Physics end-of-year advice 2015
By Craig Tilley
Craig Tilley has been a Physics examination assessor for over 16 years and is the author of the
A+ Physics Units 3 & 4 Exam Book and co-author of the A+ Physics Notes books.
Article:
The Physics examination tests all of units 3 & 4 including Motion in one and two dimensions &
Electronics and photonics (covered in the May 2015 Herald Sun article), Electric power and
Interactions of light & matter, then one out of six detailed studies.
To prepare your sheet of notes that you can take into the exam, download the VCAA formula
sheet (usually part of the exam cover PDF.) Any formula that appears on the sheet won’t be
needed on your notes but some students like to include them anyway so their notes are more
complete. As you work through past VCAA and trial exams, identify the topic in a question. If
you don’t know how to answer a particular question, use the solution and add to your notes
under the topic heading. From more and more questions your notes will fill up with things you
didn’t already know. Doing Physics exam-style questions is one of the best ways to study
Physics. Draft and re-draft your notes to help you remember things.
Electric Power
Which rule to use?
The right hand slap rule and the right hand grip rule have different applications. RH GRIP gives
the magnetic field direction for a single wire or for a solenoid. For a wire, the thumb is the
current and the curled fingers show the magnetic field direction. For a solenoid or loop use the
curled fingers for the current direction and the thumb for the straight magnetic field inside the
solenoid. RH SLAP gives the force direction for a current-carrying wire. The thumb is the
current flow and match the stretched fingers with the magnetic field lines. The palm pushes in
the direction of the force.
What is the direction of the induced current?
There are three steps to Lenz’s Law:
1. What flux change is occurring? (The flux might be reducing).
2. What would oppose this or restore the original? (Increase the flux).
3. Hence what is the current direction? (Use the RH grip rule).
How is resistance calculated?
Remember, Electric Power is an electricity topic, so Ohm’s law applies. If asked for the
resistance, use Ohm’s law to calculate it.
Where do the forces act in a motor?
All sides of the coil experience a force but sometimes the front and back sides of the coil will be
parallel to the magnetic field and no force acts. The forces acting on the sides of the coil will be
in opposite directions and will create a torque or turning effect on the coil.
How does a DC motor maintain its rotation?
As the coil passes the vertical position the current in the coil changes direction and this enables
the motor to maintain rotation. If it didn’t do this, the coil would come to a stop. This is achieved
by using a split ring commutator.
What type of output does a generator give?
A generator works in the opposite way to a motor. As a generator spins, current is induced in the
coil. This current flow will change direction every half-turn, so it is AC. Slip rings are used to
harness the AC. If a DC output is required, the reversal must be cancelled out and a split ring
commutator achieves this.
What gets ‘lost’ along transmission lines?
Voltage and power are both lost along transmission lines. Output voltage at the other end of the
lines is input voltage minus the lost voltage. Output power is input power minus lost power.
Remember to use the resistance of both power lines.
Light and Matter
In a Young’s double slit experiment, how does the path difference effect the interference pattern?
The path difference is just the difference in the distances from each slit to the pattern. Count the
bands from the centre. For example, the second bright band, use p.d. = nλ and n = 2. For the
third dark band use p.d. = (n − ½) λ and n = 3.
In the photoelectric effect, how is the kinetic energy of the photoelectrons determined?
The photoelectric effect is an energy experiment. The electrons in the metal need a minimum
amount of energy to be ejected. From the energy of the incident light (E = h f), some is used to
eject the electrons (equal to the work function) and any energy left over becomes the kinetic
energy of the photoelectrons. Different colours of light have different energies but different
intensities of the same colour do not. Different metals have different minimum energies need to
eject the electrons.
How is the shape of an electron diffraction pattern determined?
The diffraction pattern is determined by wavelength and is similar to that from X-rays only if the
de Broglie wavelength of the electron is similar to the wavelength of the X-rays. Note that
electrons do NOT have a frequency NOR do they travel at the same speed as the X-rays.