IB Physics HL common mistakes
IB Physics HL · Understand the traps and how to avoid them.
Treating 'g' as a force or writing W = mg without specifying weight as a force in newtons, then using the same symbol g in kinematics equations without distinguishing acceleration from field strength.
Textbooks use g both as gravitational field strength (N/kg) and as acceleration of free fall (m/s^2), and candidates memorize W = mg without tracking units, so they conflate the two quantities and lose method marks when asked to distinguish them.
What to do instead
Always write the unit attached to g in each context (9.81 N/kg in field problems, 9.81 m/s^2 in kinematics) and state explicitly that weight is a force while field strength is force per unit mass; when a question asks about equivalence, argue from W = mg and F = ma giving a = g.
Drawing a free-body diagram with extra arrows for 'the force of motion' or omitting the normal force/reaction, then applying F = ma with a net force that ignores friction or a component of weight along the incline.
Candidates copy the direction of velocity as if it were a force and forget that on an incline the weight component mg sin(theta) acts along the slope, so the net-force equation is wrong from the start and every subsequent number is wrong.
What to do instead
Draw only forces exerted by identifiable objects (weight from Earth, normal from surface, tension from string, friction from surface), resolve along and perpendicular to the motion, and write the perpendicular equation first to get N before computing friction; check that each arrow names its source.
Applying conservation of momentum to a collision without checking whether an external impulse acts, and computing kinetic energy as conserved in an inelastic collision because 'energy is always conserved'.
The phrase 'energy is conserved' is learned as a slogan, so candidates use KE conservation in collisions where only momentum is conserved, and they forget that momentum conservation requires zero net external force over the interaction.
What to do instead
For every collision, state the system boundary and confirm no external horizontal impulse (or account for it), then use momentum conservation; test elasticity by comparing total KE before and after and state that total energy is conserved but KE is not in inelastic collisions.
Using the ideal gas equation with temperature in degrees Celsius, or mixing p in atm with V in cm^3 and R = 8.31 J/(mol K).
Candidates plug numbers into pV = nRT straight from the stem without converting to kelvin and SI units, and they do not check that R's units force p in Pa and V in m^3.
What to do instead
Convert T to kelvin first (T/K = theta/degrees C + 273), convert volume to m^3 (1 cm^3 = 1e-6 m^3) and pressure to Pa, then substitute; as a check, estimate nRT with rounded values to see whether the magnitude is plausible.
Describing the first law of thermodynamics with the wrong sign convention, e.g. writing Q = Delta U + W and then treating work done on the gas as positive while using the same equation.
Different textbooks use Delta U = Q - W or Delta U = Q + W, and candidates memorize one form but apply the sign of W inconsistently when the gas is compressed rather than expanded.
What to do instead
Adopt one convention explicitly (Delta U = Q + W, with W the work done on the gas) and write it at the top of every thermodynamics answer; for compression W is positive, for expansion negative, and for an isothermal process Delta U = 0 so Q = -W.
Confusing the conditions for constructive and destructive interference in thin films and double slits, applying path difference = n(lambda) where the reflection phase change adds pi and the condition flips.
Candidates memorize 'path difference = n lambda for constructive' as a universal rule and do not track the pi phase shift on reflection at a higher-refractive-index boundary, so the bright/dark condition is inverted in film problems.
What to do instead
For each interface, note whether reflection is at a medium of higher n (pi phase change) or lower n (none), then add phase differences in radians before applying the condition; for two slits with no reflection, path difference = n lambda gives constructive, and for a film with one pi shift the conditions swap.
Drawing a ray diagram or diffraction pattern that violates the superposition principle, e.g. claiming single-slit minima occur at path difference = n lambda instead of a sin(theta) = n lambda.
The double-slit formula d sin(theta) = n lambda is overlearned and transferred to single slits, where minima (not maxima) occur at a sin(theta) = n lambda and the central maximum is twice as wide as the others.
What to do instead
Label the geometry: for a single slit of width b, minima at b sin(theta) = n lambda with n = 1, 2, ...; for two slits of separation d, maxima at d sin(theta) = n lambda; sketch the intensity envelope to show the single-slit pattern modulating the double-slit fringes.
Computing the electric field of a point charge or the force between charges without direction, or adding fields as scalars when they are vectors that must be added by components.
Candidates plug into E = kQ/r^2 and F = kQ1Q2/r^2 for magnitudes only, then add magnitudes of fields from two charges even when the fields point in different directions.
What to do instead
Draw the field vectors at the point of interest with arrows, resolve into components along chosen axes, and add component-wise; state the direction (e.g. 'towards the negative charge') in the final answer, and check that like charges give a field pointing away from each.
Using the right-hand rule for magnetic force on a moving charge but forgetting that a negative charge reverses the force direction, or applying F = BIL without the sin(theta) factor when the wire is not perpendicular to B.
The right-hand rule is practiced with conventional current and positive charges, so the extra minus sign for electrons is dropped, and the vector form F = BIL sin(theta) is simplified to F = BIL even at an angle.
What to do instead
Write F = qvB sin(theta) and F = BIL sin(theta) explicitly, determine the direction for a positive charge first, then reverse it for an electron; when the wire is parallel to B, state that the force is zero rather than substituting blindly.
In the photoelectric effect, treating intensity as the variable that changes maximum kinetic energy, or using the work function as the threshold frequency.
Candidates conflate intensity (photons per second) with photon energy (hf), and they mix up phi (work function, in joules or eV) with f0 (threshold frequency, in Hz), so the Einstein equation is applied with mismatched quantities.
What to do instead
Write hf = phi + E_k(max) and identify each symbol with its unit; state that increasing intensity increases the number of photoelectrons but not their maximum KE, while increasing frequency increases E_k(max); convert phi between eV and J using 1 eV = 1.6e-19 J.
In the internal assessment, collecting data without a clear independent/dependent variable structure, or reporting a conclusion that does not follow from the processed data and does not address the stated research question.
Candidates rush to take measurements before defining the variables and controls, then write a conclusion that restates the hypothesis rather than evaluating the evidence, so the analysis and evaluation criteria are not met.
What to do instead
Before collecting data, write the research question, identify the independent, dependent, and controlled variables, and design a table with units and uncertainties; in the conclusion, state the trend with a numerical gradient or fit and compare it to the theoretical prediction, then evaluate specific sources of uncertainty.