Assessment Report

Level 3 Physics 2017

Standards 91523  91524  91526 

Part A: Commentary

Most candidates attempted all questions and wrote significant amounts for the more difficult explanation questions. In general, candidates showed greater competence in solving problems that required calculations than those that required written explanations. Candidates frequently failed to fully address the specifics of the question asked, giving explanations of the general case rather than relating concepts to details of the specific case. Many candidates would also have improved their answers had they drawn diagrams to help demonstrate their understanding. Trigonometric problem solving, such as vector addition, SHM reference circles, and AC phasor diagrams are consistently poorly attempted by candidates achieving below merit level. However when attempted, they are frequently correctly drawn.

Level 3 Physics requires careful use of precise language and inclusion of relevant details linked to the specific situation given in the context to achieve with excellence.  

Part B: Report on standards

91523:  Demonstrate understanding of wave systems

Candidates who were awarded Achievement commonly:

  • calculated wavelengths and frequencies
  • related beats to interference
  • sketched wave forms of different harmonics on strings and pipes 
  • labelled nodes and antinodes on diagrams of standing waves
  • identified the speed of sound and the speed of light correctly.

Candidates who were assessed as Not Achieved commonly:

  • showed insufficient working in calculations
  • used calculators in radian mode when inputting angles in degrees
  • could not describe the Doppler effect using physics terms
  • confused the term ‘node’ with ‘destructive interference’ and ‘trough’
  • confused the term ‘antinode’ with ‘constructive interference’ and ‘crest’
  • drew fixed-end wave forms when open-end were required or visa versa
  • did not attempt more difficult questions.

Candidates who were awarded Achievement with Merit commonly:

  • communicated understanding through labelled diagrams
  • demonstrated clear logic through sufficient working for ‘show’ calculations
  • wrote accurate explanations but omitted essential details to demonstrate a thorough understanding of wave concepts
  • explained how the movement of a wave source changes the wavelength in Doppler effect applications
  • linked the colour of light to wavelength to explain changes to patterns formed by passing different colours of light through diffraction gratings
  • failed to distinguish between the central maxima and the first order antinode.

Candidates who were awarded Achievement with Excellence commonly:

  • explained all aspects of standing wave formation paying close attention to detail
  • used accurate, labelled diagrams, equations, in addition to text in explanations
  • explained that spectra are seen because different wavelengths of light interfere differently and applied this understanding to calculations
  • applied understanding of physics concepts to unfamiliar situations to solve problems and explain observations
  • related physics concepts to provide explanation for specific situations or general cases as required.

Standard specific comments

This standard requires very precise use of language and detail.  For example, many candidates say; “the path difference is a whole number” rather than “the path difference is a whole number multiple of wavelength”, ‘the first harmonic is a quarter of a wavelength’ rather than ‘the length of the pipe is one quarter of the wavelength”, and “out of phase” (which could mean anything not 100% in phase) rather than “180out of phase”.  Candidates must carefully read the question and ensure they have fully answered the question and with sufficient detail.



91524:  Demonstrate understanding of mechanical systems

Candidates who were awarded Achievement commonly:

  • used relevant equations in a one-step calculation, sometimes when amore complex calculation was required
  • selected correct problem solving methods, but made calculation mistakes
  • recognised the centre of mass moves with constant velocity before and after a collision with no external forces
  • described changes in force and potential energy as orbit radii decreased
  • equated gravitational force with centripetal force for orbiting objects
  • described relationships between mass, radii, rotational inertia, and angular velocity in situations where angular momentum is conserved
  • described conditions required for simple harmonic motion
  • drew accurate reference circle diagrams
  • identified limitations of a model compared to the reality of a situation.

Candidates who were assessed as Not Achieved commonly:

  • struggled to select and apply equations in single step calculations
  • could not calculate the position of the centre of mass or describe the motion of the centre of mass
  • failed to recognise the vector nature of momentum, attempting calculations in only one dimension
  • confused distance, speed, and momentum definitions and interchanged values in calculations
  • selected incorrect equations to solve particular problems
  • used irrelevant equations in explanations
  • failed to describe relationships between mass distribution, rotational inertia, torque, and angular acceleration
  • thought that rotational kinetic energy is conserved rather than angular momentum 
  • lacked knowledge of the conditions of SHM, SHM equations, or SHM graphs.

Candidates who were awarded Achievement with Merit commonly:

  • calculated position of the centre of mass of a system
  • added momentum vectors using vector addition processes
  • had correct method, but made minor errors in substitution or algebra
  • linked changes in one quantity to changes in another for applications involving a conservation law
  • drew velocity vs time graphs for SHM including a calculated maximum velocity
  • used linking words in explanations showing that one event causes another event
  • did not fully address questions, such as simply drawing a sinusoidal graph for SHM when the question asked to start the graph from the lowest point.

Candidates who were awarded Achievement with Excellence commonly:

  • were able to perform multi-step calculations using correct formula, selecting appropriate data, and gave answers with correct units
  • distinguished between different conservation laws and clearly stated which quantities were increasing, decreasing, and staying constant in particular situations
  • justified use of specific conservation laws by identifying that no external, unbalanced forces and/or torques existed
  • were able to articulate the assumptions made to simplify calculations.

Standard specific comments

Many candidates struggled with the concept of centre of mass, frequently misidentified forces – particularly gravitational force and tension, and held significant misconceptions around the application of conservation of energy when applied to rotational kinematics. Algebraic skills involving trigonometry were challenging for many candidates, particularly the concept that  sin ωt ≠sin t norω sin t nor t sinω.



91526:  Demonstrate understanding of electrical systems

Candidates who were awarded Achievement commonly:

  • used formula to calculate basic values
  • described and calculated time constant
  • described how current changes as a capacitor charges
  • calculated combined capacitance 
  • knew that a changing current creates a changing magnetic flux
  • explained how an inductor induced an opposing EMF
  • converted between RMS and peak voltage
  • calculated the impedance of a LR circuit
  • described the effect of increasing the dielectric constant for a capacitor on capacitance
  • showed basic understanding of characteristics of LCR circuits.

Candidates who were assessed as Not Achieved commonly:

  • did not know the meaning of time constant for a capacitor
  • thought that energy was lost in a circuit because the voltage had to pass through a resistor first
  • stated that protons move onto the positive plate of a capacitor to give it a positive charge
  • described charging a capacitor as increasing its internal resistance
  • made calculation errors such as forgetting to square or square root values in equations
  • square rooted entire blocks of numbers instead of just the values stated in the formula
  • confused charge with capacitance
  • confused inductance with reactance
  • thought that inductors create an opposing current to oppose a supply voltage
  • could not draw a phasor diagram to represent relationships in AC circuits.

Candidates who were awarded Achievement with Merit commonly:

  • described energy, charge, and current relationships for charging capacitors
  • linked changes in capacitance and time constant to the adding capacitors in parallel
  • explained how voltage is induced in the secondary coil of a transformer
  • explained why current slowly increases when an inductor is connected in parallel
  • calculated the inductance of circuits by determining the rate of chance of current from d vs t graphs
  • explained why a spark could be produced when a switch is opened in a circuit
  • calculated resonant frequencies of LCR circuits and explained why current is maximum at resonance.

Candidates who were awarded Achievement with Excellence commonly:

  • demonstrated a good understanding of the three stages of charging a capacitor and explained the exponential shape of the sketched graph 
  • gave a thorough explanation of how an inductor affects the current and why a spark is produced when the switch is opened
  • calculated the resonant frequency of an LCR circuit and explain why the current is maximum at resonance.

Standard specific comments

Many candidates demonstrated misconceptions in their understanding of inductors. Many described an ‘induced current’ opposing the circuit current rather than an induced opposing voltage.

 

Physics subject page

 

Previous years' reports
2016 (PDF, 243KB)

 
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