ORELA Physics (NT308) common mistakes
ORELA Physics (NT308) · Understand the traps and how to avoid them.
Reads a described experiment and dismisses the conclusion as "just a theory," or accepts a claim simply because it is called a law, without asking what evidence could count against it.
Everyday English uses "theory" to mean a guess, and test prep often drills the hypothesis/theory/law definitions as vocabulary instead of as reasoning about evidence. Candidates then answer definition-recall items correctly but fail items that ask them to evaluate a described investigation.
What to do instead
Practise converting each described study into three explicit statements: the testable hypothesis, the prediction, and the observation that would falsify it. Sort sample claims into hypothesis, model, law, and observation, and justify each label by pointing to the evidence offered rather than to the label's prestige.
Assigns a = +9.8 m/s² to gravity no matter which way the axis points, then reports a negative displacement as a negative distance or a negative speed.
The value 9.8 is memorised as a number rather than as the magnitude of a vector whose sign depends on the chosen coordinate system. Distance versus displacement and speed versus velocity are treated as synonyms under time pressure.
What to do instead
Before writing any equation, draw the axis and write "up is positive" or "down is positive" on the page. Then substitute a = −g only when up is positive, and check every sign against a physical statement (is the object still rising or already falling?). Report magnitudes for distance and speed, signed values for displacement and velocity.
Assumes kinetic energy is conserved in every collision, or conserves momentum in one dimension while ignoring the perpendicular component of a two-dimensional collision.
Both energy and momentum are labelled "conserved," so candidates apply both indiscriminately. Momentum's vector nature is forgotten because most practice problems are one-dimensional, and the collision is not classified before calculating.
What to do instead
Classify the collision first: elastic, inelastic, or perfectly inelastic. Conserve momentum as separate x and y components in every collision; conserve kinetic energy only when the problem states elastic or gives no energy loss. Sanity-check with the perfectly inelastic case, where final kinetic energy must be lower.
Substitutes Celsius temperatures into the ideal gas law or into kinetic-theory expressions, and treats heat and temperature as interchangeable when predicting the final temperature of two objects placed in contact.
Everyday language says objects are "hotter" or "colder" without distinguishing energy transfer from average molecular kinetic energy. Formulas are memorised with symbols but without the unit requirement, so 20 °C goes straight into pV = nRT.
What to do instead
Convert every temperature to kelvin before substituting, and write the conversion in the margin as a habit. Keep Q (energy transferred, joules) distinct from T (average kinetic energy, kelvin). Write the first law as ΔU = Q − W with the sign convention stated explicitly, then verify that an expanding gas doing work loses internal energy if no heat is added.
Plugs distances into 1/f = 1/do + 1/di using a mixture of sign conventions, then calls a virtual image real, or reports a negative image height as "inverted" without checking the geometry.
Different textbooks use different sign conventions, and candidates memorise whichever formula they saw last. Under time pressure the ray diagram is skipped, so there is no independent check on whether the image is on the same side as the object or on the opposite side.
What to do instead
Choose one convention and write it at the top of the scratch page (for example, real object distance positive, virtual image distance negative, converging focal length positive). Always draw two principal rays - parallel then through the focal point, and through the centre - and let the sketch confirm side, orientation, and whether the image is real.
Claims a wave's speed changes when its frequency changes as it enters a new medium, or applies open-pipe harmonic patterns (all integer multiples) to a pipe closed at one end.
v = fλ is memorised as a single relationship without knowing which quantity is fixed by the source and which by the medium. Boundary conditions are treated as a list to memorise rather than as constraints that determine which standing waves can exist.
What to do instead
State the rule out loud: frequency is set by the source and is unchanged at a boundary; speed is set by the medium, so wavelength adjusts. Then derive allowed wavelengths from the boundary conditions - nodes at closed ends and at fixed supports, antinodes at open ends - so a closed-open pipe is seen to support only odd harmonics.
Says current is "used up" as it passes through each resistor in a series circuit, or finds the equivalent resistance of a parallel combination by simply adding the resistances.
The water-flow analogy is overextended, and the resistance formulas are memorised without the conservation reasoning behind them. Candidates rarely check their answer by computing voltage across each element.
What to do instead
Apply Kirchhoff's junction rule explicitly: charge does not accumulate at a node, so the current entering equals the current leaving. Compute the equivalent resistance, then verify with V = IR at each element and confirm that the voltages around each loop sum to the source voltage and that parallel branches share the same voltage.
Predicts the direction of an induced current by pointing the wrong hand or thumb, or drops Lenz's law entirely and draws the induced magnetic field pointing the same way as the change in flux.
Right-hand rules are taught as isolated finger tricks with no underlying reasoning, so candidates cannot tell which rule applies to a moving charge, a current-carrying wire, or a changing flux. Lenz's law is remembered as a minus sign rather than as opposition to change.
What to do instead
Reason in ordered steps rather than by gesture memory: identify how the flux is changing, determine the direction of induced field that opposes that change, then use the right-hand rule to convert that field into a current direction. Rehearse with a magnet falling into a coil and a loop rotating in a field, checking each step against energy conservation.
Claims that brighter light ejects faster photoelectrons, or that light below the threshold frequency will eject electrons if it is intense enough.
Classical wave intuition says more intensity means more energy delivered, and the distinction between photon energy and photon number is not internalised. Intensity and frequency are both treated as measures of "how strong" the light is.
What to do instead
Anchor on E = hf as the energy of a single photon: frequency fixes the maximum kinetic energy, and the work function sets the threshold. Intensity only sets how many photons arrive per second, hence the saturation current. Sketch the stopping-potential versus frequency graph and label its slope as h/e and its intercept as −φ/e.
Calculates a mass defect but then predicts energy release for any reaction with a mass difference, or predicts that heavy nuclei fuse and light nuclei split, and substitutes grams into E = mc² expecting joules.
The binding-energy-per-nucleon curve and its peak near iron-56 are not held in memory, so the direction of energy release is guessed. Unit consistency between atomic mass units, kilograms, and electronvolts is not checked.
What to do instead
Sketch the binding-energy-per-nucleon curve and read the direction off it: reactions release energy by moving toward more tightly bound nucleons, which means fusion of light nuclei and fission of heavy ones. Convert mass to kilograms before E = mc², and cross-check the answer in MeV using 1 u ≈ 931.5 MeV.