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 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.

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

Common Errors: Reaction force and Apparent weight

Examples: 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 Newton’s 2nd Law

Consider: Question 8 from 2009 exam.

Downwards as +ive Accel’n = 2.0 m/s2

mg

Reaction force from platform, N

Net Force = ma

mg – N = ma

N = mg – ma, Solve for 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: Electric Circuits

Examples: Diode circuits

Error: Don’t realise diode limits 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 Circuits

Examples: Diode circuits

Error: Don’t realise diode limits 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 + 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?

I1

I2

I3

V1

V2

Common Errors: Electric Circuits

Examples: Diode circuits

Error: 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 marks

Many 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.

I1

I2

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

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 four 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 – 2013

• Checkpoints (Cambridge) $29 per unit

Solutions

• Vicphysics: 1999 – 2013 (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 the exam and the internal assessment for males and females for 2013.

• Vicphysics Raw score cut offs for 1999 – 2013.

Resources

Practice Papers

• STAV

• IARTV

• NEAP

• TSSM

• A+ Publishing

• TSFX

• itute