VCE Physics Common Exam Errors Revision Advice Exam

VCE Physics: Common Exam Errors Revision Advice Exam advice Resources

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. Consider: Questions 3 & 4 from 2010 Exam.

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

Common Errors: Connected Bodies Method: 1. Label all forces acting 2. The whole system and each mass of the system accelerates at the same rate 3. Apply Newton’s 2 nd Law: Net Force = Mass x Accel’n applies to each mass in the system and to the whole system as well. 4. Forces between masses in the system are examples of Newton’s 3 rd Law.

N T Accel’n= a T Accel’n = a m 1 g m 2 g Net force on m 1 equals T, but T is unknown, so use Net force on the system of two masses: Net force on (m 1 and m 2) = m 2 g, but Net F = Ma, so m 2 g = (m 1 + m 2) 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 m 1 = Gain in KE

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.

Average: 0. 9 out of 2 (45%), 51% got zero) Average: 1. 5 out of 3, 48% got zero

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 2 nd Law. Consider: Questions 5 & 6 from 2010 Exam.

N mg Vertically: mg = N Sin q, Horizontally: N Cos q = Net Force = ma = mv 2/r, so combining: Tan q = gr/v 2, now solve for angle.

Common Errors: Reaction force Examples: Platforms slowing down, landing on trampoline, going through a dip in the road Error: Confusing directions, and net force with actual forces. Consider: Question 3 from 2015 exam.

Average: 1 out of 2. 50% got zero marks. Common errors: Assume centripetal force is a force acting on the mass, rather than an expression for net force. Also ignore reaction force.

Common Errors: Reaction force Examples: Platforms slowing down, landing on trampoline, going through a dip. 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 2 nd Law

Force by track on car, N Net F = ma = mv 2/r Accel’n N – mg = mv 2/r N = mg + mv 2/r N = 2. 0 x 9. 8 + (2. 0 x 6. 02 / 4. 0) N = 19. 6 + 18 = 38 N Force by earth on car, mg

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.

36% got full marks 56% got full marks 35% got full marks

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.

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.

Average 0. 8 out of 2, 44% got zero, 26% got full marks. Common faults: Tangential force; wrong label; two arrows

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”.

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.

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.

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.

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.

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

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

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

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

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.

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

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.

Average: 0. 6 out of 3, 20%

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.

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

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

Revision Advice for Students Vicphysics The Students page has: • Several links to websites with advice of learning physics, • Advice of revising and preparing for exams, • Topic summaries.

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.

Revision Advice for Students In the two weeks before the exam: • Use the draft with past exam and trail papers, • Evaluate performance after each exam and revise draft of two page summary accordingly.

Exam Advice for Students • Use the 15 minute reading productively, • Attitude: Remember if you are finding the exam fairly hard, don’t panic, because the rest of the state is probably also finding it hard. The reverse also applies. • Read the Question Carefully: The exam will have many instances where you have to read a graph or interpret data. In many cases the values will need to be converted to SI units, e. g. cm m, k. N N, MPa Pa.

Exam Advice for Students • Highlight data and important information as you read the question. • Show working every time. • Written response: Answer in point form. • Don’t look for complexities in the question. Assume the simplest explanation.

Resources • • Past Exam Papers and Solutions Chief Assessor Reports Exam Statistics Practice Papers

Resources Past Exam Papers and Solutions Papers • VCAA: 2002 – 2016 • Checkpoints (Cambridge) $29 per unit Solutions • Vicphysics: 1999 – 2016 (for students with marking scheme)

Resources Chief Assessor Reports • VCAA General comments including: * Areas of concern, * Advice. For each question: * The percentage getting each score from 0 to max, * Specific comment, e. g. common errors

Resources Exam Statistics • VCAA Score ranges for each letter grade and grade distributions for both exams and the internal assessment for males and females for 2011. • Vicphysics Raw score cut offs for 1999 – 2016.

Resources Practice Papers • STAV • NEAP • TSSM • Physics Trial Test Team • Itute • QATs • Kilbaha • Access Education • Lisachem