is then transmitted to the oscilloscope.

26. Question 12d: Common errors The most common error was to rely on stock answers from the A3 sheet regarding the difference between slip rings and a split ring commutator. Stock descriptions did not respond to the question being asked.

27. Question 13a & b: Correct response

28. Question 13a & b: Common errors Over half the responses scored no marks. Most diagrams were nonsensical and showed that students had a poor understanding of electromagnetic induction.

29. Question 13c: Correct response • The initial flux is down and increasing. • Lenz’s Law states that the induced current will give rise to a change in flux that opposes the change that induced it. • The induced flux will be up and increasing. • The right hand grip rule shows a current from y-x in the loop will give an upwards flux. • Therefore the current will flow from x to y through the voltmeter.

30. Question 13c: Common errors Marks awarded for this question varied. Most students were able to identify some of the required points but most were unable to provide a thorough response. Students should practice responding to these high scoring questions.

31. Question 16f: Correct response • A real situation would be a power station supplying power to a town. • For a constant power delivery, transformers can be used to increase the transmission voltage and decrease the transmission current. • Since power loss = I2R, reducing the current will significantly reduce the power loss.

32. Question 16f: Common errors Marks awarded for this question varied. Most students were able to identify some of the required points but most were unable to provide a thorough response. The most commonly omitted point was the need to maintain the delivered power while varying the supply parameters. Students should practice responding to these high scoring questions.

33. Question 18d: Correct response • Same gradient. • Intercept at 7.5

34. Question 18d: Common errors The most common errors were the wrong gradient or an intercept of 10.

35. Question 20b: Correct response • Electrons diffract when they interact with things like crystals or narrow slits. • Diffraction is a wave phenomenon.

36. Question 20b: Common errors Many students refered to Young’s Double Slit experiment. Young experimented with light not electrons so the argument is not valid.

37. Question 21a: Correct response • Electrons exhibit a wave behavior. • Allowed orbits are where the circumference is a whole multiple of the electron wavelength and a standing wave can be formed. • Different orbits have different whole multiples of the electron wavelength.

38. Question 21a: Common errors Students had trouble linking the concept of a standing wave to the integer multiple of the electron wavelength. Students seem to think that electrons follow a sinusoidal path around the nucleus.

39. Question 21b: Correct response

40. Question 21b: Common errors Students who tried to draw a sinusoidal pattern on a strip of paper had trouble demonstrating the standing wave nature.

41. 2015 Examination: Chief Assessor’s Report. Andrew R. Hansen Ringwood Secondary College