Sound And Electric Power Among Problem Areas
The Age
Sunday May 21, 1995
BEFORE I get into specific detail regarding physics content, I will make some general comments that you should take note of when answering questions from the test.
The majority of the 45 marks should be relatively straightforward; however, you should expect some questions which may appear different from anything you have seen in your classwork or from trial tests that you should have attempted in your practice for this important CAT.
Some questions are of the multiple-choice type and require you to write the letter corresponding to the correct answer. This should be done clearly and any later changes should be done by crossing out the initial choice, rather than writing over.
Other questions require you to simply write the result of a calculation and this should be done with attention to the correct number of significant figures, with too few or too many significant figures possibly being penalised. Other questions will require you to provide a written explanation and you will need to think carefully about the structure of such written explanations.
The quality of this response and the detail needed, should be related to the marks available. For example, an answer to a two-mark question will require you to make at least two significant points about the physics concepts related to the question. When preparing such answers, take time to plan the answer with due regard to the key points that you intend to make. Other questions require you to provide a diagram and such diagrams should be neatly drawn or sketched and with clear annotations to highlight key points.
The quality of sketches in previous years was quite unsatisfactory and the 1995 physics cohort is advised to take make greater care in this. Finally, you are able to bring in an A4 sheet, written on both sides, and many candidates have tended to provide far too much detail, thus making it difficult to locate key information. In the preparation of your sheet for 1995, pay particular attention to careful setting out and to the practice of highlighting the key points and avoiding unnecessary detail.
Now for some specific advice about difficult concepts from previous years. The area of sound has traditionally provided some difficult questions, particularly the content areas of standing waves, sound intensity and questions related to the modelling of a physical situation. For standing waves on a string, or an air column, be aware of the difference between an instantaneous snapshot of the motion and the overall envelope within which the motion occurs over an extended period of time.
Traditionally, students have had difficulty in the calculation of dB values and you should revise the method of calculating dB as the logarithm of the relative sound intensity ratio. You should also be familiar with some common-ratio equivalents, such as doubling the sound intensity results in a three-dB increase and that increasing sound intensity by a factor of 10 results in an increase of 10 dB.
The concept of modelling is more difficult to address and you should be aware of the various string and air column modes for the fundamental and overtones of the various modes. An idea of some common values for the length of such physical quantities as the ear canal, the length of a musical instrument, the size of a loud speaker, needs to be incorporated in order to maximise your performance on a modelling type question.
The second area of the test covers the topic of electric power and there are some common areas of misunderstanding that you should be aware of. These areas of misunderstanding are regularly addressed in examiners' reports and in advice to candidates and yet there has not been an improvement shown in these areas over recent years.
When discussing the operation of electric motors, the concept of torque would normally be addressed. Torque should be defined in terms of the size of the force, multiplied by the length of the lever arm, and candidates regularly forget to do so. To maximise the torque of an electric motor requires either the force or the lever arm to be increased.
At this stage, you need to further apply your knowledge of the force on a current carrying wire via the formula BIl and thus the torque is related to the magnetic field strength, the current in the rotor coil, the length of the rotor coil and the size of the lever arm. Avoid simply quoting formulas without appropriate explanations as to the meaning of the quantities in the formula.
Again, students are advised to formulate answers in terms of basic physics quantities rather than falling back to blind use of a formula.
In the past, many candidates have related torque in motors to the flux change as the coil rotates, and this is clearly incorrect. Many candidates have been confused about the difference between electric motors and electric generators, and flux change should only be addressed in questions relating to electric generators.
As mentioned above, when answering questions relating to electric power generation, the concept of magnetic flux change and induced EMF is critical. Such questions may require a calculation of the magnitude of the induced EMF, or alternatively, may require a written explanation of the physics behind the operation of a generator.
Make sure that you are able to perform calculations using Lenz's and Faraday's laws and that any written explanations clearly highlight the method by which the change in magnetic flux is achieved. Of importance in the rotation of the generator coil, is to remember that any induced currents flowing in the coil will result in forces which oppose the motion and you should be able to relate this to the laws for conservation of energy.
Finally, generators using a split ring commutator result in a varying DC output and those with slip rings result in an AC output, with the size of the output EMF being related to the rate of rotation, number of coils and change in magnetic flux.
The operation of transformers also causes problems and the key principle to remember here is that the primary and the secondary coils are linked solely by changes in magnetic flux and that the iron core of the transformer provides for efficient linking of this magnetic flux change. You should understand the method of transmission from the generating station to city consumers and should be able to explain the physics behind transmission at high voltage and therefore low line currents as a method for the reduction of energy loss in the transmission wires.
The third area for this Test involves electronic systems.
Generally, this area has been well done in previous years; however, last year the question involving non-ohmic devices was poorly done.
This can be partly attributed to the use of Ohm's law itself; that is, V=IR can always be used in the calculation of resistance values for ohmic and non-ohmic devices. However, for non-ohmic devices the resistance values are a function of both voltage and current and students often have difficulty in understanding the implications of changes in resistance. For circuits involving non-ohmic devices, the voltages and currents have to be analysed according to the series or parallel components within the system, and hence you should gain practice at such problems prior to the test CAT.
The charge and discharge of capacitors have also proved difficult in previous years. You should know how to calculate the time-constant for a resistor-capacitor series circuit and have an awareness of what the implications of these time-constant values have in the operation of the circuit. Furthermore, the use of capacitors as a smoothing device in the conversion of varying DC into constant DC should also be understood. Again, the value of the time constant is significant in the level of smoothing achieved. You should also remember that for a simple RC-series circuit, the voltage sum across the capacitor and resistor should be the same as the input voltage from the generating device.
The other area of concern in the past has been the operation of voltage amplifiers and you need to remember that clipping occurs for input voltages that exceed the conditions for linear gain of the given amplifier. You need to have practice in the sketching of the output voltage for the two cases in which clipping does or does not occur.
In questions related to flip-flops, you will need to read carefully the question in order to decide whether the flip-flop is rising-edge or falling-edge triggered.
You should take heart in the observation over many years that the majority of available marks will clearly and directly relate to the concepts outlined in the study design and you should make an effort to read through the study design in your revision for the exam. Finally, the assessors who will be marking your paper are instructed to reward questions which demonstrate an understanding of the key Physics concepts, so ensure that you highlight these whenever possible. This approach will result in a successful outcome in this Physics CAT.
Roy Burrows is deputy head at Wesley College, Prahran, and chief assessor for VCE physics.
© 1995 The Age