# Assessment Report

Level 3 Physics 2018

### Part A: Commentary

Most candidates achieving with Excellence in these standards gave detailed explanations that linked relevant physics concepts to the specifics of the question’s context. They used precise language and demonstrated concise, logical reasoning. These candidates were well-acquainted with a wide range of physics concepts, allowing them to select and combine appropriate formulae to solve problems involving unfamiliar contexts.

Candidates achieving these standards regularly explain what they are doing at each step in calculation questions. Writing the relevant formula, then substituting numbers then giving the correct numerical solution with correct units, is expected. “Show” questions particularly require some sort of explanation. Many candidates would have improved their answers had they drawn diagrams to help process the information given in the question and to clarify the communication of their responses.

Most candidates not achieving these standards often lacked sufficient algebraic and trigonometric skills to solve problems. Many demonstrated insufficient familiarity with physics concepts to be able to identify which concepts were relevant and select appropriate formulae to solve problems.

## 91523:  Demonstrate understanding of wave systems

Candidates who were awarded Achievement commonly:

• used equations correctly but not demonstrate understanding of fundamental concepts
• had shown competence in more than one aspect of wave systems
• related wavelength of standing wave to length of medium
• recognised and could describe the Doppler effect accurately.

Candidates whose work was assessed as Not Achieved commonly:

• missed crucial parts of “show” questions
• confused diffraction and interference
• did not correctly apply the relationship between wave speed, wavelength and frequency
• described the Doppler effect as an observed change in volume
• used the speed of the wave source where the speed of sound was required
• described wave-fronts slowly separating as they radiated from a source.

Candidates who were awarded Achievement with Merit commonly:

• were accurate in the completion of calculations requiring multiple steps
• explained that interference, rather than diffraction, causes maxima
• described cause of the the Doppler effect clearly and correctly
• expressed familiarity with fundamental concepts.

Candidates who were awarded Achievement with Excellence commonly:

• had enough depth of understanding to explain all the outcomes of a change to a wave system
• answered the questions by linking concepts with equations
• explained in detail the effect on frequency, wavelength & velocity when higher harmonics are played.

Standard specific comments

• Candidates often did not show an understanding of the separate roles of diffraction and interference in producing the interference pattern when light is passed through a diffraction grating.
• Many candidates ascribed the Doppler Effect to position (distance) rather than velocity.
• Terms such as “relative velocity” were often used in ways that did not demonstrate a clear understanding of the term’s meaning.

## 91524:  Demonstrate understanding of mechanical systems

Candidates who were awarded Achievement commonly:

• used relevant equations in one-step calculations, sometimes when more complex calculations were required
• recognised that gravitational force provides the centripetal force for orbital motion
• used correct formulae, with working, to show how given values are determined
• used the correct formulae and method with incorrect calculation
• described position of the centre of the mass and its constant velocity if no external forces act
• described the effects of balanced/unbalanced forces separately from unbalanced torques
• recognised physics concepts and selected appropriate formulae to solve problems
• stated the conditions required for the law of conservation of angular momentum and simple harmonic motion
• drew a correct diagram using the reference circle or used an appropriate equation for SHM
• gave a reasonable description (or graph) of damping indicating reduced amplitude or mentioned constant period/frequency or identified damping force as friction with the liquid.

Candidates whose work was assessed as Not Achieved commonly:

• confused the use of values of G and g in calculations
• did not recognise the force due to gravity as providing the centripetal force for orbital motion
• attempted to calculate the orbital velocity of satellites using only the radius of the earth
• could not calculate the position of the centre of mass of a system or describe its motion
• used one-dimensional momentum calculations for the two-dimensional momentum problems
• did not recognise relevant physics concepts or select the appropriate equations to solve problems
• failed to convert minutes to seconds before substitution into formulae
• failed to distinguish between forces and torques
• attempted to use irrelevant concepts and equations in explain questions
• confused conservation of rotational kinetic energy with conservation of angular momentum
• did not recognise the conditions needed for the law of conservation of angular momentum
• lacked knowledge of Simple Harmonic Motion, reference circles, equations, and graphs
• were unfamiliar with the concept of damping.

Candidates who were awarded Achievement with Merit commonly:

• combined relevant formulae to derive a formula for orbital velocity
• calculated the position of the centre of mass of a system and described its motion, but failed to justify the description
• used vector mathematics to calculate the total momentum in two-dimensional problems
• described the differing effects of unbalanced torques and unbalanced forces on an object
• selected and applied appropriate formulae in multi-step calculations for various physics concepts
• justified the use of the law of conservation of angular momentum, linked to no external torques
• used reference circles or correct equations to determine the velocity of SHM oscillators
• explained damping effects on amplitude and period of harmonic motions
• used linking words in explanations, showing that one event causes another event.

Candidates who were awarded Achievement with Excellence commonly:

• selected appropriate data and formulae to solve problems involving multi-step calculations and gave the correct units
• showed understanding of the conservation of angular momentum, used relevant equations to calculate the angular velocities and stated assumptions used
• showed clear understanding of how damping affects amplitude and period, with descriptions
• of energy changes in a damped system, the damping force in laboratory models. Descriptions were clearly linked with the impacts on the motion
• sketched graphs of the position of the mass vs. time starting at the moment the mass was released.

Standard specific comments

• Many candidates displayed misconceptions around conservation of energy when applied to rotational kinematics.
• Frequently, no assumptions or irrelevant assumptions were given when asked for in questions.
• Many candidates were not able to state the conditions required for simple harmonic motion.
• Many candidates were not able to calculate the spring constant of the spring.
• Phasors on reference circles and the selection of the appropriate SHM equations are problematic for many candidates.
• Many candidates did not fully address details within the questions. For example, question 3d asked to draw the graph starting at the moment the mass is released from the maximum downward displacement and candidates drew the graph starting at equilibrium or the top.

## 91526:  Demonstrate understanding of electrical systems

Candidates who were awarded Achievement commonly:

• described the relationship between EMF, terminal voltage and internal resistance for a battery
• knew the meaning of time constant of a RC circuit for charge, current or voltage
• could describe the effect of changing resistance on a RC circuit
• evaluated the current in DC and AC LR circuits
• selected appropriate formula to solve single-step calculation problems
• knew some of electrical energy in a working circuit will be lost to heat due to resistance
• sketched I-t graphs of charging RC circuits with accurate starting points
• calculated maximum currents and time constants of LR circuits
• evaluated the total impedance of simple LRC circuits
• knew the conditions of resonance in a LCR series circuit.

Candidates whose work was assessed as Not Achieved commonly:

• did not know how the internal resistance of a battery affects simple circuits as current increases
• did not know the definition of time constant of a RC circuit or the factors that effect it
• did not describe the effects of an inductor in either DC or AC LR circuits
• did not select appropriate formulae to calculate capacitance, impedance, and reactance
• did not realise the sudden change in current causes an induced EMF (self-induction)
• did not calculate the resonant frequency of a simple LCR circuit
• did not sketch or interpret the information on I-t graphs of RC circuits
• failed to calculate the impedance of a simple LRC circuit
• did not know the condition of resonance in LCR series circuits
• failed to recognise that some of electrical energy in a working circuit will be lost to heat due to resistance in the circuit (or resistors)
• did not know the voltage across an ideal inductor is zero when the current becomes steady
• did not recognise the role of Faraday’s law LR circuits when current suddenly drops to zero.

Candidates who were awarded Achievement with Merit commonly:

• evaluated the internal resistance of a battery given EMF, terminal voltage and current
• determined time constants from charging graphs to determine the resistance of RC circuits
• identified relevant physics concepts to explain the effect of changes in resistance to RC circuits
• described the role of Faraday’s law in the production of large induced voltages when a switch is opened in a RL circuit
• evaluated current in two-loop circuits with two power sources
• plotted I-T decay graphs, including time constant and maximum current values
• calculated capacitance of a capacitor or inductance of an inductor with given reactance at a certain frequency
• calculated impedance of LRC circuits with capacitance, inductance and frequency given
• knew the factors that affect capacitance and inductance
• calculated the resonant frequency of LCR circuits with given inductance and capacitance.

Candidates who were awarded Achievement with Excellence commonly:

• linked decreases in resistance of a circuit to increases in current and decreases in the time taken to charge. Identified resulting energy changes, including where wasted energy goes and in what proportion, in order to justify why energy stored in a fully charged capacitor is independent of circuit resistance
• linked significant reductions in resistance to significant increases in time constants to explain why an LR circuit with a 20 ohm resistor produces only a small induced voltage when disconnected from a source of power, with reference to Faraday’s law
• recognised when a resonant frequency needed to be raised or lowered in order to match a set frequency, then recommended correct physical changes to the capacitor and the inductor to adjust their capacitance and inductance to do so.

Standard specific comments

• Many candidates lacked the depth of understanding of electromagnetic induction expected for this standard. Frequently, induced voltage and induced current are confused. Candidates are seemingly unaware of the difference between them or the conditions for each to occur.
• Many candidate responses appeared to be answers from previous examination questions which have been memorised and repeated without the relevant adjustment needed to make the response fit the details of the current examination question.

### Physics subject page

Previous years' reports
2016 (PDF, 243KB)

2017 (PDF, 50KB)