Post on 07-Jan-2016
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VCE Physics:
Common Exam ErrorsRevision Advice
Exam adviceResources
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 acting2. The whole system and each mass of the
system accelerates at the same rate3. 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.
4. Forces between masses in the system are examples of Newton’s 3rd Law.
Accel’n = a
Accel’n= a
m2g
T
T
m1g
N
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
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 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 2nd Law.
Consider: Questions 5 & 6 from 2010 Exam.
Vertically: mg = N Sin , Horizontally: N Cos = Net Force = ma = mv2/r,
so combining: Tan = gr/v2, now solve for angle.
mg
N
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 2nd Law
Accel’n
Force by earth on car, mg
Force by track on car, N
Net F = ma = mv2/r
N – mg = mv2/r
N = mg + mv2/r
N = 2.0 x 9.8 + (2.0 x 6.02 / 4.0)
N = 19.6 + 18 = 38 N
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 FluxError: 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.
Time Flux Induced EMF
0 – 1 Increasing steadily Constant andbut rapidly large
1 – 2 Constant Zero
2 – 4 Decreasing steadily Constant and but slowly smaller
Two possible answers
Magnetic Flux through loop
Before After Change
Change = Final – Initial or Initial + Change = Final
Induced Magnetic Field Direction of Current
Opposes change Use Hand Rule
Down the front: Q to P
Common Errors: Path Difference
Examples: Double Slit Interference of Light
Error: Assuming path difference is along the screen
Consider: Question 4 from 2010 Exam.
Average: 1 out of 2 = 50%. 48% got zero.
Two students are studying interference of light. They use a laser of wavelength 580 nm.
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 /2. Avoid stock formulae.
Consider: Question 4 from 2010 Exam.
Y is the next node after the antinode at X, so the path difference has increased by/2.Path diff for Y = Path Diff for X + (580 / 2)
= 1160 + 290 nm = 1450 nm
Two students are studying interference of light. They use a laser of wavelength 580 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 Photon
Momentum Wavelength Momentum
Velocity, v Kinetic Energy Energy Speed, c
p = h/p p = h/ p
p = mv E = p2/2m E = hc/ E = pc
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.
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
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, kN 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