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VCE Physics: Common Exam Errors Revision Advice Exam advice

VCE Physics: Common Exam Errors Revision Advice Exam advice. Common Exam Errors. Common Errors: Connected Bodies. Examples : Horse pulling a cart, Mass over edge pulls another along table. Error : Apply net force on system to each mass. Misuse internal forces.

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VCE Physics: Common Exam Errors Revision Advice Exam advice

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  1. VCE Physics: Common Exam Errors Revision Advice Exam advice

  2. Common Exam Errors

  3. Common Errors: Connected Bodies Examples: Horse pulling a cart, Mass over edge pulls another along table. Error: Apply net force on system to each mass. Misuse internal forces. Consider: Questions 3 & 4 from 2010 Exam.

  4. Average: 0.7 out of 2 (35%), Only 10% got 2 marks. Average: 1.1 out of 2 (55%), 51% got 2 marks. Consequential on Q’n 3. Common errors: assume a = g or loss of GPE of mass 2 = gain in KE of mass 1

  5. Common Errors: Connected Bodies Method: • Label all forces acting • The whole system and each mass of the system accelerates at the same rate • Apply Newton’s 2nd Law: Net Force = Mass x Accel’n applies to each mass in the system and to the whole system as well. • Forces between masses in the system are examples of Newton’s 3rd Law.

  6. N Accel’n= a T T Accel’n = a m1g m2g Net force on m1 equals T, but T is unknown, so use Net force on the system of two masses: Net force on (m1 and m2) = m2g, but Net F = Ma, so m2g = (m1 + m2) x a. Substituting, 0.10 x 10 = (0.1 + 0.4) x a, solve for a. Conseq Q’n: use value of accel’n. Two methods: i) Use equations of motion to find v, the calculate KE, or ii) Use Work Done by force on m1 = Gain in KE

  7. Common Errors: Circular Motion Examples: Banked curves, crests in road Error: Include centripetal force as another force, misunderstand reaction force. Consider: Questions 5 & 6 from 2010 Exam.

  8. Average: 0.9 out of 2 (45%), 51% got zero) Average: 1.5 out of 3, 48% got 3, 48% got zero

  9. Common Errors: Circular Motion Examples: Banked curves, crests in road Error: Include centripetal force as another force, misunderstand reaction force. Method: Better to only refer to centripetal acceleration and then apply Newton’s 2nd Law. Consider: Questions 5 & 6 from 2010 Exam.

  10. N mg Vertically: mg = N Sin q, Horizontally:N Cos q = Net Force = ma = mv2/r, so combining: Tan q = gr/v2, now solve for angle.

  11. Common Errors: Reaction force and Apparent weight Examples: Platforms slowing down, landing on trampoline Error: Confusing directions, and net force with actual forces. Consider: Question 8 from 2009 exam.

  12. Average: 1.4 out of 3, 43% full marks, 54% got zero.

  13. Common Errors: Reaction force and Apparent weight Examples: Platforms slowing down, landing on trampoline Error: Confusing directions, and net force with actual forces. Method: • Define a direction as positive, • Determine sign of acceleration, • Draw in acting forces, • Apply Newton’s 2nd Law Consider: Question 8 from 2009 exam.

  14. Net Force = ma mg – N = ma N = mg – ma, Solve for N Reaction force from platform, N Downwards as +ive Accel’n = 2.0 m/s2 mg

  15. Common Errors: Collisions Examples: Elastic, and inelastic collisions (both sticky and non-sticky) Error: Assume momentum temporarily goes into ‘storage’; assume all rebound collisions are elastic. Consider: Questions 15 - 17 from 2010 Exam.

  16. 36% got full marks 56% got full marks 35% got full marks

  17. Common Errors: Collisions Examples: Elastic, and inelastic collisions (both sticky and non-sticky) Error: Assume momentum temporarily goes into ‘storage’; assume all rebound collisions are elastic. Method: Include graphs on 2 page summary. Consider: Questions 15 - 17 from 2010 Exam.

  18. Common Errors: Gravitation Examples: Satellite motion Error: Confusion of net force with actual forces. Need for a tangential force. Consider: Question 13 from 2009 Exam.

  19. Average 0.8 out of 2, 44% got zero, 26% got full marks. Common faults: Tangential force; wrong label; two arrows

  20. Common Errors: Gravitation Examples: Satellite motion Error: Confusion of net force with actual forces. Need for a tangential force. Method: Emphasise: a force inwards is all that is necessary for circular motion - “Keys on a string”. Consider: Question 13 from 2009 Exam.

  21. Common Errors: Electric Circuits Examples: Diode circuits Error: Don’t realise diode restricts voltage. Consider: Question 2 from 2010 Exam.

  22. Average: 0.7 out of 2, 62% got zero, most assumed Total R = 700 ohm

  23. Common Errors: Electric Circuits Examples: Diode circuits Error: Don’t realise diode restricts voltage. Method: Do prac showing voltage across a LED is constant and independent of resistor values, while the LED is on. Draw current arrows and voltage drops, apply current and voltage rules.

  24. V1 I1 = I2 + I3 , 6 V = V1 + V2 From graph V2 = 1.0 V, so V1 = 5.0 V, So using V = IR on 500 ohm resistor, current = 5.0 / 500 = 0.01 A, I = 10 mA Any follow up questions? I2 I1 I3 V2

  25. Common Errors: Electric Circuits Examples: Diode circuits Error: Confused about voltage and current in series and parallel circuits. Consider: Question 4 from 2009 Exam.

  26. Average: 1.4 out of 3, 46% got zero, 41% got full marks Many said I = 12/60

  27. Common Errors: Electric Circuits Examples: Diode circuits Error: Confused about voltage and current in series and parallel circuits. Method: Do prac measuring current and voltages in complex circuits, e.g. students each design and assemble a circuit for a set of LEDs and a resistor and measure voltage values and current through battery, then swap circuits and calculate the values for the new circuit. Draw current arrows and voltage drops, then apply current and voltage rules.

  28. I2 I1 V1 V2 I1 = I2 + I2, 12 V = V1 + V2. From diode graph V2 = 3 x 3.0 V = 9.0 V, so V1 = 12 - 9 = 3.0 V. Using V = IR, current through R2 = 3.0 / 60 I = 0.050 A = 50mA

  29. Common Errors: Electromagnetic Induction Examples: Induced EMF with changing Flux Error: Don’t realise that only a changing magnetic flux induces an EMF. Consider: Questions 8, 9 and 11 from 2010 Exam.

  30. Average: 0.8 out of 2,51% got zero,some drew sine waves, many missed the point of different gradients. Average: 0.4 out of 1, both Faraday and Lenz were accepted.

  31. Average: 1.0 out of 2, 33% get zero, Many said the induced flux opposed the original flux rather than the change in flux. Others had right reason, but wrong direction.

  32. Common Errors: Electromagnetic Induction Examples: Induced EMF with changing Flux Error: Don’t realise that only a changing magnetic flux induces an EMF. Method: Demonstration as an POE, with students drawing graph of flux (with direction) and observing induced EMF. Consider: Questions 8, 9 and 11 from 2010 Exam.

  33. Time Flux Induced EMF 0 – 1 Increasing steadily Constant and but rapidly large 1 – 2 Constant Zero 2 – 4 Decreasing steadily Constant and but slowly smaller Two possible answers

  34. Magnetic Flux through loop Before After Change Induced Magnetic Field Direction of Current Use Hand Rule Down the front: Q to P Opposes change Change = Final – Initial or Initial + Change = Final

  35. Common Errors: Path Difference Examples: Double Slit Interference of Light Error: Assuming path difference is along the screen Consider: Question 4 from 2010 Exam.

  36. Two students are studying interference of light. They use a laser of wavelength 580 nm. Average: 1 out of 2 = 50%. 48% got zero.

  37. Common Errors: Path Difference Examples: Double Slit Interference of Light Error: Assuming path difference is along the screen Method: Draw the light paths. Determine the path difference at specific points in multiples of l/2. Avoid stock formulae. Consider: Question 4 from 2010 Exam.

  38. Two students are studying interference of light. They use a laser of wavelength 580 nm. Y is the next node after the antinode at X, so the path difference has increased by l/2. Path diff for Y = Path Diff for X + (580 / 2) = 1160 + 290 nm = 1450 nm

  39. Common Errors: Momentum and energy of photons and electrons Examples: Comparing diffraction patterns between electrons and X-rays. Error: Assume if momenta of both are the same, then their energies are also the same. Consider: Question 10 from 2010 Exam.

  40. Average: 0.6 out of 3, 20%

  41. Common Errors: Momentum and energy of photons and electrons Examples: Comparing diffraction patterns between electrons and X-rays. Error: Assume that if momenta of both are the same, then their energies are also the same. Method: Draw up concept map linking quantities with relationships. Consider: Question 10 from 2010 Exam.

  42. Electron Photon Momentum Wavelength Momentum p l = h/p l p = h/l p p = mv E = p2/2m E = hc/l E = pc Velocity, v Kinetic Energy Energy Speed, c KE = ½ mv2 For Electrons: KE has to be in Joules to determine its momentum and wavelength. For Photons: Energy can be in either eV or Joules to determine its wavelength.

  43. Steps: KE in eV to KE in Joules to electron momentum to electron wavelength. Wavelengths are the same, so then Wavelength to photon energy in eV

  44. Revision Advice for Students

  45. Revision Advice for Students • Prepare a one page summary as soon as you finish an Area of Study, • Use it with extra problems until the weeks before the exam, • Maintain a regular routine of doing exam type problems after finishing an Area of Study, • Two weeks before the exam prepare the first draft of the two page summary.

  46. Revision Advice for Students In the two weeks before the exam: • Use the draft with past exam and trail papers, • For June exam, one exam 90 min paper per day under exam conditions at home, for a fortnight is doable, • Evaluate performance after each exam and revise draft of two page summary accordingly.

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