Common Mistakes in HSC physics

6 Common Mistakes HSC Physics Students

Make in Exams

Posted on August 15, 2012 by DJ Kim • 0 Comments

Read the 6 Common Mistakes HSC Physics Students Make in Exams. Learn the errors HSC students are making and avoid them.

Mistake #1: Incorrect interpretation of Key Verbs

 List  Describe  Compare  Explain  Assess  Evaluate

A large number of students fail to respond to questions involving „Compare‟ or „Explain‟ or „Assess‟ correctly.

For example students are listing instead of describing or giving a description not an explanation.

Did you know that HSC Marking Centre advises the use of tables to answer questions involving compare, assess, discuss and evaluate?

Consider the Sample Responses to 2006 HSC question given below by the Board of Studies: Assess the impact on society and the environment of the potential applications of superconductors.

Sample Response 1

Application of

superconductor Impact on Society

Impact on environment Assessment Maglev Trains  Provides faster, more energy-efficient transport  More expensive to build – i.e. higher fares  Less use of fossil fuels to power train, therefore less random emissions  Less coal needs to be mined for  Beneficial to both society and environment – however is costly

power stations Transporting Electricity  More efficient transportation  Zero power losses  Safer as DC is used  No need to have expensive transformers  Cheaper to transport  Smaller cables means more aesthetic benefits  Less fossil fuels emitted into atmosphere – therefore, less air pollution  Reduced likelihood of acid rain  Beneficial to society as a cheaper, more efficient transportation takes place, less harm to society in the form of pollution Superconducting generation of power  More efficient power production  No need for AC and transformers  Cheaper electricity  Cleaner energy  No fossil fuels used for power generation  Less environmental impacts, society gets cheaper energy OR Sample Response 2

Applications Advantages Disadvantages

Maglev

Trains Society

 Frictionless form of transport allowing super fast speeds

 The costs of implementation are detrimental maintenance is a  The costs of implementation are detrimental maintenance is a problem

problem

Environment

 Does not use fossil fuels to provide transport

 The superconductors must be kept at a critical temperature which is difficult to maintain and uses a large amount of energy

Transformers

Society

 Allows for large scale distribution of energy without power loss as there is no resistance in cables.

 Replacement of manual labour causing unemployment lack of technology to maintain a large distribution grid  Replacement of manual labour causing unemployment lack of technology to maintain a large distribution grid

Environment

 Power lines are not aesthetically pleasing

 Large amounts of energy used to keep cables at critical temperatures costs and wastage of materials in creating cables for large scale supply

Mistake #2: Insufficient Workings for Calculations

 1 mark for correct formula and substitution and

 1 mark for the correct answer

In questions requiring numerical answers, you should always

 show the working in a clear and logical fashion

 round-off answers at the end of the calculation instead of during the parts of the calculation.

In the example below, the student made the correct substitution but did not square the denominator and as a result got an incorrect answer. However the student was still awarded one mark.

If you have made a calculation mistake, you may still be awarded marks for your working, so it must always be clearly shown.

Mistake #3: Write units. I mean correct SI

Units!

Students forget to provide the correct units with their numerical answers.

What is wrong with this student‟s answer regarding the magnitude of acceleration? “The acceleration of the rocket is 15 m/s.”

Make sure you know the SI units for the following:

 mass  velocity  acceleration  force  momentum  energy  frequency  wavelength  current  potential difference  resistance

Mistake #4: Line of Best Fit?

When drawing a line of best fit, students must identify and eliminate any outliers before determining the trend. In the diagram shown below, if a student does not recognise the outlier then he/she will draw an incorrect line of best fit (dotted line). However once you remove the outlier from trend, then you can draw a correct line of best fit (solid line).

Mistake #5: Confusion about Validitiy, Reliability and

Accuracy?

When students are asked how you would improve the accuracy of your experimental results, they often reply with a statement such as “repeat the experiment many times”. Does this sound like you? If so, then expect to lose at 3 – 4 marks in your HSC Exam this year. Here is an outline of the differences between validity, reliability and accuracy:

Definition Technique used in pendulum experiment

Validity

Validity is how appropriate the procedure and materials are to achieve a desired experimental result.

Swinging the pendulum through small amplitudes to ensure the equation given could be applied.

Reliability

Reliability is how repeatable the experment is. Do you get very similar results every time?

Starting and stopping the stopwatch at the extremes of the motion ensures more repeatability than trying to start and stop mid-swing.

Accuracy

Accuracy is how close the value calculated from the experiment is to the accepted true value.

Use of more precise measuring devices such as a data logger and a sensor would improve accuracy.

Mistake #6: Human error is not a valid type of

experimental error!

When students are asked about how to improve the reliability and accuracy of the

experimental results, most state “reduce human error!” Human error is not a valid type of experimental error! The two types of experimental error explored in physics experiments are random and systematic errors.

Random errors are caused by unknown and unpredictable changes in the experiment, e.g.

due to the instruments or environmental conditions. These are statistical fluctuations in both directions about the true value thus repetition and statistical analysis can reduce the

effects. To help reduce random errors:

 Make sure you know how to read the scales on the instruments and that you align yourself properly each time you take a measurement.

 Take multiple measurements (repetition increases reliability!) and then take an average for the result

Systematic errors are caused by measuring instruments being used incorrectly or problems

with the instrument itself. Systematic errors limit accuracy. To help reduce systematic errors:

 All instruments should be checked against a standard before use. Zero settings should be checked and adjusted (calibration!).

 Instructions for the use of the instrument should be read and followed.