Catalonia PAU Physics / Física 2º Bachillerato (CIC Generalitat) common mistakes
Catalonia PAU Physics / Física 2º Bachillerato (CIC Generalitat) · Understand the traps and how to avoid them.
When a problem gives a mass in grams, a distance in cm or a charge in μC, the candidate substitutes the number into the formula without converting, obtaining energies or forces that are off by powers of ten (e.g. a gravitational force 10^4 times too small because r was left in cm).
PAU statements routinely mix units to keep numbers 'nice', and the formula sheet gives constants in SI. Candidates focus on choosing the right equation and treat unit conversion as a formality done at the end, or they convert only the first magnitude and forget the rest.
What to do instead
Before substituting, write a one-line 'data in SI' list on the left margin: m = ... kg, r = ... m, q = ... C, and cross out the original statement values. Make the conversion part of the same step as reading the data, and do a magnitude sanity check at the end (a 1 kg mass and the Earth should give ~10 N, not 10^-3 N).
The candidate computes gravitational potential energy as U = −GMm/r but then adds kinetic energy with the wrong sign or uses U = +GMm/r, and concludes that a satellite in a higher orbit has less total energy than one in a lower orbit without justification.
The negative sign of the gravitational potential is memorised as a formula decoration rather than understood as a zero reference at infinity. Students also confuse 'potential energy' with 'potential' and mix up the bound-state logic (E = −GMm/2r for circular orbits).
What to do instead
Always fix the reference explicitly: U(∞) = 0, so U is negative and increases (becomes less negative) with r. Derive E_total = U + K = −GMm/2r for a circular orbit once by hand, and in each exercise state whether the quantity asked is potential, potential energy or total energy before substituting.
In electrostatics, the candidate computes the field or potential at a point created by two or three charges by adding the magnitudes arithmetically, ignoring that E is a vector and that potentials from negative charges must be subtracted.
School exercises often start with single-charge fields, so superposition is practised late. The candidate also treats 'potential' as if it were a vector and tries to take components of V, or forgets that V is a scalar sum with signs.
What to do instead
Adopt a fixed routine: draw the point, draw each field vector with its direction (away from +, towards −), decompose into x/y, then sum components; for potential, sum q_i/r_i algebraically with signs and never take components. State in one sentence why E is a vector and V is a scalar before calculating.
The candidate answers questions about a conductor in electrostatic equilibrium by saying the field inside is zero 'because there is no charge', or claims the charge distributes uniformly on any conductor, including one with sharp points.
The standard results (E = 0 inside, V constant, charge on the surface) are memorised as slogans without the reasoning from Gauss's law, so the candidate cannot adapt when the question asks where charge accumulates or what happens to the potential inside a hollow conductor.
What to do instead
Learn the three consequences as a chain: charges repel and move until E = 0 inside; E = 0 implies V is constant throughout the conductor; since like charges repel, surface density is highest where curvature is greatest (points). Practise explaining each link in two sentences, and connect to the capacitor case (uniform field between plates, E = 0 in the metal).
When a charged particle enters a magnetic field, the candidate sets the magnetic force equal to qvB and then treats it as if it did work, computing an increase in kinetic energy, or uses the right-hand rule and gets the direction reversed for a negative charge.
The formula F = qv×B is applied as a magnitude formula, and the cross product's perpendicularity is not internalised. The sign of the charge is easy to drop, and students carry over the habit from electric fields where the force does do work.
What to do instead
Anchor two facts: F is always perpendicular to v, so magnetic force does zero work and speed (kinetic energy) is constant - only the direction changes; and for negative charges reverse the result of the right-hand rule (or use the left hand). For circular motion set qvB = mv²/r and check that r depends on momentum, not on speed alone.
In an induction problem (a bar moving on rails, a coil entering a magnetic field), the candidate computes the emf with Faraday's law but then assigns the direction of the induced current by guessing, or by applying Lenz's law to the magnetic field rather than to the change in flux.
Lenz's law is stated as 'opposes the change' but candidates apply it to the flux itself, not to its variation, and they skip drawing the field lines. The minus sign in ε = −dΦ/dt is treated as a formality.
What to do instead
Use a three-step written procedure every time: (1) draw B and mark whether Φ is increasing or decreasing; (2) the induced B must oppose that change, so draw it; (3) use the right-hand grip rule to get the current sense. Then verify with the force on the induced current (it must oppose the motion). Write the minus sign and say what it means.
For a wave, the candidate confuses the speed of the wave with the speed of the oscillating particles, or claims that when the frequency doubles the wavelength also doubles because 'they are proportional'.
The relation v = λf is memorised as a proportionality between all three quantities, and the fact that v is fixed by the medium (for a given wave) is not emphasised. Students also mix up transverse displacement with propagation.
What to do instead
State explicitly: v is set by the medium (string tension and linear density, or the properties of the material); changing f changes λ inversely. Practise with y(x,t) = A sin(ωt − kx): identify which parameter is the particle velocity (∂y/∂t, maximum ωA) and which is the propagation speed (ω/k).
In a thin-lens problem, the candidate substitutes object distance as negative, or applies the magnification formula with the wrong sign, and then reports a virtual image as real (or vice versa) without checking with a ray diagram.
The Cartesian sign convention (real object distance positive, image distance negative for virtual images, f negative for diverging lenses) is applied inconsistently, and students rely on the formula alone because they cannot sketch the three principal rays quickly.
What to do instead
Fix one convention and write it at the top of the answer: for a thin lens, 1/f = 1/s' − 1/s with the signs of the convention used in class; then always draw the three rays (parallel, through the centre, through the focus) and check that the sign of s' matches the diagram. If the image is on the same side as the object for a lens, it is virtual.
In simple harmonic motion, the candidate writes x = A sin(ωt) with an arbitrary phase and then computes the velocity or acceleration at t = 0 as zero (or maximum) without checking the initial conditions given in the statement.
The general solution is memorised in one form only, and the phase constant is treated as decoration. Students also forget that a = −ω²x, so acceleration is maximum at the extremes, not at equilibrium, and they mix up the position of maximum speed.
What to do instead
Always derive the phase from the initial conditions: write x = A sin(ωt + φ), impose x(0) and v(0) from the statement, and solve for φ. Memorise the energy/kinematics picture: at x = ±A, v = 0 and |a| maximum; at x = 0, |v| maximum and a = 0. Check the answer against that picture.
In relativity or the photoelectric effect, the candidate applies the classical Doppler or Galilean addition of velocities to light, or computes the photoelectric kinetic energy as hf without subtracting the work function, and ignores the threshold frequency.
The relativistic results (time dilation, length contraction, E = hf − W₀) are learned as isolated formulas without the physical statement that c is invariant and that emission requires f > f₀. Students also forget to convert eV to J or to use h in the right units.
What to do instead
Write the two postulates at the top of relativity problems and never add velocities linearly with c. For the photoelectric effect, always write E_photon = W₀ + K_max, compute f₀ = W₀/h first, and check f > f₀ before computing K_max; keep h in J·s and convert eV with 1 eV = 1.6×10⁻¹⁹ J.
When reporting an experimental result, the candidate writes a value with six decimal places from a calculator and an uncertainty with two significant figures, or quotes an uncertainty without stating whether it is absolute or relative.
Calculator output is copied verbatim, and the rules for significant figures of uncertainties (one, at most two, significant figures, with the value rounded to the same decimal place) are not practised. Students also confuse the resolution of an instrument with the standard deviation of repeated measurements.
What to do instead
Apply the reporting rule mechanically: round the uncertainty to one significant figure (two if the leading digit is 1), round the central value to the same decimal place, and give units. Distinguish instrumental resolution (half the smallest division, or the division itself) from the standard deviation of repeats, and state which one you are quoting.
When asked to design or evaluate an experiment, the candidate lists the apparatus and the steps but never identifies the independent, dependent and controlled variables, and never says how the data will be plotted to test the hypothesis.
Experimental questions are treated as a memory test of apparatus names rather than as a design task. Students also omit repeats and the treatment of outliers, and do not linearise the expected relation before plotting.
What to do instead
Use a fixed template: hypothesis → variables (independent, dependent, controlled, with how each is controlled) → procedure with repeats → table of data → linearised graph (state what you plot against what so the slope gives the constant of interest) → conclusion linked back to the hypothesis. Practise linearising, e.g. plotting T² against L for a pendulum to get g from the slope.