Transcript Exam and Revision 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.
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 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.
N
T
Accel’n= a
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
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.
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.
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 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.
Net Force = ma
Reaction force
from platform, N
mg
mg – N = ma
Downwards as +ive N = mg – ma, Solve for N
Accel’n = 2.0 m/s2
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.
Consider: Questions 15 - 17 from 2010 Exam.
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”.
Consider: Question 13 from 2009 Exam.
Common Errors: Electric Circuits
Examples: Diode circuits
Error: Don’t 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 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.
V1
I2
I1 = I2 + I3 ,
6 V = V1 + V2
From graph V2 = 1.0 V, so V1 = 5.0 V,
I1
I3
So using V = IR on 500 ohm resistor,
current = 5.0 / 500 = 0.01 A,
V2
I = 10 mA
Any follow up questions?
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 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.
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
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.
Consider: Question 10 from 2010 Exam.
Electron
Momentum
Photon
Wavelength
l = h/p
p
p = mv
Velocity, v
E = p2/2m
Kinetic Energy
l
Momentum
p = h/l
E = hc/l
Energy
p
E = pc
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.
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
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,
• 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.
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.