Physics end of year exam advice 2014

Start preparing your notes now, by Craig Tilley

The Physics examination tests units 3 & 4. Topics are Motion in one and two dimensions and

Electronics and photonics (covered in the Herald Sun article 6th

May 2014), Electric Power and

Interactions of light and matter, then one of six Detailed Studies. Total is 150 marks in 150 minutes

so pace yourself and try to leave some time to check your answers.

You are allowed one folded A3 sheet or two taped A4 sheets of notes and one scientific calculator

in the exam. Start preparing your notes now. Use the VCAA formula sheet, or download one - it is

at the back of last year’s exam PDF. Have this handy when you start drafting your notes. VCAA

number the formulas, but they are grouped by topic and there is some overlap between topics.

Generally: formulas 1-11 go with Motion in one and two dimensions, 12-14 go with Electronics and

Photonics, 15-19 go with Electric Power, 20-28 go with Interactions of light and matter. Then the

Detailed Studies follow with 29-33 going with Synchrotron and its applications, 34 & 35 go with

Photonics, 36 is for Sound, 37-41 go with Einstein’s special relativity, 42-44 go with Materials and

their use in structures and formula 45 goes with Further Electronics. Formulas 46 to 51 may be used

by different topics.

The formula sheet only lists some of the formulas for each topic. To draft your notes, work through

some past VCAA papers or the commercial ones bought by your school, or the A+ Exam practice

book. Attempt a question and identify the topic: Motion in one and two dimensions, Electronics and

photonics etc. Try to get the answer using your existing knowledge, don’t look anything up yet. If

you can, that’s good news. Correct your answer from the solutions. If you can’t do the question, this

tells you something and it’s good news too. Learn from it. Consult the solution and work out what it

tells you. Add any formulas to your draft notes under the topic heading. Any concepts you didn’t

recall? Add them to your notes too. Continue with the next question. Don’t try to do a whole exam

in a set amount of time just yet, that’s for closer to exam time. Doing questions and adding to your

notes is one of the best ways to study Physics (and other subjects too)! So, keep going. Get more

past and trial papers. Revise your notes from time to time and delete anything you remember well to

make more space. It is a work in progress and might not be finished until right before the exam.

Now to complete my Unit 4 advice for this year.

Electric Power

To determine the magnetic field direction at a point, determine the field from each magnet

separately and add as vectors.

Voltage from a generator is given by Faraday’s law and current direction is given by Lenz’s law.

The direction of induced current has three steps:

1. What flux change is occurring?

2. What would oppose this (or restore the original)?

3. Hence what is the current direction?

DC motors and generators utilise a split ring commutator while AC utilise slip rings.

A shape of a voltage-time graph follows the negative gradient of the flux-time graph.

Transmission lines form series circuits with long wires that lose voltage and power. The voltage of

the power supply (e.g. 200 V) is lost along the sum of both lines and the output (e.g. 12 V along

each transmission line and 176 V at the load). Power values follow the same pattern. Current is the

same in each.

Interactions of light and matter

Use h = 4.14 1015

for electron volt energy (eV). Otherwise use h = 6.63 1034

Js. For

momentum h = 6.63 1034

Js & energy must be in joules (J).

Energy level diagrams: longest wavelength will have lowest energy and lowest frequency. Shortest

wavelength will have highest energy and highest frequency.

Photoelectric effect involves energy. Incident light has energy (hf), and if above the minimum for

the electrons (W), any excess energy becomes the electrons’ kinetic energy (KEmax).

So: hf = W + KEmax

And: hf = hfo + qVo where fo = threshold frequency & Vo = stopping voltage.

If light intensity increases, everything stays the same, simply more current flows.

If light frequency increases, light energy & KEmax increase, W remains the same.

If the metal changes, f stays the same, W changes & so KEmax changes.

If f < fo ; Elight < W so no photoelectrons are ejected.

In Young’s experiment the central max is bright as PD = 0. Other maximums are bright as PD = n.

Minima are dark as PD = (n ½ ) . The ‘n’ number is the line number from the central maximum

where n = 0. The band spacing (x) is determined from: x ( L) / d and x L / (f d).

Electron diffraction compares to X-ray diffraction if the electron’s de Broglie wavelength compares

to the X-ray wavelength. Electrons have NO frequency & do NOT travel at ‘c’. Wavelength and

momentum are linked for electrons but NOT wavelength and energy.

Craig Tilley has been a Physics examination assessor for over 15 years and is the author of the

A+ Physics Units 3 & 4 Exam Book and co-author of A+ Physics Notes.