Experiment one: The Motor Effect

For the HSC Physics syllabus dot-points:

• Perform a first-hand investigation to demonstrate the motor effect
• Perform an investigation to model the generation of an electric current by moving a magnet in a coil or a coil near a magnet
• Plan, choose equipment or resources for, and perform a first-hand investigation to predict and verify the effect on a generated electric current when: the distance between the coil and magnet is varied, the strength of the magnet is varied, the relative motion between the coil and the magnet is varied
• Plan, choose equipment or resources for, and perform a first-hand investigation to demonstrate the production of an alternating current

For the first dot-point, we wound wires five times, threading each loop through the region between two strong permanent (neodymium) magnets. We then applied a current of 3 amps through the wire (our power supply was auto current-limiting so no excessive heat was produced). With 5 loops threaded, each with 3 amps, the total effective current was 3×5 = 15amps, and a movement of the wires were observed. The direction of force on the wire was as predicted by the right-hand push rule (aka right-hand palm rule).

For the second dot-point, we tested each condition (distance, strength and speed of magnet) on a coil connected to an ammeter and we observed a direct correlation between strength of magnet and induced current, a direct correlation between speed of magnet and induced current, and an inverse correlation between distance of magnet and induced current.

For the final dot-point, we moved a magnet in and out of a coil connected to an ammeter. The ammeter needle’s direction of movement continually reversed, indicating that an alternating current was produced. We also used a hand-wound AC generator to power an incandescent light bulb.

Experiment two: Transformers and Induction motors

For the HSC Physics ( http://www.duxcollege.com.au/ ) syllabus dot-points:

• Perform an investigation to model the structure of a transformer to demonstrate how a secondary voltage is produced
• Perform an investigation to demonstrate the principle of an AC induction motor

For the first dot-point, we constructed a model transformer using a 300 turn coil as the primary and a 600 turn coil as the secondary. Primary voltage was 18V AC and the secondary voltage was measured (by an AC-capable voltmeter) to be approx 35V AC when using laminated iron cores. The secondary voltage was observed to fall significantly (around 22V) when laminated iron core was replaced with a solid bar of iron (more eddy currents were possible with unlaminated iron core, decreasing efficiency of the magnetic flux transfer). We then switched the primary and secondary coil so that the primary was now 600 turns and the secondary was 300 turns. The secondary voltage was measured to be 8.5V AC. The percentage inefficiency was calculated to be the same in the step up and step down versions.

For the secondary dot-point, we constructed a single phase squirrel cage induction motor. Single phase induction motors generate initial torque due to the presence of shading coils. These coils delay the flux transfer at carefully chosen parts of the surrounding stator so that the initial change in flux produces a torque on the rotor. The squirrel cage core is not connected to any electricity — it moves only due to Lenz Law — it chases the external rotating magnetic field. We observed that the speed of the induction motor was at its highest if using a laminated core. When we changed the orientation of the rotor so that flux has to pass through a section of air, we observed the motor slow down significantly. When we bridged this gap with a solid iron bar, the rotor sped up slightly, but was still slower than its original speed. This again illustrates that air and iron bars are not as effective in transferring magnetic flux as laminated iron.

HSC Biology first-hand investigations coming soon!

Doing well in HSC Physics requires a good mix of different skills. You’ll mainly need to be great at understanding physical concepts, understanding their impacts on society / environmental issues, being able to form cohesive arguments to support your answers, and command some basic maths skill. There’s already a tonne of articles on the blog or forum posts dealing the more conventional study tips, so we thought we might cover some often missed points on the subject. So doing well in HSC physics involves:

Always refer back to the syllabus

All HSC physics exams, whether internal or external, will have to test students within the bounds of the syllabus. Syllabus dot-points are worded in a way that makes them look like exam questions (or the exam questions you get are simply paraphrasing certain syllabus dot-points). If you prepare brief notes covering every dot-point before each exam, you’ll guarantee yourself the knowledge needed to score a decent mark at the least. It is therefore a great idea to make yourself syllabus dot-point summary notes for this subject. Unlike English or Maths, the HSC sciences syllabi are extremely prescriptive, which means everything that can and will be examined are written in black and white on the syllabus for all to see. Know all your dot-points and you won’t go wrong.

One caveat to this approach however are sometimes school teachers may insert ‘creative questions’ that may exceed the bounds of the syllabus because it was covered specifically in class. Can’t blame your physics teacher if this happens (firstly because it’s hard to prove as the syllabus is open to interpretation and secondly the teacher has the last word anyway so even if you’re right, you still won’t get the marks). The best thing to do is to always pay attention in class in addition to knowing the syllabus back to front.

Don’t forget the prac exam

It’s easy to forget the practical / first-hand-investigation requirements of the syllabus. For most students, your internal assessment consists of: term 4 assessment, half yearlies, trials paper exam and prac exam (not necessarily in this order – the prac exam could be the first thing or the last thing you do in the year). The prac exam will be worth around 15-25% of your entire internal assessment mark, so it’s something worth studying for. The best way to prepare for this is to ask students in years above what their prac exam was. Because prac exams require equipment, (and unless you go to a private school with unlimited faculty budgets) chances are each year will be the same prac exam. So if you’ve got friends that graduated in recent years, ask them (if not, ask friends of friends – knowledge is power!) You can also deduce that some pracs won’t be the subject of your exam as they require dangerous activities (projectile motion – they don’t want 20 students in a class throwing projectiles around for 90 minutes) or costly breakable equipment (cathode ray tubes).

Once you know what experiment you’ll be doing for your prac exam (probably the pendulum experiment) you’ll need to familiarise yourself with every aspect of the experiment so that the day will go smoothly when it comes to actually doing the prac. You’ll also have written response questions as part of your prac exam, so prepare answers to the following questions:

1. Were there any safety issues you needed to manage / be aware of?
2. What about sources of error? How would you account for differences between actual and theoretical values?
3. Identify the dependent and independent variables. Identify the control and test variables
4. Think about any graphs you may need to draw. If it involves a line of best fit over collected data, would it make sense to pass through the origin or any particular data point?

Power of the thought experiment

One of the most important factors to any part of HSC Physics is your ability to conduct thought experiments. Thought experiments are when you play a scenario out in your imagination to test the validity or absurdity of a concept you want to test. This technique is useful in many parts of the course. For example, suppose you forgot details of the effects of re-entry (Space module). You could derive everything again on the spot just by thinking it through step by step:

• The shuttle begins in space, and as it enters the atmosphere, friction with air particles slow it down. The friction heats up the air particles to a plasma (and we should know ionised gas blocks radio waves) so this leads to the loss of radio communications during re-entry (called ionisation blackout). The extreme heat of re-entry requires deflection (shuttle needs heat shields). Also the rate of deceleration depends on the angle of re-entry, so there must be an optimum angle.

Another useful application of this is where you forget how to use a formula because of some minor confusion. All you need to do is to apply a hypothetical situation to how you think the formula works – if it leads to an absurd result, you’ll know it’s incorrect and that you should apply the formula differently.