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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. - PowerPoint PPT Presentation

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• VCE Physics:

• Common Exam Errors

• Common Errors: Connected BodiesExamples: 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 Qn 3.Common errors: assume a = g or loss of GPE of mass 2 = gain in KE of mass 1

• Common Errors: Connected BodiesMethod:Label all forces actingThe whole system and each mass of the system accelerates at the same rateApply Newtons 2nd Law: Net Force = Mass x Acceln applies to each mass in the system and to the whole system as well.Forces between masses in the system are examples of Newtons 3rd Law.

• Acceln = aAcceln= am2gTTm1gNNet 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 Qn: use value of acceln. 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 MotionExamples: Banked curves, crests in roadError: 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 MotionExamples: Banked curves, crests in roadError: Include centripetal force as another force, misunderstand reaction force.

Method: Better to only refer to centripetal acceleration and then apply Newtons 2nd Law.

Consider: Questions 5 & 6 from 2010 Exam.

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

• Common Errors: Reaction force and Apparent weightExamples: Platforms slowing down, landing on trampolineError: Confusing directions, and net force with actual forces.

Consider: Question 8 from 2009 exam.

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

• Common Errors: Reaction force and Apparent weightExamples: Platforms slowing down, landing on trampolineError: Confusing directions, and net force with actual forces.Method: Define a direction as positive, Determine sign of acceleration, Draw in acting forces, Apply Newtons 2nd Law

Consider: Question 8 from 2009 exam.

• Downwards as +ive Acceln = 2.0 m/s2mgReaction force from platform, NNet Force = mamg N = maN = mg ma, Solve for N

• Common Errors: CollisionsExamples: 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 marks56% got full marks35% got full marks

• Common Errors: CollisionsExamples: 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.

• Common Errors: GravitationExamples: Satellite motionError: 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: GravitationExamples: Satellite motionError: 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.

• Common Errors: Electric CircuitsExamples: Diode circuitsError: Dont realise diode restricts voltage.

Consider: Question 2 from 2010 Exam.

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

• Common Errors: Electric CircuitsExamples: Diode circuitsError: Dont 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.

• I1 = I2 + I3 , 6 V = V1 + V2From 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 mAAny follow up questions?I1I2I3V1V2

• Common Errors: Electric CircuitsExamples: Diode circuitsError: Confused about voltage and current in series and parallel circuits.

Consider: Question 4 from 2009 Exam.

• Average: 1.4 out of 3, 46% got zero, 41% got full marksMany said I = 12/60

• Common Errors: Electric CircuitsExamples: Diode circuitsError: 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.

• I1I2V1V2I1 = 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

• Common Errors: Electromagnetic InductionExamples: Induced EMF with changing FluxError: Dont 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 InductionExamples: Induced EMF with changing FluxError: Dont 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.

• Magnetic Flux through loopBeforeAfterChangeChange = Final Initial or Initial + Change = FinalInduced Magnetic FieldDirection of CurrentOpposes changeUse Hand Rule

Down the front: Q to P

• Common Errors: Path DifferenceExamples: Double Slit Interference of LightError: 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 DifferenceExamples: Double Slit Interference of LightError: 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.

• 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 nmTwo students are studying interference of light. They use a laser of wavelength 580 nm.

• Common Errors: Momentum and energy of photons and electronsExamples: 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 electronsExamples: 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.

• ElectronPhotonMomentumWavelengthMomentumVelocity, vKinetic EnergyEnergySpeed, cpl = h/plp = h/lpp = mvE = p2/2mE = hc/lE = pcKE = mv2For 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
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